# 74 AREP

The share package \AREP provides an infrastructure and high level functions to do efficient calculations in constructive representation theory. By the term constructive" we mean that group representations are constructed and manipulated up to equality -- not only up to equivalence as it is done by using characters. Hence you can think of it as working with matrix representations, but in a very efficient way using the special structure of the matrices occuring in representation theory of finite groups. The package is named after its most important class ARep (see AReps) ( Abstract Representations) \footnote A note on the name: We have chosen abstract" because we manipulate expressions for representations, not constants. However, concrete" would also be right because the representations are given with respect to a fixed basis of the underlying vector space. The name ARep is thus, for historical reasons, somewhat misleading. implementing this idea.

A striking application of constructive representation theory is the decomposition of matrices representing discrete signal transforms into a product of highly structured sparse matrices (realized in Matrix Decomposition). This decomposition can be viewed as a fast algorithm for the signal transform. Another application is the construction of fast Fourier transforms for solvable groups (realized in DecompositionMonRep). The package has evolved out of this area of application into a more general tool.

The package \AREP consists of the following parts:

Monomial Matrices: A monomial matrix is matrix containing exactly one non-zero entry in every row and column. Hence storing and computing with monomial matrices can be done efficiently. This is realized in the class Mon, Sections Mons -- CharPolyCyclesMon.

Structured Matrices: The class AMat, Sections AMats -- UpperBoundLinearComplexityAMat, is created to represent and calculate with structured matrices, like e.g. 2.(A⊕ B)C⊗ D. E2, where A, B, C, D, E are matrices of compatible size and characteristic.

Group Representations: The class ARep, Sections AReps -- DecompositionMonRep, is created to represent and manipulate structured representations up to equality, like e.g. (φ\uparrowT G)M⊗ψ. Special care is taken of monomial representations.

Symmetry of Matrices: In Sections Symmetry of Matrices -- PermIrredSymmetry functions are provided to compute certain kinds of symmetry of a given matrix. Symmetry allows to describe structure contained in a matrix.

Discrete Signal Transforms: Sections Discrete Signal Transforms -- InverseRationalizedHaarTransform describe functions to construct many well-known discrete signal transforms.

Matrix Decomposition: Sections Matrix Decomposition -- MatrixDecompositionByMonMonSymmetry describe functions to decompose a discrete signal transform into a product of highly structured sparse matrices.

Tools for Complex Numbers, Matrices and Permutations: Sections Complex Numbers -- TensorProductPerm describe useful tools for the computation with complex numbers, matrices and permutations.

All functions described are written entirely in the GAP3 language. The functions for the computation of the symmetry of a matrix (see Symmetry of Matrices) may use the external C program desauto written by J. Leon and contained in the share package \sf GUAVA. However, the use of this program is optional and will only influence the speed and not the executability of the functions.

The package \AREP was created in the framework of our theses where the background of constructive representation theory (see Pue98) and searching for symmetry of matrices (see Egn97) can be found.

### Subsections

After having started GAP3 the \AREP package needs to be loaded. This is done by typing:

\renewcommand\baselinestretch0.8

    gap> RequirePackage("arep");

___   ___   ___  ___
|   | |   | |    |   |   Version 1.0, 16 Mar 1998
|___| |___| |___ |___|
|   | |  \  |    |       by Sebastian Egner
|   | |   \ |___ |          Markus Pueschel

Abstract REPresentations 

\renewcommand\baselinestretch1

If \AREP isn't already in memory it is loaded and its banner is displayed. If you are a frequent user of \AREP you might consider putting this line into your .gaprc file.

## 74.2 Mons

The class Mon is created to represent and calculate efficiently with monomial matrices. A monomial matrix is a matrix which contains exactly one non-zero entry in every row and every column. Hence monomial matrices are always invertible and a generalization of permutation matrices. The elements of the class Mon are called mons". A mon m is a record with at least the following fields.

\begintabularlll isMon & := & true
isGroupElement & := & true
domain & := & GroupElements
operations & := & MonOps
char & : & characteristic of the base field
perm & : & a permutation
diag & : & a list of non-zero field elements \endtabular

The MonOps class is derived from the GroupElementOps class, so that groups of mons can be constructed. The monomial matrix represented by a mon m is given by

 [δip j| i,j∈{1,...,Length( m.diag )}]. ApplyFunc(DiagonalMat, m.diag ),
where p = m.perm and δkl denotes the Kronecker symbol (δkl = 1 if k = l and = 0 else). Mons are created using the function Mon. The following sections describe functions used for the calculation with mons.

Some remarks on the design of Mon: Mons cannot be mixed with GAP3-matrices (which are just lists of lists of field elements); use MonMat (MonMat) and MatMon (MatMon) to convert explicitly. Mons are lightweighted, e.g. only the characteristic of the base field is stored. Mons are group elements but there are no efficient functions implemented to compute with mon groups. You should think of mons as being a similar thing as integers or permutations: They are just fundamental objects to work with.

The functions concerning mons are implemented in the file "arep/lib/mon.g".

## 74.3 Comparison of Mons

m1 = m2
m1 <> m2

The equality operator = evaluates to true if the mons m1 and m2 are equal and to false otherwise. The inequality operator <> evaluates to true if the mons m1 and m2 are not equal and to false otherwise.

Two mons are equal iff they define the same monomial matrix. Note that the monomial matrix being represented has a certain size. The sizes must agree, too.

m1 < m2
m1 <= m2
m1 >= m2
m1 > m2

The operators <, <=, >=, and > evaluate to true if the mon m1 is strictly less than, less than or equal to, greater than or equal to, and strictly greater than the mon m2.

The ordering of mons m is defined via the ordering of the pairs [m.perm, m.diag].

## 74.4 Basic Operations for Mons

The MonOps class is derived from the GroupElementsOps class.

m1 * m2
m1 / m2

The operators * and / evaluate to the product and quotient of the two mons m1 and m2. The product is defined via the product of the corresponding (monomial) matrices. Of course the mons must be of equal size and characteristic otherwise an error is signaled.

m1 ^ m2

The operator ^ evaluates to the conjugate <m2>-1* m_1* m_2 of m1 under m2 for two mons m1 and m2. The mons must be of equal size and characteristic otherwise an error is signaled.

m ^ i

The powering operator ^ returns the i-th power of the mon m and the integer i.

Comm( m1, m2 )

Comm returns the commutator <m1>-1* m_2-1* m_1*m_2 of two mons m1 and m2. The operands must be of equal size and characteristic otherwise an error is signaled.

LeftQuotient( m1, m2 )

LeftQuotient returns the left quotient <m1>-1*m_2 of two mons m1 and m2. The operands must be of equal size and characteristic otherwise an error is signaled.

## 74.5 Mon

Mon( p, D )

Let p be a permutation and D a list of field elements ≠ 0 of the same characteristic. Mon returns a mon representing the monomial matrix given by ip j| i,j∈{1,...,Length( D )}]. ApplyFunc(DiagonalMat, D ), where δkl denotes the Kronecker symbol. The function will signal an error if the length of D is less than the largest moved point of p.

    gap> Mon( (1,2), [1, 2, 3] );
Mon(
(1,2),
[ 1, 2, 3 ]
)
gap> Mon( (1,3,4), [Z(3)^0, Z(3)^2, Z(3), Z(9)]);
Mon(
(1,3,4),
[ Z(3)^0, Z(3)^0, Z(3), Z(3^2) ]
) 

Mon( D, p )

Mon returns a mon representing the monomial matrix given by ApplyFunc(DiagonalMat, D ). [δip j| i,j∈{1,...,Length( D )}], where δkl denotes the Kronecker symbol. Note that in the output the diagonal is commuted to the right side, but it still represents the same monomial matrix.

    gap> Mon( [1,2,3], (1,2) );
Mon(
(1,2),
[ 2, 1, 3 ]
)
gap> Mon( [Z(3)^0, Z(3)^2, Z(3), Z(9)], (1,3,4) );
Mon(
(1,3,4),
[ Z(3^2), Z(3)^0, Z(3)^0, Z(3) ]
) 

Mon( D )

Mon returns a mon representing the (monomial) diagonal matrix given by the list D.

    gap> Mon( [1, 2, 3, 4] );
Mon( [ 1, 2, 3, 4 ] ) 

Mon( p, d )
Mon( p, d, char )
Mon( p, d, field )

Let p be a permutation and d a positive integer. Mon returns a mon representing the (d×d) permutation matrix corresponding to p using the convention ip j| i,j∈{1,...,d}], where δkl denotes the Kronecker symbol. As optional parameter a characteristic char or a field can be supplied. The default characteristic is zero. The function will signal an error if the degree d is less than the largest moved point of p.

    gap> Mon( (1,2), 3 );
Mon( (1,2), 3 )
gap> Mon( (1,2,3), 3, 5 );
Mon( (1,2,3), 3, GF(5) ) 

Mon( m )

Let m a mon. Mon returns m.

    gap> Mon( Mon( (1,2), [1, 2, 3] ) );
Mon(
(1,2),
[ 1, 2, 3 ]
) 

## 74.6 IsMon

IsMon( obj )

IsMon returns true if obj, which may be an object of arbitrary type, is a mon, and false otherwise. The function will signal an error if obj is an unbound variable.

    gap> IsMon( Mon( (1,2), [1, 2, 3] ) );
true
gap> IsMon( (1,2) );
false 

## 74.7 IsPermMon

IsPermMon( m )

IsPermMon returns true if the mon m represents a permutation matrix and false otherwise.

    gap> IsPermMon( Mon( (1,2), [1, 2, 3] ) );
false
gap> IsPermMon( Mon( (1,2), 2) );
true 

## 74.8 IsDiagMon

IsDiagMon( m )

IsDiagMon returns true if the mon m represents a diagonal matrix and false otherwise.

    gap> IsDiagMon( Mon( (1,2), 2) );
false
gap> IsDiagMon( Mon( [1, 2, 3, 4] ) );
true 

## 74.9 PermMon

PermMon( m )

PermMon converts the mon m to a permutation if possible and returns false otherwise.

    gap> PermMon( Mon( (1,2), 5) );
(1,2)
gap> PermMon( Mon( [1,2] ) );
false 

## 74.10 MatMon

MatMon( m )

MatMon converts the mon m to a matrix (i.e. a list of lists of field elements).

    gap> MatMon( Mon( (1,2), [1, 2, 3] ) );
[ [ 0, 2, 0 ], [ 1, 0, 0 ], [ 0, 0, 3 ] ]
gap> MatMon( Mon( (1,2), 3) );
[ [ 0, 1, 0 ], [ 1, 0, 0 ], [ 0, 0, 1 ] ] 

## 74.11 MonMat

MonMat( M )

MonMat converts the matrix M to a mon if possible and returns false otherwise.

    gap> MonMat( [ [ 0, 1, 0 ], [ 1, 0, 0 ], [ 0, 0, 1 ] ] );
Mon( (1,2), 3 )
gap> MonMat( [ [ 0, 1, 0 ], [ E(3), 0, 0 ], [ 0, 0, 4 ] ] );
Mon(
(1,2),
[ E(3), 1, 4 ]
) 

## 74.12 DegreeMon

DegreeMon( m )

DegreeMon returns the degree of the mon m. The degree is the size of the represented matrix.

    gap> DegreeMon( Mon( (1,2), [1, 2, 3] ) );
3 

## 74.13 CharacteristicMon

CharacteristicMon( m )

CharacteristicMon returns the characteristic of the field from which the components of the mon m are.

    gap> CharacteristicMon( Mon( [1,2] ) );
0
gap> CharacteristicMon( Mon( (1,2), 4, 5) );
5 

## 74.14 OrderMon

OrderMon( m )

OrderMon returns the order of the monomial matrix represented by the mon m. The order of m is the least positive integer r such that <m>r is the identity. Note that the order might be infinite.

    gap> OrderMon( Mon( [1,2] ) );
"infinity"
gap> OrderMon( Mon( (1,2), [1, E(3), E(3)^2] ) );
6 

## 74.15 TransposedMon

TransposedMon( m )

TransposedMon returns a mon representing the transposed monomial matrix of the mon m.

    gap> TransposedMon( Mon( [1,2] ) );
Mon( [ 1, 2 ] )
gap> TransposedMon( Mon( (1,2,3), 4 ) );
Mon( (1,3,2), 4 ) 

## 74.16 DeterminantMon

DeterminantMon( m )

DeterminantMon returns the determinant of the monomial matrix represented by the mon m.

    gap> DeterminantMon( Mon( (1,2), [1, E(3), E(3)^2] ) );
-1
gap> DeterminantMon( Mon( [1,2] ) );
2 

## 74.17 TraceMon

TraceMon( m )

TraceMon returns the trace of the monomial matrix represented by the mon m.

    gap> TraceMon( Mon( (1,2), 4, 5) );
Z(5)
gap> TraceMon( Mon( [1,2] ) );
3 

## 74.18 GaloisMon

GaloisMon( m, aut )
GaloisMon( m, k )

GaloisMon returns a mon which is a galois conjugate of the mon m. This means that each component of the represented matrix is mapped with an automorphism of the underlying field. The conjugating automorphism may either be a field automorphism aut or an integer k specifying the automorphism x -> GaloisCyc(x, k) in the case characteristic = 0 or x -> x^(FrobeniusAut^k) in the case characteristic = p prime.

    gap> GaloisMon( Mon( (1,2), [1, E(3), E(3)^2] ), -1 );
Mon(
(1,2),
[ 1, E(3)^2, E(3) ]
)
gap> aut := FrobeniusAutomorphism( GF(4) );
FrobeniusAutomorphism( GF(2^2) )
gap> GaloisMon( Mon( [ Z(2)^0, Z(2^2), Z(2^2)^2 ] ), aut );
Mon( [ Z(2)^0, Z(2^2)^2, Z(2^2) ] ) 

## 74.19 DirectSumMon

DirectSumMon( m1, ..., mk )

DirectSumMon returns the direct sum of the mons m1, ..., mk. The direct sum of mons is defined via the direct sum of the represented matrices. Note that the mons must have the same characteristic.

    gap> m1 := Mon( (1,2), [1, E(3), E(3)^2] );
Mon(
(1,2),
[ 1, E(3), E(3)^2 ]
)
gap> m2 := Mon( (1,2), 3);
Mon( (1,2), 3 )
gap> DirectSumMon( m1, m2 );
Mon(
(1,2)(4,5),
[ 1, E(3), E(3)^2, 1, 1, 1 ]
) 

DirectSumMon( list )

DirectSumMon returns a mon representing the direct sum of the mons in list.

    gap> m1 := Mon( (1,2), [1, E(3), E(3)^2] );
Mon(
(1,2),
[ 1, E(3), E(3)^2 ]
)
gap> m2 := Mon( (1,2), 3);
Mon( (1,2), 3 )
gap> DirectSumMon( [m1, m2] );
Mon(
(1,2)(4,5),
[ 1, E(3), E(3)^2, 1, 1, 1 ]
) 

## 74.20 TensorProductMon

TensorProductMon( m1, ..., mk )

TensorProductMon returns the tensor product of the mons m1, ..., mk. The tensor product of mons is defined via the tensor product (or Kronecker product) of the represented matrices. Note that the mons must have the same characteristic.

    gap> m1 := Mon( (1,2), [1, E(3), E(3)^2] );
Mon(
(1,2),
[ 1, E(3), E(3)^2 ]
)
gap> m2 := Mon( (1,2), 3);
Mon( (1,2), 3 )
gap> TensorProductMon( m1, m2 );
Mon(
(1,5)(2,4)(3,6)(7,8),
[ 1, 1, 1, E(3), E(3), E(3), E(3)^2, E(3)^2, E(3)^2 ]
) 

TensorProductMon( list )

TensorProductMon returns a mon representing the tensor product of the mons in list.

    gap> m1 := Mon( (1,2), [1, E(3), E(3)^2] );
Mon(
(1,2),
[ 1, E(3), E(3)^2 ]
)
gap> m2 := Mon( (1,2), 3);
Mon( (1,2), 3 )
gap> TensorProductMon( [m1, m2] );
Mon(
(1,5)(2,4)(3,6)(7,8),
[ 1, 1, 1, E(3), E(3), E(3), E(3)^2, E(3)^2, E(3)^2 ]
) 

## 74.21 CharPolyCyclesMon

CharPolyCyclesMon( m )

CharPolyCyclesMon returns the sorted list of the characteristic polynomials of the cycles of the mon m. All polynomials are written in a common polynomial ring. Applying Product to the result yields the characteristic polynomial of m.

    gap> CharPolyCyclesMon( Mon( (1,2), 3 ) );
[ X(Rationals) - 1, X(Rationals)^2 - 1 ]
gap> CharPolyCyclesMon( Mon( (1,2), [1, E(3), E(3)^2] ) );
[ X(CF(3)) + (-E(3)^2), X(CF(3))^2 + (-E(3)) ] 

## 74.22 AMats

The class AMat ( Abstract Matrices) is created to represent and calculate efficiently with structured matrices like e.g. 2.(A⊕ B)C⊗ D. E2, where A, B, C, D, E are matrices of compatible size/characteristic and ⊕, ⊗ denote the direct sum and tensor product (Kronecker product) resp. of matrices. The elements of the class AMat are called amats" and implement a recursive datastructure to form expressions like the one above. Basic constructors for amats allow to create permutation matrices (see AMatPerm, AMatPerm), monomial matrices (see AMatMon, AMatMon) and general matrices (see AMatMat, AMatMat) in an efficient way (e.g. a permutation matrix is defined by a permutation, the degree and the characteristic). Higher constructors allow to construct direct sums (see DirectSumAMat, DirectSumAMat), tensor products (see TensorProductAMat, TensorProductAMat) etc. from given amats. Note that while building up a highly structured amat from other amats no computation is done beside checks for compatibility. To obtain the matrix represented by an amat the appropiate function has to be applied (e.g. MatAMat, MatAMat).

Some remarks on the design of AMat: The class AMat is what is called a term algebra for expressions representing highly structured matrices over certain base fields. Amats are not necessarily square but can also be rectangular. Hence, if an amat must be invertible (e.g. when it shall conjugate another amat) this has to be proven by computation. To avoid many of these calculations the result (the inverse) is stored in the object and many functions accept a hint". E.g. by supplying the hint invertible" in the example above the explicit check for invertibility is suppressed. Using and passing correct hints is essential for efficient computation. A common problem in the design of non-trivial term algebras is the simplification strategy: Aggressive or conservative simplification? Our approach here is extremely conservative. This means even trivial subexpressions like 1*A are not automatically simplified. This allows the user to write functions that return their result always in a fixed structure, e.g. the result is always a conjugated direct sum of tensor products even though the conjugation might be trivial. Finally, note that amats and normal matrices (i.e. lists of lists of field elements) do not mix -- you have to convert explicitly with AMatMat, MatAMat etc. This greatly simplifies the amat module.

We define an amat recursively in Backus-Naur-Form as the disjoint union of the following cases.

\raggedbottom

beg-tabularlllll \multicolumn5lamat ::=
; &\multicolumn4latomic cases
& & perm & ; & perm" (invertible)
& | & mon & ; & mon" (invertible)
& | & mat & ; & mat"

; &\multicolumn4lcomposed cases
& | & scalar . amat & ; & scalarMultiple"
& | & amat . ... . amat & ; & product"
& | & amat \^ int & ; & power"
& | & amat \^ amat & ; & conjugate"
& | & amat ... amat & ; & directSum"
& | & amat ... amat& ; & tensorProduct"
& | & GaloisConjugate(amat, aut) & ; & galoisConjugate". end-tabular

An amat A is a record with at least the following fields:

\begintabularlll isAMat & := & true
operations & := & AMatOps
type & : & a string identifying the type of A
dimensions & : & size of the matrix represented ( = [rows, columns] )
char & : & characteristic of the base field \endtabular

The cases as stated above are distinguished by the field .type of an amat. Depending on the type additional fields are mandatory as follows:

beg-tabular{p2.5cmp9cm} \multicolumn2ltype = "perm":
element & defining permutation end-tabular

beg-tabular{p2.5cmp9cm} \multicolumn2ltype = "mon":
element & defining mon-object (see Mons) end-tabular

beg-tabular{p2.5cmp9cm} \multicolumn2ltype = "mat":
element & defining matrix (list of lists of field elements) end-tabular

beg-tabular{p2.5cmp9cm} \multicolumn2ltype = "scalarMultiple":
element & the AMat multiplied
scalar & the scalar
end-tabular

beg-tabular{p2.5cmp9cm} \multicolumn2ltype = "product":
factors & list of AMats of compatible dimensions and the same characteristic
end-tabular

beg-tabular{p2.5cmp9cm} \multicolumn2ltype = "power":
element & the square AMat to be raised to exponent
exponent & the exponent (an integer) end-tabular

beg-tabular{p2.5cmp9cm} \multicolumn2ltype = "conjugate":
element & the square AMat to be conjugated
conjugation & the conjugating invertible AMat end-tabular

beg-tabular{p2.5cmp9cm} \multicolumn2ltype = "directSum":
summands & List of AMats of the same characteristic end-tabular

beg-tabular{p2.5cmp9cm} \multicolumn2ltype = "tensorProduct":
factors & List of AMats of the same characteristic end-tabular

beg-tabular{p2.5cmp9cm} \multicolumn2ltype = "galoisConjugate":
element & the AMat to be Galois conjugated
galoisAut & the Galois automorphism end-tabular

Note that there is an important difference between the type of an amat and the type of the matrix being represented by the amat: An amat can be of type mat" but the matrix is in fact a permutation matrix. This distinction is refelcted in the naming of the functions: XAMat" refers to the type of the amat, XMat" to the type of the matrix being represented,

Here a short overview of the functions concerning amats. sections AMatPerm -- Comparison of AMats are concerned with the construction of amats, sections Converting AMats -- MatAMatAMat with the convertability and conversion of amats to permutations, mons and matrices, sections Functions for AMats -- UpperBoundLinearComplexityAMat contain functions for amats, e.g. computation of the determinant or simplification of amats.

The functions concerning amats are implemented in the file "arep/lib/amat.g".

## 74.23 AMatPerm

AMatPerm( p, d )
AMatPerm( p, d, char )
AMatPerm( p, d, field )

AMatPerm returns an amat of type "perm" representing the (d×d) permutation matrix ip j| i,j∈{1,...,d}] corresponding to the permutation p. As optional parameter a characteristic char or a field can be supplied. The default characteristic is zero. The function will signal an error if the degree d is less than the largest moved point of p.

    gap> AMatPerm( (1,2), 5 );
AMatPerm((1,2), 5)
gap> AMatPerm( (1,2,3), 5 , 3);
AMatPerm((1,2,3), 5, GF(3))
gap> A := AMatPerm( (1,2,3), 5 , Rationals);
AMatPerm((1,2,3), 5)
gap> A.type;
"perm" 

## 74.24 AMatMon

AMatMon( m )

AMatMon returns an amat of type "mon" representing the monomial matrix given by the mon m. For the explanation of mons please refer to Mons.

    gap> AMatMon( Mon( (1,2), [1, E(3), E(3)^2] ) );
AMatMon( Mon(
(1,2),
[ 1, E(3), E(3)^2 ]
) )
gap> A := AMatMon( Mon( (1,2), 3) );
AMatMon( Mon( (1,2), 3 ) )
gap> A.type;
"mon" 

## 74.25 AMatMat

AMatMat( M )
AMatMat( M, hint )

AMatMat returns an amat of type "mat" representing the matrix M. If the optional hint "invertible" is supplied then the field .isInvertible of the amat is set to true (without checking) indicating that the matrix represented is invertible.

    gap> AMatMat( [ [1,2], [3,4] ] );
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
)
gap> A := AMatMat( [ [1,2], [3,4] ] , "invertible");
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ],
"invertible"
)
gap> A.isInvertible;
true 

## 74.26 IsAMat

IsAMat( obj )

IsAMat returns true if obj, which may be an object of arbitrary type, is an amat, and false otherwise.

    gap> IsAMat( AMatPerm( (1,2,3), 3 ) );
true
gap> IsAMat( 1/2 );
false 

## 74.27 IdentityPermAMat

IdentityPermAMat( n )
IdentityPermAMat( n, char )
IdentityPermAMat( n, field )

IdentityPermAMat returns an amat of type "perm" representing the (n×n) identity matrix. As optional parameter a characteristic char or a field can be supplied to obtain the identity matrix of arbitrary characteristic. The default characteristic is zero. Note that the same result can be obtained by using AMatPerm.

    gap> IdentityPermAMat( 3 );
IdentityPermAMat(3)
gap> AMatPerm( ( ), 3);
IdentityPermAMat(3)
gap> IdentityPermAMat( 3 , GF(3) );
IdentityPermAMat(3, GF(3)) 

## 74.28 IdentityMonAMat

IdentityMonAMat( n )
IdentityMonAMat( n, char )
IdentityMonAMat( n, field )

IdentityMonAMat returns an amat of type "mon" representing the (n×n) identity matrix. As optional parameter a characteristic char or a field can be supplied to obtain the identity matrix of arbitrary characteristic. The default characteristic is zero. Note that the same result can be obtained by using AMatMon.

    gap> IdentityMonAMat( 3 );
IdentityMonAMat(3)
gap> AMatMon( Mon( ( ), 3 ) );
IdentityMonAMat(3)
gap> IdentityMonAMat( 3, 3 );
IdentityMonAMat(3, GF(3)) 

## 74.29 IdentityMatAMat

IdentityMatAMat( n )
IdentityMatAMat( n, char )
IdentityMatAMat( n, field )

IdentityMatAMat returns an amat of type "mat" representing the (n×n) identity matrix. As optional parameter a characteristic char or a field can be supplied to obtain the identity matrix of arbitrary characteristic. The default characteristic is zero. Note that the same result can be obtained by using AMatMat.

    gap> IdentityMatAMat( 3 );
IdentityMatAMat(3)
gap> AMatMat( [ [1, 0, 0], [0, 1, 0], [0, 0, 1] ]);
IdentityMatAMat(3)
gap> IdentityMatAMat( 3, GF(3) );
IdentityMatAMat(3, GF(3)) 

IdentityMatAMat( dim )
IdentityMatAMat( dim, char )
IdentityMatAMat( dim, field )

Let dim be a pair of positive integers. IdentityMatAMat returns an amat of type "mat" representing the rectangular identity matrix with dim[1] rows and dim[2] columns. A rectangular identity matrix has the entry 1 at the position (i,j) if i = j and 0 else. As optional parameter a characteristic char or a field can be supplied to obtain the identity matrix of arbitrary characteristic. The default characteristic is zero.

    gap> IdentityMatAMat( [2, 3] );
IdentityMatAMat([ 2, 3 ])
gap> IdentityMatAMat( [2, 3], 3 );
IdentityMatAMat([ 2, 3 ], GF(3)) 

## 74.30 IdentityAMat

IdentityAMat( dim )
IdentityAMat( dim, char )
IdentityAMat( dim, field )

Let dim be a pair of positive integers. IdentityAMat returns an amat of type "perm" if dim[1] = dim[2] and an amat of type "mat" else, representing the identity matrix with dim[1] rows and dim[2] columns. A rectangular identity matrix has the entry 1 at the position (i,j) if i = j and 0 else. Use this function if you do not know whether the matrix is square and you do not care about the type. As optional parameter a characteristic char or a field can be supplied to obtain the identity matrix of arbitrary characteristic. The default characteristic is zero.

    gap> IdentityAMat( [2, 2] );
IdentityPermAMat(2)
gap> IdentityAMat( [2, 3] );
IdentityMatAMat([ 2, 3 ]) 

## 74.31 AllOneAMat

AllOneAMat( n )
AllOneAMat( n, char )
AllOneAMat( n, field )

AllOneAMat returns an amat of type "mat" representing the (n×n) all-one matrix. An all-one matrix has the entry 1 at each position. As optional parameter a characteristic char or a field can be supplied to obtain the all-one matrix of arbitrary characteristic. The default characteristic is zero.

    gap> AllOneAMat( 3 );
AllOneAMat(3)
gap> AllOneAMat( 3, 3);
AllOneAMat(3, GF(3)) 

AllOneAMat( dim )
AllOneAMat( dim, char )
AllOneAMat( dim, field )

Let dim a pair of positive integers. AllOneAMat returns an amat of type "mat" representing the rectangular all-one matrix with dim[1] rows and dim[2] columns. As optional parameter a characteristic char or a field can be supplied to obtain the all-one matrix of arbitrary characteristic. The default characteristic is zero.

    gap> AllOneAMat( [3, 2] );
AllOneAMat([ 3, 2 ])
gap> AllOneAMat( [3, 2], GF(5) );
AllOneAMat([ 3, 2 ], GF(5)) 

## 74.32 NullAMat

NullAMat( n )
NullAMat( n, char )
NullAMat( n, field )

NullAMat returns an amat of type "mat" representing the (n×n) all-zero matrix. An all-zero matrix has the entry 0 at each position. As optional parameter a characteristic char or a field can be supplied to obtain the all-zero matrix of arbitrary characteristic. The default characteristic is zero.

    gap> NullAMat( 3 );
NullAMat(3)
gap> NullAMat( 3, 3);
NullAMat(3, GF(3)) 

NullAMat( dim )
NullAMat( dim, char )
NullAMat( dim, field )

Let dim a pair of positive integers. NullAMat returns an amat of type "mat" representing the rectangular all-zero matrix with dim[1] rows and dim[2] columns. As optional parameter a characteristic char or a field can be supplied to obtain the all-zero matrix of arbitrary characteristic. The default characteristic is zero.

    gap> NullAMat( [3, 2] );
NullAMat([ 3, 2 ])
gap> NullAMat( [3, 2], GF(5) );
NullAMat([ 3, 2 ], GF(5)) 

## 74.33 DiagonalAMat

DiagonalAMat( list )

Let list contain field elements of the same characteristic. DiagonalAMat returns an amat representing the diagonal matrix with diagonal entries in list. If all elements in list are ≠ 0 the returned amat is of type "mon", else of type "directSum" (see AMats).

    gap> DiagonalAMat( [2, 3] );
DiagonalAMat([ 2, 3 ])
gap> DiagonalAMat( [0, 2, 3] );
DirectSumAMat(
NullAMat(1),
AMatMat(
[ [ 2 ] ]
),
AMatMat(
[ [ 3 ] ]
)
) 

## 74.34 DFTAMat

DFTAMat( n )
DFTAMat( n, char )
DFTAMat( n, field )

DFTAMat returns a special amat of type "mat" representing the matrix

 DFTn = [ωn i. j| i,j∈{0,...,n-1}],
with ωn being a certain primitive n-th root of unity. DFTn represents the Discrete Fourier Transform on n points (see DiscreteFourierTransform). As optional parameter a characteristic char or a field can be supplied to obtain the DFT of arbitrary characteristic. The default characteristic is zero. Note that for characteristic p prime the DFTn exists iff gcd(p, n) = 1. For a given finite field the DFTn exists iff n|Size( F ). If these conditions are violated an error is signaled. The choice of ωn is E(n) if <char> = 0 and Z(q)^((q-1)/n) for <char> = p, q an appropiate p-power.

    gap> DFTAMat(3);
DFTAMat(3)
gap> DFTAMat(3, 7);
DFTAMat(3, 7) 

## 74.35 SORAMat

SORAMat( n )
SORAMat( n, char )
SORAMat( n, field )

SORAMat returns a special amat of type "mat" representing the matrix

SORn = [
 rrrrr 1 1 1 ... 1 1 -1 0 ... 0 1 0 -1 ... 0 ⋅ ⋅ ⋅ \ddots 0 1 0 0 -1
].
The SORn is the sparsest matrix that splits off the one- representation in a permutation representation. As optional parameter a characteristic char or a field can be supplied to obtain the SOR of arbitrary characteristic. The default characteristic is zero.

    gap> SORAMat( 4 );
SORAMat(4)
gap> SORAMat( 4, 7);
SORAMat(4, 7) 

## 74.36 ScalarMultipleAMat

ScalarMultipleAMat( s, A )  or  s * A

Let s be a field element and A an amat. ScalarMultipleAMat returns an amat of type "scalarMultiple" representing the scalar multiple of s with A, which must have common characteristic otherwise an error is signaled. Note that s and A can be accessed in the fields .scalar resp. .element of the result.

    gap> A := AMatPerm( (1,2,3), 4);
AMatPerm((1,2,3), 4)
gap> ScalarMultipleAMat( E(3), A );
E(3) * AMatPerm((1,2,3), 4)
gap> 2 * A;
2 * AMatPerm((1,2,3), 4) 

## 74.37 Product and Quotient of AMats

A * B

Let A and B be amats. A * B returns an amat of type "product" representing the product of A and B, which must have compatible sizes and common characteristic otherwise an error is signaled. Note that the factors can be accessed in the field .factors of the result.

    gap> A := AMatPerm( (1,2,3), 4);
AMatPerm((1,2,3), 4)
gap> B := AMatMat( [ [1, 2], [3, 4], [5, 6], [7, 8] ] );
AMatMat(
[ [ 1, 2 ], [ 3, 4 ], [ 5, 6 ], [ 7, 8 ] ]
)
gap> A * A;
AMatPerm((1,2,3), 4) *
AMatPerm((1,2,3), 4)
gap> C := A * B;
AMatPerm((1,2,3), 4) *
AMatMat(
[ [ 1, 2 ], [ 3, 4 ], [ 5, 6 ], [ 7, 8 ] ]
)
gap> C.type;
"product" 

A / B

Let A and B be amats. A / B returns an amat of type "product" representing the quotient of A and B. The sizes and characteristics of A and B must be compatible, B must be square and invertible otherwise an error is signaled.

    gap> A := AMatPerm( (1,2,3), 4);
AMatPerm((1,2,3), 4)
gap> B := DiagonalAMat( [1, E(3), 1, 3] );
DiagonalAMat([ 1, E(3), 1, 3 ])
gap> A / B;
AMatPerm((1,2,3), 4) *
DiagonalAMat([ 1, E(3), 1, 3 ]) ^ -1 

## 74.38 PowerAMat

PowerAMat( A, n )  or  A ^ n
PowerAMat( A, n, hint )

Let A be an amat and n an integer. PowerAMat returns an amat of type "power" representing the power of A with n. A must be square otherwise an error is signaled. If n is negative then A is checked for invertibility if the hint "invertible" is not supplied. Note that A and n can be accessed in the fields .element resp. .exponent of the result.

    gap> A := AMatPerm( (1,2,3), 4);
AMatPerm((1,2,3), 4)
gap> B := PowerAMat(A, 3);
AMatPerm((1,2,3), 4) ^ 3
gap> B ^ -2;
( AMatPerm((1,2,3), 4) ^ 3
) ^ -2 

## 74.39 ConjugateAMat

ConjugateAMat( A, B )  or  A ^ B
ConjugateAMat( A, B, hint )

Let A and B be amats. ConjugateAMat returns an amat of type "conjugate" representing the conjugate of A with B (i.e. the matrix defined by <B>-1.A.B). A and B must be square otherwise an error is signaled. B is checked for invertibility if the hint "invertible" is not supplied. Note that A and B can be accessed in the fields .element resp. conjugation of the result.

    gap> A := AMatMon( Mon( (1,2), [1, E(4), -1] ) );
AMatMon( Mon(
(1,2),
[ 1, E(4), -1 ]
) )
gap> B := DFTAMat( 3 );
DFTAMat(3)
gap> ConjugateAMat( A, B, "invertible" );
ConjugateAMat(
AMatMon( Mon(
(1,2),
[ 1, E(4), -1 ]
) ),
DFTAMat(3)
)
gap> B ^ SORAMat( 3 );
ConjugateAMat(
DFTAMat(3),
SORAMat(3)
) 

## 74.40 DirectSumAMat

DirectSumAMat( A1, ..., Ak )

DirectSumAMat returns an amat of type "directSum" representing the direct sum of the amats A1, ..., Ak, which must have common characteristic otherwise an error is signaled. Note that the direct summands can be accessed in the field .summands of the result.

    gap> A1 := AMatMat( [ [1, 2] ] );
AMatMat(
[ [ 1, 2 ] ]
)
gap> A2 := DFTAMat( 2 );
DFTAMat(2)
gap> A3 := AMatPerm( (1,2), 2 );
AMatPerm((1,2), 2)
gap> DirectSumAMat( E(3) * A1, A2 ^ 2, A3 );
DirectSumAMat(
E(3) * AMatMat( [ [ 1, 2 ] ] ),
DFTAMat(2) ^ 2,
AMatPerm((1,2), 2)
) 

DirectSumAMat( list )

DirectSumAMat returns an amat of type "directSum" representing the direct sum of the amats in list. The amats must have common characteristic otherwise an error is signaled. The direct summands can be accessed in the field .summands of the result.

    gap> A := DiagonalAMat( [ Z(3), Z(3)^2 ]);
DiagonalAMat([ Z(3), Z(3)^0 ])
gap> B := AMatPerm( (1,2), 3, 3);
AMatPerm((1,2), 3, GF(3))
gap> DirectSumAMat( [A, B] );
DirectSumAMat(
DiagonalAMat([ Z(3), Z(3)^0 ]),
AMatPerm((1,2), 3, GF(3))
) 

## 74.41 TensorProductAMat

TensorProductAMat( A1, ..., Ak )

TensorProductAMat returns an amat of type "tensorProduct" representing the tensor product (or Kronecker product) of the amats A1, ..., Ak, which must have common characteristic otherwise an error is signaled. Note that the tensor factors can be accessed in the field .factors of the result.

    gap> A := IdentityPermAMat( 2 );
IdentityPermAMat(2)
gap> B := AMatMat( [ [1, 2, 3], [4, 5, 6] ] );
AMatMat(
[ [ 1, 2, 3 ], [ 4, 5, 6 ] ]
)
gap> TensorProductAMat( A, B );
TensorProductAMat(
IdentityPermAMat(2),
AMatMat(
[ [ 1, 2, 3 ], [ 4, 5, 6 ] ]
)
) 

TensorProductAMat( list )

TensorPoductAMat returns an amat of type "tensorProduct" representing the tensor product of the amats in list. The amats must have common characteristic otherwise an error is signaled. The tensor factors can be accessed in the field .factors of the result.

    gap> A := AMatPerm( (1,2), 3 );
AMatPerm((1,2), 3)
gap> B := AMatMat( [ [1], [2] ]);
AMatMat(
[ [ 1 ], [ 2 ] ]
)
gap> TensorProductAMat( [A ^ 2, 2 * B] );
TensorProductAMat(
AMatPerm((1,2), 3) ^ 2,
2 * AMatMat(
[ [ 1 ], [ 2 ] ]
)
) 

## 74.42 GaloisConjugateAMat

GaloisConjugateAMat( A, k )
GaloisConjugateAMat( A, aut )

GaloisConjugateAMat returns an amat which represents a Galois conjugate of the amat A. The conjugating automorphism may either be a field automorphism aut or an integer k specifying the automorphism x -> GaloisCyc(x, k) in the case characteristic = 0 or x -> x^(FrobeniusAut^k) in the case characteristic = p prime. Note that A and k/aut can be accessed in the fields .element resp. .galoisAut of the result.

    gap> A := DiagonalAMat( [1, E(3)] );
DiagonalAMat([ 1, E(3) ])
gap> GaloisConjugateAMat( A, -1 );
GaloisConjugateAMat(
DiagonalAMat([ 1, E(3) ]),
-1
)
gap> aut := FrobeniusAutomorphism( GF(4) );
FrobeniusAutomorphism( GF(2^2) )
gap> B := AMatMon( Mon( (1,2), [ Z(2)^0, Z(2^2) ] ) );
AMatMon( Mon(
(1,2),
[ Z(2)^0, Z(2^2) ]
) )
gap> GaloisConjugateAMat( B, aut );
GaloisConjugateAMat(
AMatMon( Mon(
(1,2),
[ Z(2)^0, Z(2^2) ]
) ),
FrobeniusAutomorphism( GF(2^2) )
) 

## 74.43 Comparison of AMats

A = B
A <> B

The equality operator = evaluates to true if the amats A and B are equal and to false otherwise. The inequality operator <> evaluates to true if the amats A and B are not equal and to false otherwise.

Two amats are equal iff they define the same matrix.

    gap> A := DiagonalAMat( [E(3), 1] );
DiagonalAMat([ E(3), 1 ])
gap> B := A ^ 3;
DiagonalAMat([ E(3), 1 ]) ^ 3
gap> B = IdentityPermAMat( 2 );
true 

A < B
A <= B
A >= B
A > B

The operators <, <=, >=, and > evaluate to true if the amat A is strictly less than, less than or equal to, greater than or equal to, and strictly greater than the amat B.

The ordering of amats is defined via the ordering of records.

## 74.44 Converting AMats

The following sections describe the functions for the convertability and conversion of amats to permutations, mons (see Mons) and matrices.

The names of the conversion functions are chosen according to the usual GAP3-convention: ChalkCheese makes chalk from cheese. The parts in the name (chalk, cheese) are

\begintabular{l@ -- l} Perm & a GAP3-permutation, e.g. (1,2)
Mon & a mon object, e.g. Mon( (1,2), 2 ) (see Mons)
Mat & a GAP3-matrix, e.g. [[1,2],[3,4]]
AMat & an amat of any type
PermAMat & an amat of type perm"
MonAMat & an amat of type mon"
MatAMat & an amat of type mat" \endtabular

## 74.45 IsIdentityMat

IsIdentityMat( A )

IsIdentityMat returns true if the matrix represented by the amat A is the identity matrix and false otherwise. Note that the name of the function is not IsIdentityAMat since A can be of any type but represents an identity matrix in the mathematical sense.

    gap> IsIdentityMat(AMatPerm( (1,2), 3 ));
false
gap> A := DiagonalAMat( [Z(3), Z(3)] ) ^ 2;
DiagonalAMat([ Z(3), Z(3) ]) ^ 2
gap> IsIdentityMat(A);
true 

## 74.46 IsPermMat

IsPermMat( A )

IsPermMat returns true if the matrix represented by the amat A is a permutation matrix and false otherwise. The name of the function is not IsPermAMat since A can be of any type but represents a permutation matrix in the mathematical sense. Note that IsPermMat sets and tests A.isPermMat.

    gap> IsPermMat( AMatMon( Mon( (1,2), [1, -1] )));
false
gap> IsPermMat( DiagonalAMat( [Z(3), Z(9)] ) ^ 8);
true 

## 74.47 IsMonMat

IsMonMat( A )

IsMonMat returns true if the matrix represented by the amat A is a monomial matrix (a matrix containing exactly one entry ≠ 0 in every row and column) and false otherwise. The name of the function is not IsMonAMat since A can be of any type but represents a monomial matrix in the mathematical sense. Note that IsMonMat sets and tests A.isMonMat.

    gap> IsMonMat( AMatPerm( (1,2), 3 ));
true
gap> IsMonMat( AMatPerm( (1,2,3), 3 ) ^ DFTAMat(3) );
true 

## 74.48 PermAMat

PermAMat( A )

Let A be an amat. PermAMat returns the permutation represented by A if A is a permutation matrix (i.e. IsPermMat( A ) = true) and false otherwise. Note that PermAMat sets and tests A.perm.

    gap> PermAMat(AMatPerm( (1,2), 5 ));
(1,2)
gap> A := AMatMat( [ [Z(3)^0, Z(3)], [0*Z(3), Z(3)^0] ] );
AMatMat(
[ [ Z(3)^0, Z(3) ], [ 0*Z(3), Z(3)^0 ] ]
)
gap> PermAMat(A);
false
gap> PermAMat(A ^ 3);
() 

## 74.49 MonAMat

MonAMat( A )

Let A be an amat. MonAMat returns the mon (see Mons) represented by A if A is a monomial matrix (i.e. IsMonMat( A ) = true) and false otherwise. Note that MonAMat sets and tests A.mon.

    gap> MonAMat(AMatPerm( (1,2,3), 5 ));
Mon( (1,2,3), 5 )
gap> MonAMat(AMatPerm( (1,2,3), 3 ) ^ DFTAMat(3) );
Mon( [ 1, E(3), E(3)^2 ] )
gap> MonAMat( AMatMat( [ [1, 2] ] ));
false 

## 74.50 MatAMat

MatAMat( A )

MatAMat returns the matrix represented by the amat A. Note that MatAMat sets and tests A.mat.

    gap> MatAMat( AMatPerm( (1,2), 3, 2 ));
[ [ 0*Z(2), Z(2)^0, 0*Z(2) ], [ Z(2)^0, 0*Z(2), 0*Z(2) ],
[ 0*Z(2), 0*Z(2), Z(2)^0 ] ]
gap> MatAMat(DFTAMat(3));
[ [ 1, 1, 1 ], [ 1, E(3), E(3)^2 ], [ 1, E(3)^2, E(3) ] ]
gap> A := IdentityPermAMat(2);
IdentityPermAMat(2)
gap> B := AMatMat( [ [1,2], [3,4] ] );
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
)
gap> MatAMat(TensorProductAMat(A, B));
[ [ 1, 2, 0, 0 ], [ 3, 4, 0, 0 ], [ 0, 0, 1, 2 ], [ 0, 0, 3, 4 ] ] 

## 74.51 PermAMatAMat

PermAMatAMat( A )

Let A be an amat. PermAMatAMat returns an amat of type "perm" equal to A if A is a permutation matrix (i.e. IsPermMat( A ) = true) and false otherwise.

    gap> PermAMatAMat(AMatMon(Mon( (1,2), 3 )));
AMatPerm((1,2), 3)
gap> PermAMatAMat(DiagonalAMat( [E(3), 1] ) ^ 3);
IdentityPermAMat(2)
gap> PermAMatAMat(AMatMat( [ [1,2] ] ));
false 

## 74.52 MonAMatAMat

MonAMatAMat( A )

Let A be an amat. MonAMatAMat returns an amat of type "mon" equal to A if A is a monomial matrix (i.e. IsMonMat( A ) = true) and false otherwise.

    gap> MonAMat(AMatPerm( (1,2), 3 ));
Mon( (1,2), 3 )
gap> MonAMat(DFTAMat(3)^2);
Mon(
(2,3),
[ 3, 3, 3 ]
)
gap> MonAMat(AMatMat( [ [1, 2] ] ));
false 

## 74.53 MatAMatAMat

MatAMatAMat( A )

MatAMatAMat returns an amat of type "mat" equal to A.

    gap> A := AMatPerm( (1,2), 2 );
AMatPerm((1,2), 2)
gap> B := AMatMat( [ [1,2] ] );
AMatMat(
[ [ 1, 2 ] ]
)
gap> MatAMatAMat(DirectSumAMat(A, B));
AMatMat(
[ [ 0, 1, 0, 0 ], [ 1, 0, 0, 0 ], [ 0, 0, 1, 2 ] ]
) 

## 74.54 Functions for AMats

The following sections describe useful functions for the calculation with amats (e.g. calculation of the inverse, determinant of an amat as well as simplifying amats). Most of these functions can take great advantage of the highly structured form of the amats.

## 74.55 InverseAMat

InverseAMat( A )

InverseAMat returns an amat representing the inverse of the amat A. If A is not invertible, an error is signaled. The function uses mathematical rules to invert the direct sum, tensor product etc. of matrices. Note that InverseAMat sets and tests A.inverse.

    gap> A := AMatPerm( (1,2), 3);
AMatPerm((1,2), 3)
gap> B := AMatMat( [ [1,2], [3,4] ]);
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
)
gap> C := DiagonalAMat( [ E(3), 1] );
DiagonalAMat([ E(3), 1 ])
gap> D := DirectSumAMat(A, TensorProductAMat(B, C));
DirectSumAMat(
AMatPerm((1,2), 3),
TensorProductAMat(
AMatMat( [ [ 1, 2 ], [ 3, 4 ] ] ),
DiagonalAMat([ E(3), 1 ])
)
)
gap> InverseAMat(D);
DirectSumAMat(
AMatPerm((1,2), 3),
TensorProductAMat(
AMatMat(
[ [ -2, 1 ], [ 3/2, -1/2 ] ],
"invertible"
),
DiagonalAMat([ E(3)^2, 1 ])
)
) 

## 74.56 TransposedAMat

TransposedAMat( A )

TransposedAMat returns an amat representing the transpose of the amat A. The function uses mathematical rules to transpose the direct sum, tensor product etc. of matrices.

    gap> A := AMatPerm( (1,2,3), 3);
AMatPerm((1,2,3), 3)
gap> B := AMatMat( [ [1, 2] ] );
AMatMat(
[ [ 1, 2 ] ]
)
gap> TransposedAMat(TensorProductAMat(A, B));
TensorProductAMat(
AMatPerm((1,3,2), 3),
AMatMat(
[ [ 1 ], [ 2 ] ]
)
) 

## 74.57 DeterminantAMat

DeterminantAMat( A )

DeterminantAMat returns the determinant of the amat A. If A is not square an error is signaled. The function uses mathematical rules to calculate the determinant of the direct sum, tensor product etc. of matrices. Note that DeterminantAMat sets and tests A.determinant.

    gap> A := AMatMat( [ [1,2], [3,4] ] );
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
)
gap> B := AMatPerm( (1,2), 2 );
AMatPerm((1,2), 2)
gap> DeterminantAMat(TensorProductAMat(A, B));
4 

## 74.58 TraceAMat

TraceAMat( A )

TraceAMat returns the trace of the amat A. If A is not square an error is signaled. The function uses mathematical rules to calculate the trace of direct sums, tensor product etc. of matrices. Note that TraceAMat sets and tests A.trace.

    gap> A := DFTAMat(2);
DFTAMat(2)
gap> B := DiagonalAMat( [1, 2, 3] );
DiagonalAMat([ 1, 2, 3 ])
gap> TraceAMat(DirectSumAMat( A^2, B ));
10 

## 74.59 RankAMat

RankAMat( A )

RankAMat returns the rank of the amat A. Note that RankAMat sets and tests A.rank.

    gap> RankAMat(AllOneAMat(100));
1
gap> RankAMat(AMatPerm( (1,2), 10 ));
10 

## 74.60 SimplifyAMat

SimplifyAMat( A )

SimplifyAMat returns a simplified amat representing the same matrix as the amat A. The simplification is performed recursively according to certain rules. E.g. the following simplifications are performed:

• If A represents a permutation matrix, monomial matrix then an amat of type perm", mon" resp. is returned.

• In a product resp. tensor product, trivial factors are omitted.

• Trivial conjugation is omitted.

• In a direct sum adjacent permutation/monomial matrices are put together.

• In a product adjacent permutation/monomial matrices are multiplied together.

• Successive scalars are multiplied together.

• Successive exponents are multiplied together, negative exponents are evaluated using InverseAMat. Note that important information about the matrix is shifted to the simplification.

    gap> A := IdentityPermAMat( 3 );
IdentityPermAMat(3)
gap> B := DiagonalAMat( [E(3), 1, 1] );
DiagonalAMat([ E(3), 1, 1 ])
gap> C := AMatMat( [ [1,2], [3,4] ] );
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
)
gap> D := DirectSumAMat(A ^ -1, 1 * B * A, C);
DirectSumAMat(
IdentityPermAMat(3) ^ -1,
( 1 * DiagonalAMat([ E(3), 1, 1 ])
) *
IdentityPermAMat(3),
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
)
)
gap> SimplifyAMat(D);
DirectSumAMat(
IdentityPermAMat(3),
DiagonalAMat([ E(3), 1, 1 ]),
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
)
) 

## 74.61 kbsAMat

kbsAMat( A1, ..., Ak )

kbsAMat returns the joined kbs (conjugated blockstructure) of the amats A1, ..., Ak. The amats must be square and of common size and characteristic otherwise an error is signaled. The joined kbs of a list of (n× n)-matrices is a partition of {1,...,n} representing their common blockstructure. For an exact definition see kbs.

    gap> A := IdentityPermAMat(2);
IdentityPermAMat(2)
gap> B := AMatMat( [ [1,2], [3,4] ] );
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
)
gap> kbsAMat(TensorProductAMat(A, B));
[ [ 1, 2 ], [ 3, 4 ] ]
gap> kbsAMat(AMatPerm( (1,3)(2,4), 5 ));
[ [ 1, 3 ], [ 2, 4 ], [ 5 ] ] 

kbsAMat( list )

kbsAMat returns the joined kbs of the amats in list (see above).

## 74.62 kbsDecompositionAMat

kbsDecompositionAMat( A )

kbsDecompositionAMat decomposes the amat A into a conjugated (by an amat of type "perm") direct sum of amats of type "mat" as far as possible. If A is not square an error is signaled. The decomposition is performed according to the kbs (see kbs) of A which is a partition of {1,...,n} (n = number of rows of A) describing the blockstructure of A.

    gap> A := AMatMat( [[1,0,2,0], [0,1,0,2], [3,0,4,0], [0,3,0,4]] );
AMatMat(
[ [ 1, 0, 2, 0 ], [ 0, 1, 0, 2 ], [ 3, 0, 4, 0 ], [ 0, 3, 0, 4 ] ]
)
gap> kbsDecompositionAMat(A);
ConjugateAMat(
DirectSumAMat(
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
),
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
)
),
AMatPerm((2,3), 4)
) 

## 74.63 AMatSparseMat

AMatSparseMat( M ) AMatSparseMat( M, match-blocks )

Let M be a sparse matrix (i.e. containing entries ≠ 0). AMatSparseMat returns an amat of the form <P>1. E1. D. E2. P2 where (for i = 1,2) <P>i are amats of type "perm", <E>i are identity-amats (might be rectangular) and D is an amat of type "directSum". If match-blocks is true or not provided then, furthermore, the permutations p1 and p2 are chosen such that equivalent summands of D are equal and collected together by a tensor product. If match-blocks is false this is not done. The major part of the work is done by the function DirectSummandsPermutedMat (see DirectSummandsPermutedMat). Use the function SimplifyAMat (see SimplifyAMat) for simplification of the result.

For an explanation of the algorithm see Egn97.

    gap> M := [[0,0,0,0],[0,1,0,2],[0,0,3,0],[0,4,0,5]];;
gap> PrintArray(M);
[ [  0,  0,  0,  0 ],
[  0,  1,  0,  2 ],
[  0,  0,  3,  0 ],
[  0,  4,  0,  5 ] ]
gap> AMatSparseMat(M);
AMatPerm((1,4,3), 4) *
IdentityMatAMat([ 4, 3 ]) *
DirectSumAMat(
TensorProductAMat(
IdentityPermAMat(1),
AMatMat(
[ [ 3 ] ]
)
),
TensorProductAMat(
IdentityPermAMat(1),
AMatMat(
[ [ 1, 2 ], [ 4, 5 ] ]
)
)
) *
IdentityMatAMat([ 3, 4 ]) *
AMatPerm((1,3,4), 4) 

## 74.64 SubmatrixAMat

SubmatrixAMat( A, inds )

Let A be an amat and inds a set of positive integers. SubmatrixAMat returns an amat of type "mat" representing the submatrix of A defined by extracting all entries with row and column index in inds.

    gap> A := AMatPerm( (1,2), 2 );
AMatPerm((1,2), 2)
gap> B := AMatMat( [ [1,2], [3,4] ] );
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
)
gap> SubmatrixAMat(TensorProductAMat(A, B), [2,3] );
AMatMat(
[ [ 0, 3 ], [ 2, 0 ] ]
) 

## 74.65 UpperBoundLinearComplexityAMat

UpperBoundLinearComplexityAMat( A )

UpperBoundLinearComplexityAMat returns an upper bound for the linear complexity of the amat A according to the complexity model L of Clausen/Baum, CB93. The linear complexity is a measure for the complexity of the matrix-vector multiplication of a given matrix with an arbitrary vector.

    gap> UpperBoundLinearComplexityAMat(DFTAMat(2));
2
gap> UpperBoundLinearComplexityAMat(DiagonalAMat( [2, 3] ));
2
gap> A := AMatPerm( (1,2), 3);
AMatPerm((1,2), 3)
gap> B := AMatMat( [ [1,2], [3,4] ] );
AMatMat(
[ [ 1, 2 ], [ 3, 4 ] ]
)
gap> UpperBoundLinearComplexityAMat(TensorProductAMat(A, B));
24 

## 74.66 AReps

The class ARep ( Abstract Representations) is created to represent and calculate efficiently with structured matrix representations of finite groups up to equality, e.g. expressions like (φ\uparrowT G)M⊗ψ where φ, ψ are representations and \uparrow, ⊗ denotes the induction resp. inner tensor product of representations. The implementation idea is the same as with the class AMat (see AMats), i.e. a representation is a record containing the necessary information (e.g. degree, characteristic, list of images on the generators) to define a representation up to equality. The elements of ARep are called areps" and are no group homomorphisms in the sense of GAP3 (which is the reason for the term abstract" representation). Special care is taken of permutation and monomial representations, which can be represented very efficiently by storing a list of permutations or mons (see Mons) instead of matrices as images on the generators.

Areps can represent representations of any finite group and any characteristic including modular (characteristic divides group size) representations, but most of the higher functions will only work in the non-modular case or even only in the case of characteristic zero. These restrictions are always indicated in the description of the respective function.

Basic constructors allow to create areps, e.g. by supplying the list of images on the generators (see ARepByImages, ARepByImages). Since GAP3 allows the manipulation of the generators given to construct a group, it is important for consistency to have a field with generators one can rely on. This is realized in the function GroupWithGenerators, GroupWithGenerators.

Higher constructors allow to construct inductions (see InductionARep, InductionARep), direct sums (see DirectSumARep, DirectSumARep), inner tensor products (see InnerTensorProductARep, InnerTensorProductARep) etc. from given areps.

Some remarks on the design of ARep: The class ARep is a term algebra for matrix representations of finite groups (see also AMat, AMats). The simplification strategy is extremely conservative, which means that even trivial expressions like GaloisConjugate(R, id) are only simplified upon explicit request. As in AMat we use the hint"-concept extensively to suppress unnecessary expensive computations of little interest. The class AMat is used in ARep in three ways: 1. for images under areps, 2. for conjugating matrices (change of base of the underlying vector space) and 3. for elements of the intertwining space of two areps. Note that 3. requires non-invertible or even rectangular matrices to be represented. A special point that deserves mentioning is the way in which areps act as homomorphisms anf how they are defined. Areps are no GAP3-homomorphisms. We simply did not manage to implement ARep as a term algebra and as GAP3-homomorphisms in a relyable and efficient way which avoids maximal confusion. In addition, working with ARep usually involves many representations of the same group. This is supported in the most obvious way by fixing the list of generators used to create the group (see GroupWithGenerators) and only varying the list of images. Although this strategy differs from the approach in GAP3 (which deliberately manipulates the generating list used to construct the group) it turned out to be very useful and efficient in the situation at hand.

We define an arep recursively in Backus-Naur-Form as the disjoint union of the following cases.

beg-tabularlllll \multicolumn5larep ::=
; &\multicolumn4latomic cases
& & perm & ; & perm"
& | & mon & ; & mon"
& | & mat & ; & mat"

; &\multicolumn4lcomposed cases
& | & arep \^ arep & ; & conjugate"
& | & arep ... arep & ; & directSum"
& | & arep ... arep& ; & innerTensorProduct"
& | & arep # ... # arep & ; & outerTensorProduct"
& | & arep \downarrow subgrp & ; & restriction"
& | & arep \uparrow supgrp, transversal & ; & induction"
& | & Extension(arep, ext-character) & ; & extension"
& | & GaloisConjugate(arep, aut) & ; & galoisConjugate" end-tabular

An arep R is a record with the following fields mandatory to all types of areps.

\begintabularlll isARep & := & true
operations & := & AMatOps
char & : & characteristic of the base field
degree & : & degree of the representation
source & : & the group being represented, which must contain
& & the field .theGenerators, see GroupWithGenerators
type & : & a string identifying the type of R \endtabular

The cases as stated above are distinguished by the field .type of an arep R. Depending on the type additional fields are mandatory as follows.

beg-tabular{p2.5cmp10cm} \multicolumn2ltype = "perm":
theImages & list of permutations for the images of source.theGenerators end-tabular

beg-tabular{p2.5cmp10cm} \multicolumn2ltype = "mon":
theImages & list of mons (see Mons) for the images of source.theGenerators end-tabular

beg-tabular{p2.5cmp10cm} \multicolumn2ltype = "mat":
theImages & list of matrices for the images of source.theGenerators end-tabular

beg-tabular{p2.5cmp10cm} \multicolumn2ltype = "mat":
rep & an arep to be conjugated
conjugation & an amat (see AMats) conjugating rep end-tabular

beg-tabular{p2.5cmp10cm} \multicolumn2ltype = "directSum":
summands & list of areps of the same source and characteristic end-tabular

beg-tabular{p2.5cmp10cm} \multicolumn2ltype = "innerTensorProduct":
factors & list of areps of the same characteristic end-tabular

beg-tabular{p2.5cmp10cm} \multicolumn2ltype = "outerTensorProduct":
factors & list of areps of the same characteristic end-tabular

beg-tabular{p2.5cmp10cm} \multicolumn2ltype = "restriction":
rep & an arep of a supergroup of source, the group source
& and rep.source have the same parent group end-tabular

beg-tabular{p2.5cmp10cm} \multicolumn2ltype = "induction":
rep & an arep of a subgroup of source, the group source
& and rep.source have the same parent group
transversal & a right transversal of Cosets(source, rep.source) end-tabular

beg-tabular{p2.5cmp10cm} \multicolumn2ltype = "galoisConjugate":
rep & an arep to be conjugated
galoisAut & the Galois automorphism end-tabular

Note that most of the function concerning areps require calculation in the source group. Hence it is most useful to choose aggroups or permutation groups as sources if possible. Furthermore there is an important difference between the type of an arep and the type of the representation being represented by the arep: E.g. an arep can be of type induction" but the representation is in fact a permutation representation. This distinction is reflected in the naming of the functions: XARep" refers to the type of the arep, XRep" to the type of the representation being represented,

Here a short overview of the function concerning areps. sections GroupWithGenerators -- GaloisConjugateARep are concerned with the construction of areps, sections Basic Functions for AReps -- ARepWithCharacter are concerned with the evaluation of an arep at a point, tests for equivalence and irreducibility, construction of an arep with given character etc., sections Converting AReps -- MatARepARep deal with the conversion of areps to areps of type "perm", "mon", "mat". Sections Higher Functions for AReps -- DecompositionMonRep provide function for the computation of the intertwining space of areps and a plenty of functions for monomial areps. The most important function here is DecompositionMonRep (see DecompositionMonRep) performing the decomposition of a monomial arep including the computation of a highly structured decomposition matrix.

The basic functions concerning areps are implemented in the file "arep/lib/arep.g", the higher functions in "arep/lib/arepfcts.g".

For details on constructive representation theory and the theoretical background of the higher functions please refer to Pue98.

## 74.67 GroupWithGenerators

GroupWithGenerators( G )

Let G be a group. GroupWithGenerators returns G with the field G.theGenerators being set to a fixed non-empty generating set of G. This function is created because GAP3 has the freedom to manipulate the generators given to construct a group. Based on the list G.theGenerators areps can be constructed, e.g. by the images on that list (ARepByImages, ARepByImages). If an arep for a group G is constructed with the field G.theGenerators unbound a warning is signaled and the field is set.

    gap> G := Group( (1,2) );
Group( (1,2) )
gap> GroupWithGenerators(G);
Group( (1,2) )
gap> G.theGenerators;
[ (1,2) ]
gap> G := Group( () );
Group( () )
gap> GroupWithGenerators(G);
Group( () )
gap> G.theGenerators;
[ () ]
gap> G.generators;
[  ] 

GroupWithGenerators( list )

GroupWithGenerators returns the group G generated by the elements in list. The field G.theGenerators is set to list. For the reason of this function see above.

    gap> G := GroupWithGenerators( [ (), (1,2), (1,2,3) ] );
Group( (1,2), (1,2,3) )
gap> G.theGenerators;
[ (), (1,2), (1,2,3) ]
gap> G.generators;
[ (1,2), (1,2,3) ] 

## 74.68 TrivialPermARep

TrivialPermARep( G )
TrivialPermARep( G, d )
TrivialPermARep( G, d, char )
TrivialPermARep( G, d, field )

TrivialPermARep returns an arep of type "perm" representing the one representation of the group G of degree d. The default degree is 1. As optional parameter a characteristic char or a field can be supplied to obtain the one representation of arbitrary characteristic. The default characteristic is zero.

    gap> G := GroupWithGenerators( [(1,2), (3,4)] );
Group( (1,2), (3,4) )
gap> TrivialPermARep(G, 2, 3);
TrivialPermARep( GroupWithGenerators( [ (1,2), (3,4) ] ), 2, GF(3) )
gap> G := GroupWithGenerators( [(1,2), (3,4)] );
Group( (1,2), (3,4) )
gap> R := TrivialPermARep(G, 2, 3);
TrivialPermARep( GroupWithGenerators( [ (1,2), (3,4) ] ), 2, GF(3) )
gap> R.degree;
2
gap> R.char;
3 

## 74.69 TrivialMonARep

TrivialMonARep( G )
TrivialMonARep( G, d )
TrivialMonARep( G, d, char )
TrivialMonARep( G, d, field )

TrivialMonARep returns an arep of type "mon" representing the one representation of the group G of degree d. The default degree is 1. As optional parameter a characteristic char or a field can be supplied to obtain the one representation of arbitrary characteristic. The default characteristic is zero.

    gap> G := GroupWithGenerators( [(1,2), (3,4)] );
Group( (1,2), (3,4) )
gap> R := TrivialMonARep(G, 2);
TrivialMonARep( GroupWithGenerators( [ (1,2), (3,4) ] ), 2 )
gap> R.theImages;
[ Mon( (), 2 ), Mon( (), 2 ) ] 

## 74.70 TrivialMatARep

TrivialMatARep( G )
TrivialMatARep( G, d )
TrivialMatARep( G, d, char )
TrivialMatARep( G, d, field )

TrivialMatARep returns an arep of type "mat" representing the one representation of the group G of degree d. The default degree is 1. As optional parameter a characteristic char or a field can be supplied to obtain the one representation of arbitrary characteristic. The default characteristic is zero.

    gap> G := GroupWithGenerators( [(1,2), (3,4)] );
Group( (1,2), (3,4) )
gap> R := TrivialMatARep(G);
TrivialMatARep( GroupWithGenerators( [ (1,2), (3,4) ] ) )
gap> R.theImages;
[ [ [ 1 ] ], [ [ 1 ] ] ] 

## 74.71 RegularARep

RegularARep( G )
RegularARep( G, char )
RegularARep( G, field )

RegularARep returns an arep of type "induction" representing the regular representation of G. The regular representation is defined (up to equality) by the induction R = (1{\mathsfE}\uparrowT G) of the trivial representation (of degree one) of the trivial subgroup \mathsfE of G with the transversal T being the ordered list of elements of G. As optional parameter a characteristic char or a field can be supplied to obtain the regular representation of arbitrary characteristic. The default characteristic is zero.

    gap> G := GroupWithGenerators(SymmetricGroup(3));
Group( (1,3), (2,3) )
gap> RegularARep(G);
RegularARep( GroupWithGenerators( [ (1,3), (2,3) ] ) )
gap> RegularARep(G, GF(2));
RegularARep( GroupWithGenerators( [ (1,3), (2,3) ] ), GF(2) ) 

## 74.72 NaturalARep

NaturalARep( G )
NaturalARep( G, d )
NaturalARep( G, d, char )
NaturalARep( G, d, field )

Let G be a mongroup or a matrix group (for mons see Mons). NaturalARep returns an arep of type "mon" or "mat" resp. representing the representation given by G, which means that G is taken as a representation of itself.

For a permutation group G the desired degree d of the representation has to be supplied. The returned arep is of type "perm". If d is smaller than the largest moved point of G an error is signaled. As optional parameter a characteristic char or a field can be supplied (if G is a permutation group). Note that a mongroup or a matrix group as source of an arep slows down most of the calculations with it.

    gap> G := GroupWithGenerators( [ (1,2), (1,2,3) ] );
Group( (1,2), (1,2,3) )
gap> R := NaturalARep(G, 4);
NaturalARep( GroupWithGenerators( [ (1,2), (1,2,3) ] ), 4 )
gap> R.theImages;
[ (1,2), (1,2,3) ]
gap> R.degree;
4
gap> G := GroupWithGenerators( [ Mon( (1,2), [E(4), 1] ) ] );
Group( Mon(
(1,2),
[ E(4), 1 ]
) )
gap> NaturalARep(G);
NaturalARep(
GroupWithGenerators( [ Mon(
(1,2),
[ E(4), 1 ]
) ] ) ) 

## 74.73 ARepByImages

ARepByImages( G, list )
ARepByImages( G, list, hint )

ARepByImages( G, list, d )
ARepByImages( G, list, d, hint )
ARepByImages( G, list, d, char )
ARepByImages( G, list, d, field )
ARepByImages( G, list, d, char, hint )
ARepByImages( G, list, d, field, hint )

ARepByImages allows to construct an arep of the group G by supplying the list of images on the list G.theGenerators.

Let list contain mons (see Mons). ARepByImages returns an arep of type "mon" defined by mapping G.theGenerators elementwise onto list.

Let list contain matrices. ARepByImages returns an arep of type "mat" defined by mapping G.theGenerators elementwise onto list.

Let list contain permutations. ARepByImages returns an arep of type "perm" and degree d defined by mapping G.theGenerators elementwise onto list. If d is smaller than the largest moved point of G an error is signaled. As optional parameter a characteristic char or a field can be supplied to obtain an arep of arbitrary characteristic.

In all cases the hint "hom" or "faithful" can be supplied to indicate that the list of images does define a homomorphism or even a faithful homomorphism respectively. If no hint is supplied it is checked whether the list of images defines a homomorphism.

    gap> G := GroupWithGenerators( [(1,2), (1,2,3)] );
Group( (1,2), (1,2,3) )
gap> ARepByImages(G, [ Mon( [-1] ), Mon( [1] ) ] );
ARepByImages(
GroupWithGenerators( [ (1,2), (1,2,3) ] ),
[ Mon( [ -1 ] ), Mon( (), 1 ) ],
"hom"
)
gap> L := [ [ [Z(2), Z(2)], [0*Z(2), Z(2)] ], IdentityMat(2, GF(2)) ];
[ [ [ Z(2)^0, Z(2)^0 ], [ 0*Z(2), Z(2)^0 ] ],
[ [ Z(2)^0, 0*Z(2) ], [ 0*Z(2), Z(2)^0 ] ] ]
gap> ARepByImages(G, L);
ARepByImages(
GroupWithGenerators( [ (1,2), (1,2,3) ] ),
[ [ [ Z(2)^0, Z(2)^0 ], [ 0*Z(2), Z(2)^0 ] ],
[ [ Z(2)^0, 0*Z(2) ], [ 0*Z(2), Z(2)^0 ] ]
],
GF(2),
"hom"
)
gap> ARepByImages(G, [ (1,2), () ], 3);
ARepByImages(
GroupWithGenerators( [ (1,2), (1,2,3) ] ),
[ (1,2), () ],
3, # degree
"hom"
)
gap> ARepByImages(G, [ (1,2), () ], 3, "hom");
ARepByImages(
GroupWithGenerators( [ (1,2), (1,2,3) ] ),
[ (1,2), () ],
3, # degree
"hom"
) 

## 74.74 ARepByHom

ARepByHom( hom )

ARepByHom( hom, d )
ARepByHom( hom, d, char )
ARepByHom( hom, d, char )

Let hom be a homomorphism of a group into a mongroup. ARepByHom returns an arep of type "mon" corresponding to hom.

Let hom be a homomorphism of a group into a matrix group. ARepByHom returns an arep of type "mat" corresponding to hom.

Let hom be a homomorphism of a group into a permutation group and d a positive integer. ARepByHom returns an arep of type "perm" and degree d corresponding to hom. If d is smaller than the largest moved point of hom.range an error is signaled. As optional parameter a characteristic char or a field can be supplied to obtain an arep of arbitrary characteristic.

    gap> G := GroupWithGenerators(SymmetricGroup(4));
Group( (1,4), (2,4), (3,4) )
gap> phi := IdentityMapping(G);
IdentityMapping( Group( (1,4), (2,4), (3,4) ) )
gap> ARepByHom(phi, 4);
NaturalARep( GroupWithGenerators( [ (1,4), (2,4), (3,4) ] ), 4 )
gap> H := GroupWithGenerators( [ Mon( [-1] ) ] );
Group( Mon( [ -1 ] ) )
gap> psi :=
> GroupHomomorphismByImages(G, H, G.generators, [H.1, H.1, H.1]);
GroupHomomorphismByImages(
Group( (1,4), (2,4), (3,4) ),
Group( Mon( [ -1 ] ) ),
[ (1,4), (2,4), (3,4) ],
[ Mon( [ -1 ] ), Mon( [ -1 ] ), Mon( [ -1 ] ) ] )
gap> ARepByHom(psi);
ARepByImages(
GroupWithGenerators( [ (1,4), (2,4), (3,4) ] ),
[ Mon( [ -1 ] ),
Mon( [ -1 ] ),
Mon( [ -1 ] )
],
"hom"
) 

## 74.75 ARepByCharacter

ARepByCharacter( chi )

Let chi be a onedimensional character of a group. ARepByCharacter returns a onedimensional arep of type "mon" given by chi.

    gap> G := GroupWithGenerators( [ (1,2) ] );
Group( (1,2) )
gap> L := Irr(G);
[ Character( Group( (1,2) ), [ 1, 1 ] ),
Character( Group( (1,2) ), [ 1, -1 ] ) ]
gap> ARepByCharacter( L[2] );
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ Mon( [ -1 ] ) ],
"hom"
) 

## 74.76 ConjugateARep

ConjugateARep( R, A )  or  R ^ A
ConjugateARep( R, A, hint )

Let R be an arep and A an amat (see AMats). ConjugateARep returns an arep of type "conjugate" representing the conjugated representation <R>A: x→ A-1. R(x). A. The amat is tested for invertibility if the optional hint "invertible" is not supplied. R and A must be compatible in size and characteristic otherwise an error is signaled. Note that R and A can be accessed in the fields .rep and .conjugation of the result.

    gap> G := GroupWithGenerators(SymmetricGroup(4));
Group( (1,4), (2,4), (3,4) )
gap> R := NaturalARep(G, 4);
NaturalARep( GroupWithGenerators( [ (1,4), (2,4), (3,4) ] ), 4 )
gap> A := AMatPerm( (1,2,3,4), 4 );
AMatPerm((1,2,3,4), 4)
gap> R ^ A;
ConjugateARep(
NaturalARep( GroupWithGenerators( [ (1,4), (2,4), (3,4) ] ), 4 ),
AMatPerm((1,2,3,4), 4)
) 

## 74.77 DirectSumARep

DirectSumARep( R1, ..., Rk )

DirectSumARep returns an arep of type "directSum" representing the direct sum <R1>⊕...⊕ R_k of the areps R1, ..., Rk, which must have common source and characteristic otherwise an error is signaled.

The direct sum <R> = R_1⊕...⊕ R_k of representations is defined as x→ R_1(x)⊕...⊕ R_k(x).

Note that the summands R1, ..., Rk can be accessed in the field .summands of the result.

    gap> G := GroupWithGenerators( [(1,2,3,4), (1,3)] );
Group( (1,2,3,4), (1,3) )
gap> R1 := RegularARep(G);
RegularARep( GroupWithGenerators( [ (1,2,3,4), (1,3) ] ) )
gap> R2 := ARepByImages(G, [ [[1]], [[-1]] ]);
ARepByImages(
GroupWithGenerators( [ (1,2,3,4), (1,3) ] ),
[ [ [ 1 ] ], [ [ -1 ] ] ],
"hom"
)
gap> DirectSumARep(R1, R2);
DirectSumARep(
RegularARep( GroupWithGenerators( [ (1,2,3,4), (1,3) ] ) ),
ARepByImages(
GroupWithGenerators( [ (1,2,3,4), (1,3) ] ),
[ [ [ 1 ] ], [ [ -1 ] ] ],
"hom"
)
) 

DirectSumARep( list )

DirectSumARep returns an arep of type "directSum" representing the direct sum of the areps in list (see above).

## 74.78 InnerTensorProductARep

InnerTensorProductARep( R1, ..., Rk )

InnerTensorProductARep returns an arep of type "innerTensorProduct" representing the inner tensor product <R> = R_1⊗...⊗ R_k of the areps R1, ..., Rk, which must have common source and characteristic otherwise an error is signaled.

The inner tensor product <R> = R_1⊗...⊗ R_k of representations is defined as x→ R_1(x)⊗...⊗ R_k(x). Note that the inner tensor product yields a representation of the same source (in contrast to the outer tensor product, see OuterTensorProductARep).

Note that the tensor factors R1, ..., Rk can be accessed in the field .factors of the result.

    gap> G := GroupWithGenerators( [ (1,2), (3,4) ] );
Group( (1,2), (3,4) )
gap> R1 := ARepByImages(G, [ Mon( (1,2), 2 ), Mon( [-1, -1] ) ] );
ARepByImages(
GroupWithGenerators( [ (1,2), (3,4) ] ),
[ Mon( (1,2), 2 ), Mon( [ -1, -1 ] ) ],
"hom"
)
gap> R2 := NaturalARep(G, 5);
NaturalARep( GroupWithGenerators( [ (1,2), (3,4) ] ), 5 )
gap> InnerTensorProductARep(R1, R2);
InnerTensorProductARep(
ARepByImages(
GroupWithGenerators( [ (1,2), (3,4) ] ),
[ Mon( (1,2), 2 ), Mon( [ -1, -1 ] ) ],
"hom"
),
NaturalARep( GroupWithGenerators( [ (1,2), (3,4) ] ), 5 )
) 

InnerTensorProductARep( list )

InnerTensorProductARep returns an arep of type "innerTensorProduct" representing the inner tensor product of the areps in list (see above).

## 74.79 OuterTensorProductARep

OuterTensorProductARep( R1, ..., Rk )
OuterTensorProductARep( G, R1, ..., Rk )

OuterTensorProductARep returns an arep of type "outerTensorProduct" representing the outer tensor product <R> = R_1# ...# R_k of the areps R1, ..., Rk, which must have common characteristic otherwise an error is signaled.

The outer tensor product <R> = R_1# ...# R_k of representations is defined as x→ R_1(x)⊗...⊗ R_k(x). Note that the outer tensor product of representations is a representation of the direct product of the sources (in contrast to the inner tensor product, see InnerTensorProductARep).

Using the first version OuterTensorProductARep returns an arep R with R.source = DirectProduct(R1.source, ..., Rk.source) using the GAP3 function DirectProduct. In the second version the returned arep has as source the group G which must be the inner direct product <G> = R_1.source×...×R_k.source. This property is not checked.

Note that the tensor factors R1, ..., Rk can be accessed in the field .factors of the result.

    gap> G1 := GroupWithGenerators(DihedralGroup(8));
Group( (1,2,3,4), (2,4) )
gap> G2 := GroupWithGenerators( [ (1,2) ] );
Group( (1,2) )
gap> R1 := NaturalARep(G1, 4);
NaturalARep( GroupWithGenerators( [ (1,2,3,4), (2,4) ] ), 4 )
gap> R2 := ARepByImages(G2, [ [[-1]] ]);
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ [ [ -1 ] ] ],
"hom"
)
gap> OuterTensorProductARep(R1, R2);
OuterTensorProductARep(
NaturalARep( GroupWithGenerators( [ (1,2,3,4), (2,4) ] ), 4 ),
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ [ [ -1 ] ] ],
"hom"
)
) 

## 74.80 RestrictionARep

RestrictionARep( R, H )

RestrictionARep returns an arep of type "restriction" representing the restriction of the arep R to the subgroup H of R.source. Here, subgroup" means, that all elements of H are contained in R.source.

The restriction <R>\downarrow H of a representation R to a subgroup H is defined by x→ R(x), x∈ H.

Note that R can be accessed in the field .rep of the result.

    gap> G := GroupWithGenerators(SymmetricGroup(4));
Group( (1,4), (2,4), (3,4) )
gap> H := GroupWithGenerators(AlternatingGroup(4));
Group( (1,2,4), (2,3,4) )
gap> R := NaturalARep(G, 4);
NaturalARep( GroupWithGenerators( [ (1,4), (2,4), (3,4) ] ), 4 )
gap> RestrictionARep(R, H);
RestrictionARep(
NaturalARep( GroupWithGenerators( [ (1,4), (2,4), (3,4) ] ), 4 ),
GroupWithGenerators( [ (1,2,4), (2,3,4) ] )
) 

## 74.81 InductionARep

InductionARep( R, G )
InductionARep( R, G, T )

InductionARep returns an arep of type "induction" representing the induction of the arep R to the supergroup G with the transversal T of the residue classes R.source \ G. Here, supergroup" means that all elements of R.source are contained in G. If no transversal T is supplied one is chosen by the function RightTransversal. If a transversal T is given it is not checked to be one.

The induction <R>\uparrowT G of a representation R of H to a supergroup G with transversal T = {t1,...,tk} of <H> \ G is defined by x→[ \dotR(ti. x. tj-1) | i,j∈{1,...,k}], where \dotR(y) = R(y) for y∈ H and 0 else.

Note that R and T can be accessed in the fields .rep and .transversal resp. of the result.

    gap> G := GroupWithGenerators( [ (1,2,3,4), (1,2) ] );
Group( (1,2,3,4), (1,2) )
gap> H := GroupWithGenerators( [ (1,2) ] );
Group( (1,2) )
gap> R := ARepByImages(H, [ [[Z(2), Z(2)], [0*Z(2), Z(2)]] ] );
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ [ [ Z(2)^0, Z(2)^0 ], [ 0*Z(2), Z(2)^0 ] ]
],
"hom"
)
gap> R.name := "R";
"R"
gap> InductionARep(R, G);
InductionARep(
R,
GroupWithGenerators( [ (1,2,3,4), (1,2) ] ),
[ (), (3,4), (2,3), (2,3,4), (2,4,3), (2,4), (1,4,3),
(1,4), (1,4,2,3), (1,4)(2,3), (1,2,3), (1,2,3,4) ]
) 

## 74.82 ExtensionARep

ExtensionARep( R, chi )

Let R be an irreducible arep of characteristic zero and chi a character of a supergroup of R.source which extends the character of R. ExtensionARep returns an arep of type "extension" representing an extension of R to chi.source. Here, supergroup" means that all elements of R.source are contained in G. The extension is evaluated using Minkwitz's formula (see Min96).

Note that R and chi can be accessed in the fields .rep and .character of the result.

    gap> G := GroupWithGenerators( [ (1,2,3,4), (1,2) ] );
Group( (1,2,3,4), (1,2) )
gap> H := GroupWithGenerators(AlternatingGroup(4));
Group( (1,2,4), (2,3,4) )
gap> G.name := "S4";
"S4"
gap> H.name := "A4";
"A4"
gap> R := ARepByImages(H, [ Mon( (1,2,3), [ 1, -1, -1 ] ),
> Mon( (1,2,3), 3 ) ] );
ARepByImages(
A4,
[ Mon( (1,2,3), [ 1, -1, -1 ] ),
Mon( (1,2,3), 3 )
],
"hom"
)
gap> L := Irr(G);
[ Character( Group( (1,2,3,4), (1,2) ), [ 1, 1, 1, 1, 1 ] ),
Character( Group( (1,2,3,4), (1,2) ), [ 1, -1, 1, 1, -1 ] ),
Character( Group( (1,2,3,4), (1,2) ), [ 2, 0, -1, 2, 0 ] ),
Character( Group( (1,2,3,4), (1,2) ), [ 3, -1, 0, -1, 1 ] ),
Character( Group( (1,2,3,4), (1,2) ), [ 3, 1, 0, -1, -1 ] ) ]
gap> ExtensionARep(R, L[4]);
ExtensionARep(
ARepByImages(
A4,
[ Mon(
(1,2,3),
[ 1, -1, -1 ]
),
Mon( (1,2,3), 3 )
],
"hom"
),
Character( Group( (1,2,3,4), (1,2) ), [ 3, -1, 0, -1, 1 ] )
) 

## 74.83 GaloisConjugateARep

GaloisConjugateARep( R, aut )
GaloisConjugateARep( R, k )

GaloisConjugateARep returns an arep of type "galoisConjugate" representing the Galois conjugate of the arep A. The conjugating automorphism may either be a field automorphism aut or an integer k specifying the automorphism x -> GaloisCyc(x, k) in the case characteristic = 0 or x -> x^(FrobeniusAut^k) in the case characteristic = p prime.

The Galois conjugate of a representation R with a field automorphism aut is defined by x→ R(x)aut .

Note that R and aut can be accessed in the fields .rep and .galoisAut resp. of the result.

    gap> G := GroupWithGenerators( [ (1,2,3) ] );
Group( (1,2,3) )
gap> R := ARepByImages(G, [ [[E(3)]] ] );
ARepByImages(
GroupWithGenerators( [ (1,2,3) ] ),
[ [ [ E(3) ] ]
],
"hom"
)
gap> GaloisConjugateARep(R, -1);
GaloisConjugateARep(
ARepByImages(
GroupWithGenerators( [ (1,2,3) ] ),
[ [ [ E(3) ] ]
],
"hom"
),
-1
) 

## 74.84 Basic Functions for AReps

The following sections describe basic functions for areps like e.g. testing irreducibility and equivalence, evaluating an arep at a group element, computing kernel and character, and constructing an arep with given character.

## 74.85 Comparison of AReps

R1 = R2
R1 <> R2

The equality operator = evaluates to true if the areps R1 and R2 are equal and to false otherwise. The inequality operator <> evaluates to true if the amats R1 and R2 are not equal and to false otherwise.

Two areps are equal iff they define the same representation. This means that first the sources have to be equal, i.e. R1.source = R2.source and second the images are pointwise equal.

R1 < R2
R1 <= R2
R1 >= R2
R1 > R2

The operators <, <=, >=, and > evaluate to true if the arep R1 is strictly less than, less than or equal to, greater than or equal to, and strictly greater than the arep R2.

The ordering of areps is defined via the ordering of records.

## 74.86 ImageARep

ImageARep( x, R )  or x ^ R

Let R be an arep and x a group element of R.source. ImageARep returns the image of x under R as an amat (see AMats). For conversion of amats see PermAMat -- MatAMat.

    gap> G := GroupWithGenerators(SolvableGroup(8, 5));
Q8
gap> R := RegularARep(G);
RegularARep( Q8 )
gap> x := Random(G);
c
gap> ImageARep(x, R);
TensorProductAMat(
AMatPerm((1,2)(3,4)(5,6)(7,8), 8),
IdentityPermAMat(1)
) *
DirectSumAMat(
IdentityPermAMat(1),
IdentityPermAMat(1),
IdentityPermAMat(1),
IdentityPermAMat(1),
IdentityPermAMat(1),
IdentityPermAMat(1),
IdentityPermAMat(1),
IdentityPermAMat(1)
)
gap> PermAMat(last);
(1,2)(3,4)(5,6)(7,8) 

ImageARep( list, R )

ImageARep returns the list of images of the group elements in list under the arep R (see above). The images are amats (see AMats). For conversion of amats see PermAMat -- MatAMat.

## 74.87 IsEquivalentARep

IsEquivalentARep( R1, R2 )

Let R1 and R2 be two areps with Maschke condition, i.e. Size( Ri.source ) mod Ri.char ≠ 0, i = 1,2. IsEquivalentARep returns true if the areps R1 and R2 define equivalent representations and false otherwise. Two representations (with Maschke condition) are equivalent iff they have the same character. R1 and R2 must have identical source (i.e. IsIdentical(R1, R2) = true) and characteristic otherwise an error is signaled.

    gap> G := GroupWithGenerators( [ (1,2,3) ] );
Group( (1,2,3) )
gap> R1 := NaturalARep(G, 3);
NaturalARep( GroupWithGenerators( [ (1,2,3) ] ), 3 )
gap> R2 := RegularARep(G);
RegularARep( GroupWithGenerators( [ (1,2,3) ] ) )
gap> IsEquivalentARep(R1, R2);
true 

## 74.88 CharacterARep

CharacterARep( R )

CharacterARep returns the character of the arep R. Since GAP3 only provides characters of characteristic zero, CharacterARep only works in this case and will signal an error otherwise. Note that CharacterARep sets and tests R.character.

    gap> G := GroupWithGenerators( [ (1,2), (3,4) ] );
Group( (1,2), (3,4) )
gap> CharacterARep(RegularARep(G));
Character( Group( (1,2), (3,4) ), [ 4, 0, 0, 0 ] ) 

## 74.89 IsIrreducibleARep

IsIrreducibleARep( R )

Let R an arep of characteristic zero. IsIrreducibleARep returns true if R represents an irreducible arep and false otherwise. To determine irreducibility the character is used, which is the reason for the condition characteristic = 0 (see CharacterARep). Note that IsIrreducibleARep sets and tests R.isIrreducible.

    gap> G := GroupWithGenerators(SolvableGroup(12, 5));
A4
gap> L := Irr(G);
[ Character( A4, [ 1, 1, 1, 1 ] ),
Character( A4, [ 1, 1, E(3), E(3)^2 ] ),
Character( A4, [ 1, 1, E(3)^2, E(3) ] ),
Character( A4, [ 3, -1, 0, 0 ] ) ]
gap> R := ARepByCharacter(L[2]);
ARepByImages(
A4,
[ Mon( [ E(3) ] ),
Mon( (), 1 ),
Mon( (), 1 )
],
"hom"
)
gap> IsIrreducibleARep(R);
true
gap> IsIrreducibleARep(RegularARep(G));
false 

## 74.90 KernelARep

KernelARep( R )

KernelARep returns the kernel of the arep R. Note that KernelARep sets and tests R.kernel.

    gap> G := GroupWithGenerators(SymmetricGroup(3));
Group( (1,3), (2,3) )
gap> R := ARepByImages(G, [ [[-1]], [[-1]] ] );
ARepByImages(
GroupWithGenerators( [ (1,3), (2,3) ] ),
[ [ [ -1 ] ],
[ [ -1 ] ]
],
"hom"
)
gap> KernelARep(R);
Subgroup( Group( (1,3), (2,3) ), [ (1,3,2) ] ) 

## 74.91 IsFaithfulARep

IsFaithfulARep( R )

IsFaithfulARep returns true if the arep R represents a faithful representation and false otherwise. Note that IsFaithfulARep sets and tests R.isFaithful.

    gap> G := GroupWithGenerators(SolvableGroup(16, 7));
Q8x2
gap> IsFaithfulARep(TrivialPermARep(G));
false
gap> IsFaithfulARep(RegularARep(G));
true 

## 74.92 ARepWithCharacter

ARepWithCharacter( chi )

ARepWithCharacter constructs an arep with character chi. The group chi.source must be solvable otherwise an error is signaled. Note that the function returns a monomial arep if this is possible.

Attention: ARepWithCharacter only works in GAP3 3.4.4 after bugfix 9!

    gap> G := GroupWithGenerators(SolvableGroup(8, 5));
Q8
gap> L := Irr(G);
[ Character( Q8, [ 1, 1, 1, 1, 1 ] ),
Character( Q8, [ 1, 1, -1, 1, -1 ] ),
Character( Q8, [ 1, 1, 1, -1, -1 ] ),
Character( Q8, [ 1, 1, -1, -1, 1 ] ),
Character( Q8, [ 2, -2, 0, 0, 0 ] ) ]
gap> MonARepARep(ARepWithCharacter(L[5]));
ARepByImages(
Q8,
[ Mon(
(1,2),
[ -1, 1 ]
),
Mon( [ E(4), -E(4) ] ),
Mon( [ -1, -1 ] )
],
"hom"
) 

## 74.93 GeneralFourierTransform

GeneralFourierTransform( G )

GeneralFourierTransform returns an amat representing a Fourier transform over the complex numbers for the solvable group G. For an explanation of Fourier transforms see CB93. In order to obtain a fast Fourier transform for G apply the function DecompositionMonRep to any regular representation of G.

Attention: GeneralFourierTransform only works in GAP3 3.4.4 after bugfix 9!

    gap> G := SymmetricGroup(3);
Group( (1,3), (2,3) )
gap> GeneralFourierTransform(G);
AMatMat(
[ [ 1, 1, 1, 1, 1, 1 ], [ 1, -1, -1, 1, 1, -1 ],
[ 1, 0, 0, E(3), E(3)^2, 0 ], [ 0, 1, E(3)^2, 0, 0, E(3) ],
[ 0, 1, E(3), 0, 0, E(3)^2 ], [ 1, 0, 0, E(3)^2, E(3), 0 ] ],
"invertible"
) ^ -1 

## 74.94 Converting AReps

The following sections describe functions for convertibility and conversion of arbitrary areps to areps of type "perm", "mon", and "mat". As in AMat (see AMats) the naming of the functions follows the usual GAP3-convention: ChalkCheese makes chalk from cheese. The parts in the name (chalk, cheese) are:

\begintabular{l@ -- l} ARep & an arep of any type
PermARep & an arep of type perm"
MonARep & an arep of type mon"
MatARep & an arep of type mat" \endtabular

## 74.95 IsPermRep

IsPermRep( R )

IsPermRep returns true if R represents a permutation representation and false otherwise. Note that the name of this function is not IsPermARep since R can be an arep of any type but represents a permutation representation in the mathematical sense (every image is a permutation matrix). Note that IsPermRep sets and tests R.isPermRep.

    gap> G := GroupWithGenerators( [ (1,2) ] );
Group( (1,2) )
gap> R := ARepByImages(G, [ Mon( [1, -1] ) ] );
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ Mon( [ 1, -1 ] )
],
"hom"
)
gap> IsPermRep(ConjugateARep(R, DFTAMat(2)));
true 

## 74.96 IsMonRep

IsMonRep( R )

IsMonRep returns true if R represents a monomial representation and false otherwise. Note that the name of this function is not IsMonARep since R can be an arep of any type but represents a monomial representation in the mathematical sense (every image is a monomial matrix). Note that IsMonRep sets and tests R.isMonRep.

    gap> G := GroupWithGenerators(SolvableGroup(8, 5));
Q8
gap> R := RegularARep(G);
RegularARep( Q8 )
gap> IsMonRep(InnerTensorProductARep(R, R));
true 

## 74.97 PermARepARep

PermARepARep( R )

PermARepARep returns an arep of type "perm" representing the same representation as the arep R if possible. Otherwise false is returned. Note that PermARepARep sets and tests R.permARep.

    gap> G := GroupWithGenerators( [ (1,2) ] );
Group( (1,2) )
gap> R := ARepByImages(G, [ Mon( [1, -1] ) ] );
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ Mon( [ 1, -1 ] )
],
"hom"
)
gap> PermARepARep(ConjugateARep(R, DFTAMat(2)));
NaturalARep( GroupWithGenerators( [ (1,2) ] ), 2 )
gap> PermARepARep(R);
false 

## 74.98 MonARepARep

MonARepARep( R )

MonARepARep returns an arep of type "mon" representing the same representation as the arep R if possible. Otherwise false is returned. Note that MonARepARep sets and tests R.monARep.

    gap> G := GroupWithGenerators( [ (1,2,3), (1,2) ] );
Group( (1,2,3), (1,2) )
gap> R1 := ARepByImages(G, [ [[1]], [[-1]] ] );
ARepByImages(
GroupWithGenerators( [ (1,2,3), (1,2) ] ),
[ [ [ 1 ] ],
[ [ -1 ] ]
],
"hom"
)
gap> R2 := NaturalARep(G, 4);
NaturalARep( GroupWithGenerators( [ (1,2,3), (1,2) ] ), 4 )
gap> MonARepARep(InnerTensorProductARep(R1, R2));
ARepByImages(
GroupWithGenerators( [ (1,2,3), (1,2) ] ),
[ Mon( (1,2,3), 4 ),
Mon(
(1,2),
[ -1, -1, -1, -1 ]
)
],
"hom"
) 

## 74.99 MatARepARep

MatARepARep( R )

MatARepARep returns an arep of type "mat" representing the same representation as the arep R. Note that MatARepARep sets and tests R.matARep.

    gap> G := GroupWithGenerators( [ (1,2), (3,4) ] );
Group( (1,2), (3,4) )
gap> MatARepARep(RegularARep(G, 3));
ARepByImages(
GroupWithGenerators( [ (1,2), (3,4) ] ),
[ [ [ 0*Z(3), 0*Z(3), Z(3)^0, 0*Z(3) ],
[ 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0 ],
[ Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3) ],
[ 0*Z(3), Z(3)^0, 0*Z(3), 0*Z(3) ] ],
[ [ 0*Z(3), Z(3)^0, 0*Z(3), 0*Z(3) ],
[ Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3) ],
[ 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0 ],
[ 0*Z(3), 0*Z(3), Z(3)^0, 0*Z(3) ] ]
],
"hom"
) 

## 74.100 Higher Functions for AReps

The following sections describe functions allowing the structural manipulation of, mainly monomial, areps. The idea is to convert a given arep into a mathematical equal (not only equivalent!) arep having more structure. Examples are: converting a transitive monomial arep into a conjugated induction (see TransitiveToInductionMonRep), converting an induction into a conjugated double induction (see InsertedInductionARep), changing the transversal of an induction (see TransversalChangeInductionARep), decomposing a transitive monomial arep into a conjugated outer tensor product (see OuterTensorProductDecompositionMonRep) and last but not least decomposing a monomial arep into a conjugated sum of irreducibles (see DecompositionMonRep). The latter is one of the most interesting functions of the package \AREP.

## 74.101 IsRestrictedCharacter

IsRestrictedCharacter( chi, chisub )

IsRestrictedCharacter returns true if the character chisub is a restriction of the character chi to chisub.source and false otherwise. All elements of chisub.source must be contained in chi.source otherwise an error is signaled.

    gap> G := SymmetricGroup(3); G.name := "S3";
Group( (1,3), (2,3) )
"S3"
gap> H := CyclicGroup(3); H.name := "Z3";
Group( (1,2,3) )
"Z3"
gap> L1 := Irr(G);
[ Character( S3, [ 1, 1, 1 ] ), Character( S3, [ 1, -1, 1 ] ),
Character( S3, [ 2, 0, -1 ] ) ]
gap> L2 := Irr(H);
[ Character( Z3, [ 1, 1, 1 ] ), Character( Z3, [ 1, E(3), E(3)^2 ] ),
Character( Z3, [ 1, E(3)^2, E(3) ] ) ]
gap> IsRestrictedCharacter(L1[2], L2[1]);
true 

## 74.102 AllExtendingCharacters

AllExtendingCharacters( chi, G )

AllExtendingCharacters returns the list of all characters of G extending chi. All elements of chi.source must be contained in G otherwise an error is signaled.

    gap> H := AlternatingGroup(4); H.name := "A4";
Group( (1,2,4), (2,3,4) )
"A4"
gap> G := SymmetricGroup(4); G.name := "S4";
Group( (1,4), (2,4), (3,4) )
"S4"
gap> L := Irr(H);
[ Character( A4, [ 1, 1, 1, 1 ] ),
Character( A4, [ 1, 1, E(3)^2, E(3) ] ),
Character( A4, [ 1, 1, E(3), E(3)^2 ] ),
Character( A4, [ 3, -1, 0, 0 ] ) ]
gap> AllExtendingCharacters(L[4], G);
[ Character( S4, [ 3, -1, -1, 0, 1 ] ),
Character( S4, [ 3, 1, -1, 0, -1 ] ) ] 

## 74.103 OneExtendingCharacter

OneExtendingCharacter( chi, G )

OneExtendingCharacter returns one character of G extending chi if possible or returns false otherwise. All elements of chi.source must be contained in G otherwise an error is signaled.

    gap> H := Group( (1,3)(2,4) ); H.name := "Z2";
Group( (1,3)(2,4) )
"Z2"
gap> G := Group( (1,2,3,4) ); G.name := "Z4";
Group( (1,2,3,4) )
"Z4"
gap> L := Irr(H);
[ Character( Z2, [ 1, 1 ] ), Character( Z2, [ 1, -1 ] ) ]
gap> OneExtendingCharacter(L[2], G);
Character( Z4, [ 1, E(4), -1, -E(4) ] ) 

## 74.104 IntertwiningSpaceARep

IntertwiningSpaceARep( R1, R2 )

IntertwiningSpaceARep returns a list of amats (see AMats) representing a base of the intertwining space Int(R_1, R_2) of the areps R1 and R2, which must have common source and characteristic otherwise an error is signaled.

The intertwining space Int(R_1, R_2) of two representations R1 and R2 of a group G of the same characteristic is the vector space of matrices {M| R1(x). M = M . R2(x), for all x∈ G}.

    gap> G := GroupWithGenerators( [ (1,2,3) ] );
Group( (1,2,3) )
gap> R1 := NaturalARep(G, 3);
NaturalARep( GroupWithGenerators( [ (1,2,3) ] ), 3 )
gap> R2 := ARepByImages(G, [ Mon( [1, E(3), E(3)^2] ) ] );
ARepByImages(
GroupWithGenerators( [ (1,2,3) ] ),
[ Mon( [ 1, E(3), E(3)^2 ] )
],
"hom"
)
gap> IntertwiningSpaceARep(R1, R2);
[ AMatMat( [ [ 1, 0, 0 ], [ 1, 0, 0 ], [ 1, 0, 0 ] ] ),
AMatMat( [ [ 0, 1, 0 ], [ 0, E(3), 0 ], [ 0, E(3)^2, 0 ] ] ),
AMatMat( [ [ 0, 0, 1 ], [ 0, 0, E(3)^2 ], [ 0, 0, E(3) ] ] ) ] 

## 74.105 IntertwiningNumberARep

IntertwiningNumberARep( R1, R2 )

IntertwiningNumberARep returns the intertwining number of the areps R1 and R2. The Maschke condition must hold for both R1 and R2, otherwise an error is signaled. R1 and R2 must have identical source (i.e. IsIdentical(R1, R2) = true) and characteristic otherwise an error is signaled.

The intertwining number of two representations R1 and R2 (with Maschke condition) is the dimension of the intertwining space or the scalar product of the characters.

    gap> G := GroupWithGenerators(SolvableGroup(64, 12));
2^3xD8
gap> R := RegularARep(G);
RegularARep( 2^3xD8 )
gap> IntertwiningNumberARep(R, R);
64 

## 74.106 UnderlyingPermRep

UnderlyingPermRep( R )

Let R be a monomial arep (i.e. IsMonRep( R ) = true). UnderlyingPermRep returns an arep of type "perm" representing the underlying permutation representation of R.

The underlying permutation representation of a monomial representation R is obtained by replacing all entries ≠ 0 in the images <R>(x), x∈ G by 1.

    gap> G := GroupWithGenerators( [ (1,2) ] );
Group( (1,2) )
gap> R := ARepByImages(G, [ [[0, 2], [1/2, 0]] ] );
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ [ [ 0, 2 ], [ 1/2, 0 ] ]
],
"hom"
)
gap> UnderlyingPermARep(R);
NaturalARep( GroupWithGenerators( [ (1,2) ] ), 2 ) 

## 74.107 IsTransitiveMonRep

IsTransitiveMonRep( R )

Let R be a monomial arep (i.e. IsMonRep( R ) = true). IsTransitiveMonRep returns true if R is transitive and false otherwise. Note that IsTransitiveMonRep sets and tests R.isTransitive.

A monomial representation is transitive iff the underlying permutation representation is.

    gap> G := GroupWithGenerators( [ (1,2), (3,4) ] );
Group( (1,2), (3,4) )
gap> IsTransitiveMonRep(NaturalARep(G, 4));
false
gap> IsTransitiveMonRep(RegularARep(G));
true 

## 74.108 IsPrimitiveMonRep

IsPrimitiveMonRep( R )

Let R be a monomial arep (i.e. IsMonRep( R ) = true). IsPrimitiveMonRep returns true if R is primitive and false otherwise.

A monomial representation is primitive iff the underlying permutation representation is.

    gap> G := GroupWithGenerators(SymmetricGroup(4)); G.name := "S4";
Group( (1,4), (2,4), (3,4) )
"S4"
gap> H := GroupWithGenerators(SymmetricGroup(3)); H.name := "S3";
Group( (1,3), (2,3) )
"S3"
gap> L := Irr(H);
[ Character( S3, [ 1, 1, 1 ] ), Character( S3, [ 1, -1, 1 ] ),
Character( S3, [ 2, 0, -1 ] ) ]
gap> R := ARepByCharacter(L[2]);
ARepByImages(
S3,
[ Mon( [ -1 ] ),
Mon( [ -1 ] )
],
"hom"
)
gap> IsPrimitiveMonRep(InductionARep(R, G));
true 

## 74.109 TransitivityDegreeMonRep

TransitivityDegreeMonRep( R )

Let R be a monomial arep (i.e. IsMonRep( R ) = true). TransitivityDegreeMonRep returns the degree of transitivity of R as an integer. Note that TransitivityDegreeMonRep sets and tests R.transitivity.

The degree of transitivity of a monomial representation is defined as the degree of transitivity of the underlying permutation representation.

    gap> G := GroupWithGenerators(AlternatingGroup(5));
Group( (1,2,5), (2,3,5), (3,4,5) )
gap> TransitivityDegreeMonRep(NaturalARep(G, 5));
3 

## 74.110 OrbitDecompositionMonRep

OrbitDecompositionMonRep( R )

Let R be a monomial arep (i.e. IsMonRep( R ) = true). OrbitDecompositionMonRep returns an arep equal to R with structure (R1⊕...⊕ Rk)P where Ri, i = 1,...,k are transitive areps of type "mon" and P is an amat of type "perm" (for amats see AMats).

    gap> G := GroupWithGenerators( [ (1,2,3,4) ] ); G.name := "Z4";
Group( (1,2,3,4) )
"Z4"
gap> R := ARepByImages(G, [ Mon( (1,2)(3,4), [1,-1,1,1,-1] ) ] );
ARepByImages(
GroupWithGenerators( [ (1,2,3,4) ] ),
[ Mon( (1,2)(3,4), [ 1, -1, 1, 1, -1 ] ) ],
"hom"
)
gap> OrbitDecompositionMonRep(R);
ConjugateARep(
DirectSumARep(
ARepByImages(
Z4,
[ Mon( (1,2), [ 1, -1 ] ) ],
"hom"
),
ARepByImages(
Z4,
[ Mon( (1,2), 2 ) ],
"hom"
),
ARepByImages(
Z4,
[ Mon( [ -1 ] ) ],
"hom"
)
),
IdentityPermAMat(5)
) 

## 74.111 TransitiveToInductionMonRep

TransitiveToInductionMonRep( R )
TransitiveToInductionMonRep( R, i )

Let R be a transitive monomial arep of a group G. TransitiveToInductionMonRep returns an arep equal to R with structure <R> = (L\uparrowT G)D. L is an arep of degree one of the stabilizer H of the point i and T a transversal of <H> \ G. The default for i is R.degree. D is a diagonal amat (see AMats) of type "mon". Note that TransitiveToInductionMonRep sets and tests the field R.induction if i = R.degree.

    gap> G := GroupWithGenerators(DihedralGroup(8));
Group( (1,2,3,4), (2,4) )
gap> R := ARepByImages(G, [ Mon( [E(4), E(4)^-1] ), Mon( (1,2), 2 ) ]);
ARepByImages(
GroupWithGenerators( [ (1,2,3,4), (2,4) ] ),
[ Mon( [ E(4), -E(4) ] ), Mon( (1,2), 2 ) ],
"hom"
)
gap> TransitiveToInductionMonRep(R);
ConjugateARep(
InductionARep(
ARepByImages(
GroupWithGenerators( [ (1,2,3,4) ] ),
[ Mon( [ -E(4) ] ) ],
"hom"
),
GroupWithGenerators( [ (1,2,3,4), (2,4) ] ),
[ (2,4), () ]
),
IdentityMonAMat(2)
) 

## 74.112 InsertedInductionARep

InsertedInductionARep( R, H )

Let R be an arep of type "induction", i.e. <R> = L\uparrowT G where L is an arep of <U> ≤ G and <U> ≤ HG. InsertedInductionARep returns an arep equal to R with structure ( (L\uparrowT1H)\uparrowT2G)M where M is an amat (see AMats) with a structure similar to R. If R.rep is of degree 1 then M is an amat of type "mon".

    gap> G := GroupWithGenerators(SymmetricGroup(4)); G.name := "S4";
Group( (1,4), (2,4), (3,4) )
"S4"
gap> H := GroupWithGenerators(AlternatingGroup(4)); H.name := "A4";
Group( (1,2,4), (2,3,4) )
"A4"
gap> U := GroupWithGenerators(CyclicGroup(3)); U.name := "Z3";
Group( (1,2,3) )
"Z3"
gap> R := ARepByImages(U, [ [[E(3)]] ] );
ARepByImages(
Z3,
[ [ [ E(3) ] ]
],
"hom"
)
gap> InsertedInductionARep(InductionARep(R, G), H);
ConjugateARep(
InductionARep(
InductionARep(
ARepByImages(
Z3,
[ [ [ E(3) ] ] ],
"hom"
),
A4,
[ (), (2,3,4), (2,4,3), (1,4)(2,3) ]
),
S4,
[ (), (3,4) ]
),
AMatMon( Mon(
(2,4,8,7,3,5),
[ 1, 1, 1, 1, 1, 1, E(3)^2, 1 ]
) )
) 

## 74.113 ConjugationPermReps

ConjugationPermReps( R1, R2 )

Let R1 and R2 be permutation representations (i.e. IsPermRep( Ri ) = true, i = 1,2). ConjugationPermReps returns an amat A (see AMats) of type "perm" such that <R1>A = R_2. R1 and R2 must have common source and characteristic otherwise an error is signaled.

    gap> G := GroupWithGenerators( [ (1,2,3) ] );
Group( (1,2,3) )
gap> R1 := NaturalARep(G, 3);
NaturalARep( GroupWithGenerators( [ (1,2,3) ] ), 3 )
gap> R2 := ARepByImages(G, [ (1,3,2) ], 3);
ARepByImages(
GroupWithGenerators( [ (1,2,3) ] ),
[ (1,3,2)
],
3, # degree
"hom"
)
gap> A := ConjugationPermReps(R1, R2);
AMatPerm((2,3), 3)
gap> R1 ^ A = R2;
true 

## 74.114 ConjugationTransitiveMonReps

ConjugationTransitiveMonReps( R1, R2 )

Let R1 and R2 be transitive monomial representations. ConjugationTransitiveMonReps returns an amat A (see AMats) of type "mon" such that <R1>A = R_2 if possible and false otherwise. R1 and R2 must have common source otherwise an error is signaled.

Note that a conjugating monomial matrix exists iff R1 and R2 are induced from inner conjugated representations of degree one (see Pue98).

    gap> G := GroupWithGenerators( [ (1,2,3), (1,2) ] );
Group( (1,2,3), (1,2) )
gap> R1 := ARepByImages(G, [ Mon( [E(3), E(3)^2] ), Mon( (1,2), 2 ) ]);
ARepByImages(
GroupWithGenerators( [ (1,2,3), (1,2) ] ),
[ Mon( [ E(3), E(3)^2 ] ),
Mon( (1,2), 2 )
],
"hom"
)
gap> R2 := ARepByImages(G, [ Mon( [E(3)^2, E(3)] ), Mon( (1,2), 2 ) ]);
ARepByImages(
GroupWithGenerators( [ (1,2,3), (1,2) ] ),
[ Mon( [ E(3)^2, E(3) ] ),
Mon( (1,2), 2 )
],
"hom"
)
gap> ConjugationTransitiveMonReps(R1, R2);
AMatMon( Mon( (1,2), 2 ) ) 

## 74.115 TransversalChangeInductionARep

TransversalChangeInductionARep( R, T )
TransversalChangeInductionARep( R, T, hint )

Let R be an arep of type "induction", i.e. <R> = L\uparrowS G and T another transversal of L.source \ G. TransversalChangeInductionARep returns an arep equal to R with structure (L\uparrowT G)M where M is an amat (see AMats). M is of type "mon" if L is of degree 1 else M has a structure similar to R. The hint "isTransversal" suppresses checking T to be a right transversal.

    gap> G := GroupWithGenerators(SymmetricGroup(4)); G.name := "S4";
Group( (1,4), (2,4), (3,4) )
"S4"
gap> H := GroupWithGenerators(SymmetricGroup(3)); H.name := "S3";
Group( (1,3), (2,3) )
"S3"
gap> R := ARepByImages(H, [ [[-1]], [[-1]] ], "hom" );
ARepByImages(
S3,
[ [ [ -1 ] ], [ [ -1 ] ] ],
"hom"
)
gap> RG := InductionARep(R, G);
InductionARep(
ARepByImages(
S3,
[ [ [ -1 ] ], [ [ -1 ] ] ],
"hom"
),
S4,
[ (), (3,4), (2,4), (1,4) ]
)
gap> T := [(1,2,3,4), (2,3,4), (3,4), ()];;
gap> TransversalChangeInductionARep(RG, T);
ConjugateARep(
InductionARep(
ARepByImages(
S3,
[ [ [ -1 ] ], [ [ -1 ] ] ],
"hom"
),
S4,
[ (1,2,3,4), (2,3,4), (3,4), () ]
),
AMatMon( Mon( (1,4)(2,3), [ 1, 1, -1, 1 ] ) )
)
gap> last = RG;
true 

## 74.116 OuterTensorProductDecompositionMonRep

OuterTensorProductDecompositionMonRep( R )

Let R be a transitive monomial arep. OuterTensorProductDecompositionMonRep returns an arep equal to R with structure (R_1# ...#R_k)M. The Ri are areps of type "mon", M is an amat of type mon.

For a definition of the outer tensor product of representations see OuterTensorProductARep. For an explanation of the algorithm see Pue98.

    gap> G := GroupWithGenerators(SolvableGroup(48, 16));
2x4xS3
gap> R := RegularARep(G, 2);
RegularARep( 2x4xS3, GF(2) )
gap> OuterTensorProductDecompositionMonRep(R);
ConjugateARep(
OuterTensorProductARep(
2x4xS3,
ARepByImages(
GroupWithGenerators( [ c ] ),
[ Mon( (1,2), 2, GF(2) ) ],
"hom"
),
ARepByImages(
GroupWithGenerators( [ d, e ] ),
[ Mon( (1,3,2,4), 4, GF(2) ),
Mon( (1,2)(3,4), 4, GF(2) )
],
"hom"
),
ARepByImages(
GroupWithGenerators( [ a*e, b ] ),
[ Mon( (1,4)(2,6)(3,5), 6, GF(2) ),
Mon( (1,2,3)(4,5,6), 6, GF(2) )
],
"hom"
)
),
AMatMon( Mon( ( 2, 9,18,44,16,28,30,46,31, 6,42,48,47,39,23,35,37, 7)
( 3,17,36,45,24,43, 8,10,25, 5,34,29,38,15,19, 4,26,13)
(11,33,22,27,21,20,12,41,40,32,14), 48, GF(2) ) )
)
gap> last = R;
true 

## 74.117 InnerConjugationARep

InnerConjugationARep( R, G, t )

Let R be an arep with source <H> ≤ G and <t>∈ G. InnerConjugationARep returns an arep of type "perm" or "mon" or "mat", the most specific possible, representing the inner conjugate <R>t of R with t.

The inner conjugate <R>t is a representation of <H>t defined by x→ R(t. x. t-1).

    gap> G := GroupWithGenerators(SymmetricGroup(4));
Group( (1,4), (2,4), (3,4) )
gap> H := GroupWithGenerators(SymmetricGroup(3));
Group( (1,3), (2,3) )
gap> R := NaturalARep(H, 3);
NaturalARep( GroupWithGenerators( [ (1,3), (2,3) ] ), 3 )
gap> InnerConjugationARep(R, G, (1,2,3,4));
ARepByImages(
GroupWithGenerators( [ (2,4), (3,4) ] ),
[ (1,3), (2,3) ],
3, # degree
"hom"
) 

## 74.118 RestrictionInductionARep

RestrictionInductionARep( R, K )

Let R be an arep of type "induction", i.e. <R> = L\uparrowT G where L is an arep of <H> ≤ G of degree 1 and <K> ≤ G a subgroup. RestrictionInductionARep returns an arep equal to the restriction <R>\downarrow K with structure (i=1k ((Lsi\downarrow(HsiK))\uparrowTi K))M. S = {s1,..., sk} is a transversal of the double cosets H \ G/K, <L>si denotes the inner conjugate of R with si, and M is an amat (see AMats) of type "mon".

Note that this decomposition is based on a refined version of Mackey's subgroup theorem (see Pue98).

    gap> G := GroupWithGenerators(SymmetricGroup(4)); G.name := "S4";
Group( (1,4), (2,4), (3,4) )
"S4"
gap> H := GroupWithGenerators( [ (1,2) ] ); H.name := "Z2";
Group( (1,2) )
"Z2"
gap> K := GroupWithGenerators( [ (1,2,3) ] ); K.name := "Z3";
Group( (1,2,3) )
"Z3"
gap> L := ARepByImages(H, [ Mon( [-1] ) ] );
ARepByImages(
Z2,
[ Mon( [ -1 ] )
],
"hom"
)
gap> RestrictionInductionARep(InductionARep(L, G), K);
ConjugateARep(
DirectSumARep(
RegularARep( GroupWithGenerators( [ (1,2,3) ] ) ),
RegularARep( GroupWithGenerators( [ (1,2,3) ] ) ),
RegularARep( GroupWithGenerators( [ (1,2,3) ] ) ),
RegularARep( GroupWithGenerators( [ (1,2,3) ] ) )
),
AMatMon( Mon(
( 2,12, 4, 6, 9, 5, 8,10),
[ 1, 1, -1, -1, 1, 1, -1, -1, -1, -1, 1, 1 ]
) )
) 

## 74.119 kbsARep

kbsARep( R )

kbsARep returns the kbs (conjugated blockstructure) of the arep R. The kbs of a representation is a partition of the set {1,...,R.degree} representing the blockstructure of R. For an exact definition see kbs.

Note that for a monomial representation the kbs is exactly the list of orbits.

    gap> G := GroupWithGenerators( [ (1,2) ] );
Group( (1,2) )
gap> R := ARepByImages(G, [ (2,3) ], 4);
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ (2,3) ],
4, # degree
"hom"
)
gap> kbsARep(R);
[ [ 1 ], [ 2, 3 ], [ 4 ] ] 

## 74.120 RestrictionToSubmoduleARep

RestrictionToSubmoduleARep( R, list )
RestrictionToSubmoduleARep( R, list, hint )

Let R be an arep and list a subset of [1..R.degree]. RestrictionToSubmoduleARep returns an arep of type "perm" or "mon" or "mat", the most specific possible, representing the restriction of R to the submodule generated by the base vectors given through list. The optional hint "hom" avoids the check for homomorphism.

Note that the restriction to the submodule given by list defines a homomorphism iff list is a union of lists in the kbs of R (see kbsARep).

    gap> G := GroupWithGenerators( [ (1,2) ] );
Group( (1,2) )
gap> R := ARepByImages(G, [ (2,4) ], 4);
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ (2,4) ],
4, # degree
"hom"
)
gap> RestrictionToSubmoduleARep(R, [2,4]);
NaturalARep( GroupWithGenerators( [ (1,2) ] ), 2 ) 

## 74.121 kbsDecompositionARep

kbsDecompositionARep( R )

kbsDecompositionARep returns an arep equal to R with structure (R_1⊕...⊕R_k)P where P is an amat (see AMats) of type "perm"and all Ri have trivial kbs (see kbsARep).

Note that for a monomial arep kbsDecompositionARep performs exactly the same as the function OrbitDecompositionMonRep (see OrbitDecompositionMonRep).

    gap> G := GroupWithGenerators( [ (1,2) ] );
Group( (1,2) )
gap> R := ARepByImages(G,
> [ [[Z(2), Z(2), 0*Z(2), 0*Z(2)], [0*Z(2), Z(2), 0*Z(2), 0*Z(2)],
> [0*Z(2), 0*Z(2), Z(2), Z(2)], [0*Z(2), 0*Z(2), 0*Z(2), Z(2)]] ] );
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ [ [ Z(2)^0, Z(2)^0, 0*Z(2), 0*Z(2) ],
[ 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2) ],
[ 0*Z(2), 0*Z(2), Z(2)^0, Z(2)^0 ],
[ 0*Z(2), 0*Z(2), 0*Z(2), Z(2)^0 ] ]
],
"hom"
)
gap> kbsDecompositionARep(R);
ConjugateARep(
DirectSumARep(
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ [ [ Z(2)^0, Z(2)^0 ], [ 0*Z(2), Z(2)^0 ] ] ],
"hom"
),
ARepByImages(
GroupWithGenerators( [ (1,2) ] ),
[ [ [ Z(2)^0, Z(2)^0 ], [ 0*Z(2), Z(2)^0 ] ] ],
"hom"
)
),
IdentityPermAMat(4, GF(2))
) 

## 74.122 ExtensionOnedimensionalAbelianRep

ExtensionOnedimensionalAbelianRep( R, G )

Let R be an arep of the subgroup <H> ≤ G and let <G>/kernel(R) be an abelian factor group. ExtensionOnedimensionalAbelianRep returns an arep of type "mon" and degree 1 extending R to G. For the extension the smallest possible extension field is chosen.

    gap> G := GroupWithGenerators(CyclicGroup(8));
Group( (1,2,3,4,5,6,7,8) )
gap> H := GroupWithGenerators( [ G.1^2 ] );
Group( (1,3,5,7)(2,4,6,8) )
gap> R := ARepByImages(H, [ [[-1]] ] );
ARepByImages(
GroupWithGenerators( [ (1,3,5,7)(2,4,6,8) ] ),
[ [ [ -1 ] ]
],
"hom"
)
gap> ExtensionOnedimensionalAbelianRep(R, G);
ARepByImages(
GroupWithGenerators( [ (1,2,3,4,5,6,7,8) ] ),
[ Mon( [ E(4) ] )
],
"hom"
) 

## 74.123 DecompositionMonRep

DecompositionMonRep( R )
DecompositionMonRep( R, hint )

Let R be a monomial arep (i.e. IsMonRep( R ) yields true). DecompositionMonRep returns an arep equal to R with structure (R_1⊕...⊕ R_k)A-1 where all Ri are irreducible and A-1 is a highly structured amat (see AMats). A is a decomposition matrix for R and can be accessed in the field .conjugation.element of the result. The list of the Ri can be accessed in the field .rep.summands of the result. Note that any Ri is monomial if this is possible. If the hint "noOuter" is supplied, the decomposition of R is performed without any decomposition into an outer tensor product which may speed up the function. The function only works for characteristic zero otherwise an error is signaled. At least the following types of monomial areps can be decomposed: monomial representations of solvable groups, double transitive permutation representations, primitive permutation representations with solvable socle. If DecompositionMonRep is not able to decompose R then false is returned. The performance of DecompositionMonRep depends on the size of the group represented as well as on the degree of R. E.g. the decomposition of a regular representation of a group of size 96 takes less than half a minute (CPU-time on a SUN Ultra-Sparc 150\mathrmMHz) if the source group is an ag group.

Note that in the case that R is a regular representation of the solvable group G the structured decomposition matrix A computed by DecompositionMonRep represents a fast Fourier transform for G. Hence, DecompositionMonRep is able to compute a fast Fourier transform for any solvable group.

The algorithm is a major result of Pue98 where a thorough explanation can be found.

Set InfoLatticeDec := Print to obtain information on the recursive decomposition of R.

An important application of this function is the automatic generation of fast algorithms for discrete signal transforms which is realized in Matrix Decomposition. (see Min93, Egn97, Pue98).

    gap> G := GroupWithGenerators(SolvableGroup(8, 5));
Q8
gap> R := RegularARep(G);
RegularARep( Q8 )
gap> DecompositionMonRep(R);
ConjugateARep(
DirectSumARep(
TrivialMonARep( Q8 ),
ARepByImages(
Q8,
[ Mon( [ -1 ] ), Mon( [ -1 ] ), Mon( (), 1 ) ],
"hom"
),
ARepByImages(
Q8,
[ Mon( [ -1 ] ), Mon( (), 1 ), Mon( (), 1 ) ],
"hom"
),
ARepByImages(
Q8,
[ Mon( (), 1 ), Mon( [ -1 ] ), Mon( (), 1 ) ],
"hom"
),
ARepByImages(
Q8,
[ Mon( (1,2), [ -1, 1 ] ),
Mon( [ E(4), -E(4) ] ),
Mon( [ -1, -1 ] )
],
"hom"
),
ARepByImages(
Q8,
[ Mon( (1,2), [ -1, 1 ] ),
Mon( [ E(4), -E(4) ] ),
Mon( [ -1, -1 ] )
],
"hom"
)
),
( AMatPerm((7,8), 8) *
TensorProductAMat(
IdentityPermAMat(2),
AMatPerm((2,3), 4) *
TensorProductAMat(
DFTAMat(2),
IdentityPermAMat(2)
) *
DiagonalAMat([ 1, 1, 1, E(4) ]) *
TensorProductAMat(
IdentityPermAMat(2),
DFTAMat(2)
) *
AMatPerm((2,3), 4)
) *
AMatMon( Mon(
(2,5,3)(4,8,7),
[ 1, 1, 1, 1, 1, 1, -1, 1 ]
) ) *
DirectSumAMat(
TensorProductAMat(
DFTAMat(2),
IdentityPermAMat(2)
),
IdentityPermAMat(4)
) *
AMatPerm((2,4), 8)
) ^ -1
)
gap>  last = R;
true 

## 74.124 Symmetry of Matrices

The following sections describe functions for the computation of symmetry of a given matrix. A symmetry of a matrix is a pair (R1, R2) of representations of the same group G with the property R1(x). M = M. R2(x) for all x∈ G. This definition corresponds to the definition of the intertwining space of R1, R2 (see IntertwiningSpaceARep). The origin of this definition is due to Minkwitz (see Min95, Min93) and was generalized to the definition above by the authors of this package.

Restrictions on the representations R1, R2 yield special types of symmetry. We consider the following three types:

• Perm-Irred symmetry: R1 is a permutation representation, R2 is a conjugated (by a permutation) direct sum of irreducible representations

• Perm-Perm symmetry: both R1 and R2 are permutation representations

• Mon-Mon symmetry: both R1 and R2 are monomial representations

There are two implementations for the search algorithm for Perm-Perm-Symmetry. One is entirely in GAP3 by S. Egner, the other uses the external C-program desauto bei J. Leon which is distributed with the \sf GUAVA package. By default the GAP3 code is run. In order to use the much faster method of J. Leon based on partitions (see Leo91) you should set UseLeon := true and make sure that an executable version of desauto is placed in \\$GAP/pkg/arep/bin. The implementation of Leon requires the matrix to have ≤ 256 different entries. If this condition is violated the GAP3 implementation is run.

A matrix with symmetry of one of the types above contains structure in a sense and can be decomposed into a product of highly structured sparse matrices (see Matrix Decomposition).

For details on the concept and computation of symmetry see Egn97 and Pue98.

The following functions are implemented in the file "arep/lib/symmetry.g" based on functions from "arep/lib/permperm.g", "arep/lib/monmon.g", "arep/lib/permblk.g" and "arep/lib/permmat.g".

## 74.125 PermPermSymmetry

PermPermSymmetry( M )

Let M be a matrix or an amat (see AMats). PermPermSymmetry returns a pair (R1, R2) of areps of type "perm" (see AReps) of the same group G representing the perm-perm symmetry of M, i.e. R1(x). M = M. R2(x) for all x∈ G. The returned symmetry is maximal in the sense that for every pair (p1, p2) of permutations satisfying p1. M= M. p2 there is an x with p1 = R1(x) and p2 = R2(x).

To use the much faster implementation of J. Leon set UseLeon := true as explained in Symmetry of Matrices.

Set InfoPermSym1 := true to obtain information about the search.

For the algorithm see Leo91 resp. Egn97.

    gap> M := DFT(5);;
gap> PrintArray(M);
[ [       1,       1,       1,       1,       1 ],
[       1,    E(5),  E(5)^2,  E(5)^3,  E(5)^4 ],
[       1,  E(5)^2,  E(5)^4,    E(5),  E(5)^3 ],
[       1,  E(5)^3,    E(5),  E(5)^4,  E(5)^2 ],
[       1,  E(5)^4,  E(5)^3,  E(5)^2,    E(5) ] ]
gap> L := PermPermSymmetry(M);
[ ARepByImages(
GroupWithGenerators( [ g1, g2 ] ),
[ (2,3,5,4),
(2,5)(3,4)
],
5, # degree
"hom"
), ARepByImages(
GroupWithGenerators( [ g1, g2 ] ),
[ (2,4,5,3),
(2,5)(3,4)
],
5, # degree
"hom"
) ]
gap> L[1]^AMatMat(M) = L[2];
true 

## 74.126 MonMonSymmetry

MonMonSymmetry( M )

Let M be a matrix or an amat (see AMats) of characteristic zero. MonMonSymmetry returns a pair (R1, R2) of areps of type "mon" (see AReps) of the same group G representing a mon-mon symmetry of M, i.e. R1(x). M = M. R2(x) for all x∈ G.

The non-zero entries in the matrices R1(x), R2(x) are all roots of unity of a certain order d. This order is given by the lcm of all quotients of non-zero entries of M with equal absolute value. The returned symmetry is maximal in the sense that for every pair (m1, m2) of monomial matrices containing only dth roots of unity (and 0) and satisfying m1. M=M. m2 there is an x with m1 = R1(x) and m2 = R2(x).

MonMonSymmetry uses the function PermPermSymmetry. Hence you can accelerate the function using the faster implementation of J. Leon by setting UseLeon := true as explained in Symmetry of Matrices.

For an explanation of the algorithm see Pue98.

    gap> M := DFT(5);;
gap> PrintArray(M);
[ [       1,       1,       1,       1,       1 ],
[       1,    E(5),  E(5)^2,  E(5)^3,  E(5)^4 ],
[       1,  E(5)^2,  E(5)^4,    E(5),  E(5)^3 ],
[       1,  E(5)^3,    E(5),  E(5)^4,  E(5)^2 ],
[       1,  E(5)^4,  E(5)^3,  E(5)^2,    E(5) ] ]
gap> L := MonMonSymmetry(M);
[ ARepByImages(
GroupWithGenerators( [ g1, g2, g3, g4, g5 ] ),
[ Mon(
(2,3,5,4),
[ 1, E(5)^3, E(5), E(5)^4, E(5)^2 ]
),
Mon(
(2,5)(3,4),
[ 1, E(5)^2, E(5)^4, E(5), E(5)^3 ]
),
Mon(
(1,2,3,4,5),
[ E(5), E(5), E(5), E(5), E(5) ]
),
Mon( [ E(5), 1, E(5)^4, E(5)^3, E(5)^2 ] ),
Mon( [ 1, E(5), E(5)^2, E(5)^3, E(5)^4 ] )
],
"hom"
), ARepByImages(
GroupWithGenerators( [ g1, g2, g3, g4, g5 ] ),
[ Mon( (1,3,4,2), 5 ),
Mon( (1,4)(2,3), 5 ),
Mon( [ E(5), E(5)^2, E(5)^3, E(5)^4, 1 ] ),
Mon(
(1,2,3,4,5),
[ E(5), E(5), E(5), E(5), E(5) ]
),
Mon( (1,5,4,3,2), 5 )
],
"hom"
) ]
gap> L[1]^AMatMat(M) = L[2];
true 

## 74.127 PermIrredSymmetry

PermIrredSymmetry( M )
PermIrredSymmetry( M, maxblocksize )

Let M be a matrix or an amat (see AMats) of characteristic zero. PermIrredSymmetry returns a list of pairs (R1, R2) of areps (see AReps) of the same group G representing a perm-irred symmetry of M, i.e. R1(x). M = M. R2(x) for all x∈ G and R1 is a permutation representation and R2 a conjugated (by a permutation) direct sum of irreducible representations. If maxblocksize is supplied exactly those perm-irred symmetries are returned where R2 contains at least one irreducible of degree maxblocksize. The default for maxblocksize is 2.

Refer to Egn97 to understand how the search is done and how to interpret the result.

Note that the perm-irred symmetry is not symmetric. Hence it is possible that a matrix M admits a perm-irred symmetry but its transpose not.

The perm-irred symmetry is a special case of a perm-block symmetry. The perm-block symmetries admitted by a fixed matrix M can be described by two lattices which are in a certain way related to each other (semi-order preserving). To explore this structure (described in Egn97) you should refer to PermBlockSym and DisplayPermBlockSym in the file "arep/lib/permblk.g".

    gap> M := DFT(4);
[ [ 1, 1, 1, 1 ], [ 1, E(4), -1, -E(4) ], [ 1, -1, 1, -1 ],
[ 1, -E(4), -1, E(4) ] ]
gap> PermIrredSymmetry(M);
[ [ NaturalARep( G2, 4 ), ConjugateARep(
DirectSumARep(
TrivialMatARep( G2 ),
ARepByImages(
G2,
[ [ [ -1 ] ],
[ [ E(4) ] ]
],
"hom"
),
ARepByImages(
G2,
[ [ [ 1 ] ],
[ [ -1 ] ]
],
"hom"
),
ARepByImages(
G2,
[ [ [ -1 ] ],
[ [ -E(4) ] ]
],
"hom"
)
),
IdentityPermAMat(4)
) ], [ NaturalARep( G3, 4 ), ConjugateARep(
DirectSumARep(
TrivialMatARep( G3 ),
ARepByImages(
G3,
[ [ [ 0, -E(4) ], [ E(4), 0 ] ],
[ [ 0, 1 ], [ 1, 0 ] ],
[ [ 0, -1 ], [ -1, 0 ] ]
],
"hom"
),
ARepByImages(
G3,
[ [ [ -1 ] ],
[ [ 1 ] ],
[ [ 1 ] ]
],
"hom"
)
),
AMatPerm((3,4), 4)
) ], [ NaturalARep( G1, 4 ), ConjugateARep(
DirectSumARep(
TrivialMatARep( G1 ),
ARepByImages(
G1,
[ [ [ 1/2, -1/2+1/2*E(4), 1/2*E(4) ],
[ -1/2-1/2*E(4), 0, -1/2+1/2*E(4) ],
[ -1/2*E(4), -1/2-1/2*E(4), 1/2 ] ],
[ [ 0, 0, 1 ], [ 0, 1, 0 ], [ 1, 0, 0 ] ],
[ [ 1/2, 1/2+1/2*E(4), -1/2*E(4) ],
[ 1/2-1/2*E(4), 0, 1/2+1/2*E(4) ],
[ 1/2*E(4), 1/2-1/2*E(4), 1/2 ] ]
],
"hom"
)
),
IdentityPermAMat(4)
) ] ] 

## 74.128 Discrete Signal Transforms

The following sections describe functions for the construction of many well known signal transforms in matrix form, as e.g. the discrete Fourier transform, several discrete cosine transforms etc. For the definition of the mentioned signal transforms see ER82, Mal92, Mer96.

The functions for discrete signal transforms are implemented in "arep/lib/transf.g".

## 74.129 DiscreteFourierTransform

DiscreteFourierTransform( r )
DiscreteFourierTransform( n )
DiscreteFourierTransform( n, char )

shortcut: DFT

DiscreteFourierTransform or DFT returns the discrete Fourier transform from a given root of unity r or the size n and the characteristic char (see CB93). The default for char is zero. Note that the DFT on n points and characteristic char exists iff n and char are coprime. If this condition is violenced an error is signaled.

The DFTn of size n is defined as DFTn = [ωnkl| k,l∈{0,...,n-1}], ωn a primitive nth root of unity.

    gap> DFT(Z(3));
[ [ Z(3)^0, Z(3)^0 ], [ Z(3)^0, Z(3) ] ]
gap> DFT(4);
[ [ 1, 1, 1, 1 ], [ 1, E(4), -1, -E(4) ], [ 1, -1, 1, -1 ],
[ 1, -E(4), -1, E(4) ] ] 

## 74.130 InverseDiscreteFourierTransform

InverseDiscreteFourierTransform( r )
InverseDiscreteFourierTransform( n )
InverseDiscreteFourierTransform( n, char )

shortcut: InvDFT

InverseDiscreteFourierTransform or InvDFT returns the inverse of the discrete Fourier transform from a given root of unity r or the size n and the characteristic char (see DiscreteFourierTransform). The default for char is zero.

    gap> InvDFT(3);
[ [ 1/3, 1/3, 1/3 ], [ 1/3, 1/3*E(3)^2, 1/3*E(3) ],
[ 1/3, 1/3*E(3), 1/3*E(3)^2 ] ] 

## 74.131 DiscreteHartleyTransform

DiscreteHartleyTransform( n )

shortcut: DHT

DiscreteHartleyTransform or DHT returns the discrete Hartley transform on n points.

The DHTn of size n is defined by DHTn = [1/√n. (cos(2π kl/n) + sin(2π kl/n))| k,l∈{0,...,n-1}].

    gap> DHT(4);
[ [ 1/2, 1/2, 1/2, 1/2 ], [ 1/2, 1/2, -1/2, -1/2 ],
[ 1/2, -1/2, 1/2, -1/2 ], [ 1/2, -1/2, -1/2, 1/2 ] ] 

## 74.132 InverseDiscreteHartleyTransform

InverseDiscreteHartleyTransform( n )

shortcut: InvDHT

InverseDiscreteHartleyTransform or InvDHT returns the inverse of the discrete Hartley transform on n points. Since the DHT is self inverse the result is exactly the same as from DHT above.

    gap> InvDHT(4);
[ [ 1/2, 1/2, 1/2, 1/2 ], [ 1/2, 1/2, -1/2, -1/2 ],
[ 1/2, -1/2, 1/2, -1/2 ], [ 1/2, -1/2, -1/2, 1/2 ] ] 

## 74.133 DiscreteCosineTransform

DiscreteCosineTransform( n )

shortcut: DCT

DiscreteCosineTransform returns the standard cosine transform (type II) on n points.

The DCTn of size n is defined by DCTn = [√2/n. ck. (cos(k(l+1/2)π/n)| k,l∈{0,...,n-1}], ck = 1/√2 for k = 0 and ck = 1 else.

    gap> DCT(3);
[ [ 1/3*E(12)^7-1/3*E(12)^11, 1/3*E(12)^7-1/3*E(12)^11,
1/3*E(12)^7-1/3*E(12)^11 ],
[ -1/2*E(8)+1/2*E(8)^3, 0, 1/2*E(8)-1/2*E(8)^3 ],
[ -1/6*E(24)+1/6*E(24)^11+1/6*E(24)^17-1/6*E(24)^19,
1/3*E(24)-1/3*E(24)^11-1/3*E(24)^17+1/3*E(24)^19,
-1/6*E(24)+1/6*E(24)^11+1/6*E(24)^17-1/6*E(24)^19 ] ] 

## 74.134 InverseDiscreteCosineTransform

InverseDiscreteCosineTransform( n )

shortcut: InvDCT

InverseDiscreteCosineTransform returns the inverse of the standard cosine transform (type II) on n points. Since the DCT is orthogonal, the result is the transpose of the DCT, which is exactly the discrete cosine transform of type III.

    [ [ 1/3*E(12)^7-1/3*E(12)^11, -1/2*E(8)+1/2*E(8)^3,
-1/6*E(24)+1/6*E(24)^11+1/6*E(24)^17-1/6*E(24)^19 ],
[ 1/3*E(12)^7-1/3*E(12)^11, 0,
1/3*E(24)-1/3*E(24)^11-1/3*E(24)^17+1/3*E(24)^19 ],
[ 1/3*E(12)^7-1/3*E(12)^11, 1/2*E(8)-1/2*E(8)^3,
-1/6*E(24)+1/6*E(24)^11+1/6*E(24)^17-1/6*E(24)^19 ] ] 

## 74.135 DiscreteCosineTransformIV

DiscreteCosineTransformIV( n )

shortcut: DCT_IV

DiscreteCosineTransformIV returns the cosine transform of type IV on n points.

The DCT\_IVn of size n is defined by DCT\_IVn = [√2/n. (cos((k+1/2)(l+1/2)π/n)| k,l∈{0,...,n-1}].

    [ [ 1/2*E(12)^4+1/6*E(12)^7+1/2*E(12)^8-1/6*E(12)^11,
1/3*E(12)^7-1/3*E(12)^11,
1/2*E(12)^4-1/6*E(12)^7+1/2*E(12)^8+1/6*E(12)^11 ],
[ 1/3*E(12)^7-1/3*E(12)^11, -1/3*E(12)^7+1/3*E(12)^11,
-1/3*E(12)^7+1/3*E(12)^11 ],
[ 1/2*E(12)^4-1/6*E(12)^7+1/2*E(12)^8+1/6*E(12)^11,
-1/3*E(12)^7+1/3*E(12)^11,
1/2*E(12)^4+1/6*E(12)^7+1/2*E(12)^8-1/6*E(12)^11 ] ] 

## 74.136 InverseDiscreteCosineTransformIV

InverseDiscreteCosineTransformIV( n )

shortcut: InvDCT_IV

InverseDiscreteCosineTransformIV returns the inverse of the cosine transform of type IV on n points. Since the DCT\_IV is orthogonal, the result is the transpose of the DCT\_IV.

    [ [ 1/3*E(12)^7-1/3*E(12)^11, -1/2*E(8)+1/2*E(8)^3,
-1/6*E(24)+1/6*E(24)^11+1/6*E(24)^17-1/6*E(24)^19 ],
[ 1/3*E(12)^7-1/3*E(12)^11, 0,
1/3*E(24)-1/3*E(24)^11-1/3*E(24)^17+1/3*E(24)^19 ],
[ 1/3*E(12)^7-1/3*E(12)^11, 1/2*E(8)-1/2*E(8)^3,
-1/6*E(24)+1/6*E(24)^11+1/6*E(24)^17-1/6*E(24)^19 ] ] 

## 74.137 DiscreteCosineTransformI

DiscreteCosineTransformI( n )

shortcut: DCT_I

DiscreteCosineTransformI returns the cosine transform of type I on <n>+1 points.

The DCT\_In of size <n>+1 is defined by DCT\_In = [√2/n. ck. cl. (cos(klπ/n)| k,l∈{0,...,n}], ck = 1/√2 for k = 0 and ck = 1 else.

    [ [ 1/2, 1/2*E(8)-1/2*E(8)^3, 1/2 ],
[ 1/2*E(8)-1/2*E(8)^3, 0, -1/2*E(8)+1/2*E(8)^3 ],
[ 1/2, -1/2*E(8)+1/2*E(8)^3, 1/2 ] ] 

## 74.138 InverseDiscreteCosineTransformI

InverseDiscreteCosineTransformI( n )

shortcut: InvDCT_I

InverseDiscreteCosineTransformI returns the inverse of the cosine transform of type I on n points. Since the DCT\_I is orthogonal, the result is the transpose of the DCT\_I.

    [ [ 1/2, 1/2*E(8)-1/2*E(8)^3, 1/2 ],
[ 1/2*E(8)-1/2*E(8)^3, 0, -1/2*E(8)+1/2*E(8)^3 ],
[ 1/2, -1/2*E(8)+1/2*E(8)^3, 1/2 ] ] 

WalshHadamardTransform( n )

shortcut: WHT

WalshHadamardTransform returns the Walsh-Hadamard transform on n points.

Let <n> = ∏i=1k piνi be the prime factor decomposition of n. Then the WHTn is defined by WHTn = i=1k DFTpi⊗νi.

    gap> WHT(4);
[ [ 1, 1, 1, 1 ], [ 1, -1, 1, -1 ],
[ 1, 1, -1, -1 ], [ 1, -1, -1, 1 ] ] 

InverseWalshHadamardTransform( n )

shortcut: InvWHT

InverseWalshHadamardTransform returns the inverse of the Walsh-Hadamard transform on n points.

    gap> InvWHT(4);
[ [ 1/4, 1/4, 1/4, 1/4 ], [ 1/4, -1/4, 1/4, -1/4 ],
[ 1/4, 1/4, -1/4, -1/4 ], [ 1/4, -1/4, -1/4, 1/4 ] ] 

## 74.141 SlantTransform

SlantTransform( n )

shortcut: ST

SlantTransform returns the Slant transform on n points, which must be a power of 2, <n> = 2k

For a definition of the Slant transform see ER82, 10.9.

    gap> ST(4);
[ [ 1/2, 1/2, 1/2, 1/2 ],
[ 3/10*E(5)-3/10*E(5)^2-3/10*E(5)^3+3/10*E(5)^4,
1/10*E(5)-1/10*E(5)^2-1/10*E(5)^3+1/10*E(5)^4,
-1/10*E(5)+1/10*E(5)^2+1/10*E(5)^3-1/10*E(5)^4,
-3/10*E(5)+3/10*E(5)^2+3/10*E(5)^3-3/10*E(5)^4 ],
[ 1/2, -1/2, -1/2, 1/2 ],
[ 1/10*E(5)-1/10*E(5)^2-1/10*E(5)^3+1/10*E(5)^4,
-3/10*E(5)+3/10*E(5)^2+3/10*E(5)^3-3/10*E(5)^4,
3/10*E(5)-3/10*E(5)^2-3/10*E(5)^3+3/10*E(5)^4,
-1/10*E(5)+1/10*E(5)^2+1/10*E(5)^3-1/10*E(5)^4 ] ] 

## 74.142 InverseSlantTransform

InverseSlantTransform( n )

shortcut: InvST

InverseSlantTransform returns the inverse of the Slant transform on n points, which must be a power of 2, <n> = 2k. Since ST is orthogonal, this is exactly the transpose of the ST.

    gap> InvST(4);
[ [ 1/2, 3/10*E(5)-3/10*E(5)^2-3/10*E(5)^3+3/10*E(5)^4, 1/2,
1/10*E(5)-1/10*E(5)^2-1/10*E(5)^3+1/10*E(5)^4 ],
[ 1/2, 1/10*E(5)-1/10*E(5)^2-1/10*E(5)^3+1/10*E(5)^4, -1/2,
-3/10*E(5)+3/10*E(5)^2+3/10*E(5)^3-3/10*E(5)^4 ],
[ 1/2, -1/10*E(5)+1/10*E(5)^2+1/10*E(5)^3-1/10*E(5)^4, -1/2,
3/10*E(5)-3/10*E(5)^2-3/10*E(5)^3+3/10*E(5)^4 ],
[ 1/2, -3/10*E(5)+3/10*E(5)^2+3/10*E(5)^3-3/10*E(5)^4, 1/2,
-1/10*E(5)+1/10*E(5)^2+1/10*E(5)^3-1/10*E(5)^4 ] ] 

## 74.143 HaarTransform

HaarTransform( n )

shortcut: HT

HaarTransform returns the Haar transform on n points, which must be a power of 2, <n> = 2k.

For a definition of the Haar transform see ER82, 10.10.

    gap> HT(4);
[ [ 1/4, 1/4, 1/4, 1/4 ], [ 1/4, 1/4, -1/4, -1/4 ],
[ 1/4*E(8)-1/4*E(8)^3, -1/4*E(8)+1/4*E(8)^3, 0, 0 ],
[ 0, 0, 1/4*E(8)-1/4*E(8)^3, -1/4*E(8)+1/4*E(8)^3 ] ] 

## 74.144 InverseHaarTransform

InverseHaarTransform( n )

shortcut: InvHT

InverseHaarTransform returns the inverse of the Haar transform on n points, which must be a power of 2, <n> = 2k.

The inverse is exactly n times the transpose of HT.

    gap> InvHT(4);
[ [ 1, 1, E(8)-E(8)^3, 0 ], [ 1, 1, -E(8)+E(8)^3, 0 ],
[ 1, -1, 0, E(8)-E(8)^3 ], [ 1, -1, 0, -E(8)+E(8)^3 ] ] 

## 74.145 RationalizedHaarTransform

RationalizedHaarTransform( n )

shortcut: RHT

RationalizedHaarTransform returns the rationalized Haar transform on n points, which must be a power of 2, <n> = 2k.

For a definition of the rationalized Haar transform see ER82, 10.11.

    gap> RHT(4);
[ [ 1, 1, 1, 1 ], [ 1, 1, -1, -1 ],
[ 1, -1, 0, 0 ], [ 0, 0, 1, -1 ] ] 

## 74.146 InverseRationalizedHaarTransform

InverseRationalizedHaarTransform( n )

shortcut: InvRHT

InverseRationalizedHaarTransform returns the inverse of the rationalized Haar transform on n points, which must be a power of 2, <n> = 2k.

    gap> InvRHT(4);
[ [ 1/4, 1/4, 1/2, 0 ], [ 1/4, 1/4, -1/2, 0 ],
[ 1/4, -1/4, 0, 1/2 ], [ 1/4, -1/4, 0, -1/2 ] ] 

## 74.147 Matrix Decomposition

The decomposition of a matrix M with symmetry is a striking application of constructive representation theory and was the original motivation to create the package \AREP. Here, decomposition means that M is decomposed into a product of highly structured sparse matrices. Applied to matrices corresponding to discrete signal transforms such a decomposition may represent a fast algorithm for the signal transform.

For the definition of symmetry see Symmetry of Matrices.

The idea of decomposing a matrix with symmetry is due to Minkwitz Min95, Min93 and was further developed by the authors of this package. See Egn97, chapter 1 or Pue98, chapter 3 for a thorough explanation of the method.

The following three functions correspond to the three types of symmetry considered in this package (see Symmetry of Matrices). The functions are implemented in the file "arep/lib/algogen.g".

## 74.148 MatrixDecompositionByPermPermSymmetry

MatrixDecompositionByPermPermSymmetry( M )

Let M be a matrix or an amat (see AMats). MatrixDecompositionByPermPermSymmetry returns a highly structured amat of type "product" with all factors being sparse which represents the matrix M. The returned amat can be viewed as a fast algorithm for the multiplication with M.

The function uses the perm-perm symmetry (see PermPermSymmetry) to decompose the matrix (see Matrix Decomposition) and can hence be accelerated by setting UseLeon := true as described in Symmetry of Matrices.

The following examples show that MatrixDecompositionByPermPermSymmetry discovers automatically the method of Rader (see Rad68) for a discrete Fourier transform of prime degree as well as the well-known decomposition of circulant matrices.

    gap> M := DFT(5);;
gap> PrintArray(M);
[ [       1,       1,       1,       1,       1 ],
[       1,    E(5),  E(5)^2,  E(5)^3,  E(5)^4 ],
[       1,  E(5)^2,  E(5)^4,    E(5),  E(5)^3 ],
[       1,  E(5)^3,    E(5),  E(5)^4,  E(5)^2 ],
[       1,  E(5)^4,  E(5)^3,  E(5)^2,    E(5) ] ]
gap> MatrixDecompositionByPermPermSymmetry(M);
AMatPerm((4,5), 5) *
DirectSumAMat(
IdentityPermAMat(1),
TensorProductAMat(
DFTAMat(2),
IdentityPermAMat(2)
) *
DiagonalAMat([ 1, 1, 1, E(4) ]) *
TensorProductAMat(
IdentityPermAMat(2),
DFTAMat(2)
) *
AMatPerm((2,3), 4)
) *
AMatPerm((1,4,2,5,3), 5) *
DirectSumAMat(
DiagonalAMat([ E(20)^4-E(20)^13-E(20)^16+E(20)^17,
E(5)-E(5)^2-E(5)^3+E(5)^4, E(20)^4+E(20)^13-E(20)^16-E(20)^17 ]),
AMatMat(
[ [ 1, 4 ], [ 1, -1 ] ]
)
) *
AMatPerm((1,3,5,2,4), 5) *
DirectSumAMat(
IdentityPermAMat(1),
AMatPerm((2,3), 4) *
TensorProductAMat(
IdentityPermAMat(2),
DiagonalAMat([ 1/2, 1/2 ]) *
DFTAMat(2)
) *
DiagonalAMat([ 1, 1, 1, -E(4) ]) *
TensorProductAMat(
DiagonalAMat([ 1/2, 1/2 ]) *
DFTAMat(2),
IdentityPermAMat(2)
)
) *
AMatPerm((3,4,5), 5)

gap> M := [[1, 2, 3], [3, 1, 2], [2, 3, 1]];;
gap> PrintArray(M);
[ [  1,  2,  3 ],
[  3,  1,  2 ],
[  2,  3,  1 ] ]
gap> MatrixDecompositionByPermPermSymmetry(M);
DFTAMat(3) *
AMatMon( Mon(
(2,3),
[ 2, 2/3*E(3)+1/3*E(3)^2, 1/3*E(3)+2/3*E(3)^2 ]
) ) *
DFTAMat(3) 

## 74.149 MatrixDecompositionByMonMonSymmetry

MatrixDecompositionByMonMonSymmetry( M )

Let M be a matrix or an amat (see AMats). MatrixDecompositionByMonMonSymmetry returns a highly structured amat of type "product" with all factors being sparse which represents the matrix M. The returned amat can be viewed as a fast algorithm for the multiplication with M.

The function uses the mon-mon symmetry (see MonMonSymmetry) to decompose the matrix (see Matrix Decomposition) and can hence be accelerated by setting UseLeon := true as described in Symmetry of Matrices.

The following example show that MatrixDecompositionByMonMonSymmetry is able to find automatically a decomposition of the discrete cosine transform of type IV (see DiscreteCosineTransformIV).

    gap> M := DCT_IV(8);;
gap> MatrixDecompositionByMonMonSymmetry(M);
AMatMon( Mon(
(3,4,7,6,8,5),
[ E(4), E(16)^5, E(8)^3, -E(16)^7, 1, -E(16), E(8), -E(16)^3 ]
) ) *
TensorProductAMat(
DFTAMat(2),
IdentityPermAMat(4)
) *
DiagonalAMat([ 1, 1, 1, 1, 1, E(8), E(4), E(8)^3 ]) *
TensorProductAMat(
IdentityPermAMat(2),
DFTAMat(2),
IdentityPermAMat(2)
) *
DiagonalAMat([ 1, 1, 1, E(4), 1, 1, 1, E(4) ]) *
TensorProductAMat(
IdentityPermAMat(4),
DFTAMat(2)
) *
DiagonalAMat([ -E(64), -E(64), E(64)^9, -E(64)^9, E(64)^23, -E(64)^23,
E(64)^31, E(64)^31 ]) *
TensorProductAMat(
IdentityPermAMat(4),
DiagonalAMat([ 1/2, 1/2 ]) *
DFTAMat(2)
) *
DiagonalAMat([ 1, 1, 1, -E(4), 1, 1, 1, -E(4) ]) *
TensorProductAMat(
IdentityPermAMat(2),
DiagonalAMat([ 1/2, 1/2 ]) *
DFTAMat(2),
IdentityPermAMat(2)
) *
DiagonalAMat([ 1, 1, 1, 1, 1, -E(8)^3, -E(4), -E(8) ]) *
TensorProductAMat(
DiagonalAMat([ 1/2, 1/2 ]) *
DFTAMat(2),
IdentityPermAMat(4)
) *
AMatMon( Mon(
(2,6,3,4,7,5,8),
[ E(4), E(16)^5, -E(16)^7, E(8), E(8)^3, -E(16)^3, -E(16), 1 ]
) ) 

## 74.150 MatrixDecompositionByPermIrredSymmetry

MatrixDecompositionByPermIrredSymmetry( M )
MatrixDecompositionByPermIrredSymmetry( M, maxblocksize )

Let M be a matrix or an amat (see AMats). MatrixDecompositionByPermIrredSymmetry returns a highly structured amat of type "product" with all factors being sparse which represents the matrix M. The returned amat can be viewed as a fast algorithm for the multiplication with M.

The function uses the perm-irred symmetry (see PermIrredSymmetry) to decompose the matrix (see Matrix Decomposition).

If maxblocksize is supplied only those perm-irred symmetries with all irreducibles having degree less than maxblocksize are considered. The default for maxblocksize is 2.

Note that the perm-irred symmetry is not symmetric. Hence it is possible that a matrix M decomposes but its transpose not.

The following examples show that MatrixDecompositionByPermIrredSymmetry discovers automatically the Cooley-Tukey decomposition (see CT65) of a discrete Fourier transform as well as a decomposition of the transposed discrete cosine transform of type II (see DiscreteCosineTransform).

    gap> M := DFT(4);
[ [ 1, 1, 1, 1 ], [ 1, E(4), -1, -E(4) ], [ 1, -1, 1, -1 ],
[ 1, -E(4), -1, E(4) ] ]
gap> MatrixDecompositionByPermIrredSymmetry(M);
TensorProductAMat(
DFTAMat(2),
IdentityPermAMat(2)
) *
DiagonalAMat([ 1, 1, 1, E(4) ]) *
TensorProductAMat(
IdentityPermAMat(2),
DFTAMat(2)
) *
AMatPerm((2,3), 4)

gap> M := TransposedMat(DCT(8));;
gap> MatrixDecompositionByPermIrredSymmetry(M);
AMatPerm((1,2,6,7,5,3,8), 8) *
TensorProductAMat(
IdentityPermAMat(2),
AMatPerm((3,4), 4) *
TensorProductAMat(
IdentityPermAMat(2),
DFTAMat(2)
) *
AMatPerm((2,3), 4) *
DirectSumAMat(
DFTAMat(2),
IdentityPermAMat(2)
)
) *
AMatPerm((2,7,5,4,3)(6,8), 8) *
DirectSumAMat(
IdentityPermAMat(3),
DirectSumAMat(
IdentityPermAMat(1),
AMatMat(
[ [ -1/2*E(8)+1/2*E(8)^3, 1/2*E(8)-1/2*E(8)^3 ],
[  1/2*E(8)-1/2*E(8)^3, 1/2*E(8)-1/2*E(8)^3 ] ],
"invertible"
)
),
IdentityPermAMat(2)
) *
DirectSumAMat(
TensorProductAMat(
DFTAMat(2),
IdentityPermAMat(3)
),
IdentityPermAMat(2)
) *
AMatPerm((2,7,3,8,4), 8) *
DirectSumAMat(
DiagonalAMat([ 1/4*E(8)-1/4*E(8)^3, 1/4*E(8)-1/4*E(8)^3 ]),
AMatMat(
[ [ 1/4*E(16)-1/4*E(16)^7, 1/4*E(16)^3-1/4*E(16)^5 ],
[ 1/4*E(16)^3-1/4*E(16)^5, -1/4*E(16)+1/4*E(16)^7 ] ]
),
AMatMat(
[ [ -1/4*E(32)+1/4*E(32)^15, -1/4*E(32)^7+1/4*E(32)^9 ],
[ 1/4*E(32)^7-1/4*E(32)^9, -1/4*E(32)+1/4*E(32)^15 ] ]
),
AMatMat(
[ [ -1/4*E(32)^3+1/4*E(32)^13, -1/4*E(32)^5+1/4*E(32)^11 ],
[ -1/4*E(32)^5+1/4*E(32)^11, 1/4*E(32)^3-1/4*E(32)^13 ] ]
)
) *
AMatPerm((2,5)(4,7)(6,8), 8) 

## 74.151 Complex Numbers

The next sections describe basic functions for the calculation with complex numbers which are represented as cyclotomics, e.g. computation of the complex conjugate or certain sine and cosine expressions.

The following functions are implemented in the file "arep/lib/complex.g".

## 74.152 ImaginaryUnit

ImaginaryUnit( )

ImaginaryUnit returns E(4).

    gap> ImaginaryUnit();
E(4) 

## 74.153 Re

Re( z )

Re returns the real part of the cyclotomic z.

    gap> z := E(3) + E(4);
E(12)^4-E(12)^7-E(12)^11
gap> Re(z);
-1/2 

Re( list )

Re returns the list of the real parts of the cyclotomics in list.

## 74.154 Im

Im( z )

Im returns the imaginary part of the cyclotomic z.

    gap> z := E(3) + E(4);
E(12)^4-E(12)^7-E(12)^11
gap> Im(z);
-E(12)^4-1/2*E(12)^7-E(12)^8+1/2*E(12)^11 

Im( list )

Im returns the list of the imaginary parts of the cyclotomics in list.

## 74.155 AbsSqr

AbsSqr( z )

AbsSqr returns the squared absolute value of the cyclotomic z.

    gap> AbsSqr(z);
-2*E(12)^4-E(12)^7-2*E(12)^8+E(12)^11 

AbsSqr( list )

AbsSqr returns the list of the squared absolute values of the cyclotomics in list.

## 74.156 Sqrt

Sqrt( r )

Sqrt returns the square root of the rational number r.

    gap> Sqrt(1/3);
1/3*E(12)^7-1/3*E(12)^11 

## 74.157 ExpIPi

ExpIPi( r )

Let r be a rational number. ExpIPi returns eπ i r.

    gap> ExpIPi(1/5);
-E(5)^3 

## 74.158 CosPi

CosPi( r )

Let r be a rational number. CosPi( r ) returns cosr).

    gap> CosPi(1/5);
-1/2*E(5)^2-1/2*E(5)^3 

## 74.159 SinPi

SinPi( r )

Let r be a rational number. SinPi( r ) returns sinr).

    gap> SinPi(1/5);
-1/2*E(20)^13+1/2*E(20)^17 

## 74.160 TanPi

TanPi( r )

Let r be a rational number. TanPi( r ) returns tanr).

    gap> TanPi(1/5);
E(20)-E(20)^9+E(20)^13-E(20)^17 

## 74.161 Functions for Matrices and Permutations

The following sections describe basic functions for matrices and permutations, like forming the tensor product (Kronecker product) or direct sum and determination of the blockstructure of a matrix.

The following functions are implemented in the files "arep/lib/permblk.g" (kbs, see kbs), "arep/lib/summands.g" (DirectSummandsPermutedMat, see DirectSummandsPermutedMat) and the file "arep/lib/tools.g" (the other functions).

## 74.162 TensorProductMat

TensorProductMat( M1, ..., Mk )

TensorProductMat returns the tensor product of the matrices <M1>, ..., M_k.

    gap> TensorProductMat( [[1]], [[1,2], [3,4]], [[5,6], [7,8]] );
[ [ 5, 6, 10, 12 ], [ 7, 8, 14, 16 ],
[ 15, 18, 20, 24 ], [ 21, 24, 28, 32 ] ] 

TensorProductMat( list )

TensorProductMat returns the tensor product of the matrices in list.

## 74.163 MatPerm

MatPerm( p, d ) MatPerm( p, d, char )

MatPerm returns the permutation matrix of degree d corresponding to the permutation p in characteristic char. The default characteristic is 0. If d is less than the largest moved point of p an error is signaled.

We use the following convention to create a permutation

matrix from a permutation p with degree d:
ip j| i,j∈{1,...,d}].

    gap> MatPerm( (1,2,3), 4 );
[ [ 0, 1, 0, 0 ], [ 0, 0, 1, 0 ], [ 1, 0, 0, 0 ], [ 0, 0, 0, 1 ] ] 

## 74.164 PermMat

PermMat( M )

PermMat returns the permutation represented by the matrix M and returns false otherwise. For the convention see MatPerm.

    gap> PermMat( [[0,0,1], [1,0,0], [0,1,0]] );
(1,3,2) 

## 74.165 PermutedMat

PermutedMat( p1, M, p2 )

Let p1, p2 be permutations and M a matrix with r rows and c columns. PermutedMat returns MatPerm( p1, r ) . M.MatPerm( p2, c ) (see MatPerm). The largest moved point of p1 and p2 must not exceed r resp. c otherwise an error is signaled.

    gap> PermutedMat( (1,2), [[1,2,3], [4,5,6], [7,8,9]], (1,2,3) );
[ [ 6, 4, 5 ], [ 3, 1, 2 ], [ 9, 7, 8 ] ] 

## 74.166 DirectSummandsPermutedMat

DirectSummandsPermutedMat( M )
DirectSummandsPermutedMat( M, match-blocks )

Let M be a matrix. DirectSummandsPermutedMat returns the list [ p1, [ M1, ..., Mk ], p2 ] where p1, p2 are permutations and Mi, i = 1,...,k, are matrices with the property <M> = PermutedMat( p_1, DiagonalMat( M_1, ..., M_k ), p_2 ) (see PermutedMat, DiagonalMat). If match-blocks is true or not provided then the permutations p1 and p2 are chosen such that equivalent Mi are equal and occur next to each other. If match-blocks is false this is not done.

For an explanation of the algorithm see Egn97.

    gap> M := [ [ 0, 0, 0, 2, 0, 1], [ 3, 1, 0, 0, 0, 0],
> [ 0, 0, 1, 0, 2, 0], [ 1, 2, 0, 0, 0, 0],
> [ 0, 0, 0, 1, 0, 3], [ 0, 0, 3, 0, 1, 0] ];;
gap> PrintArray(M);
[ [  0,  0,  0,  2,  0,  1 ],
[  3,  1,  0,  0,  0,  0 ],
[  0,  0,  1,  0,  2,  0 ],
[  1,  2,  0,  0,  0,  0 ],
[  0,  0,  0,  1,  0,  3 ],
[  0,  0,  3,  0,  1,  0 ] ]
gap> DirectSummandsPermutedMat(M);
[ (2,4,3,5),
[ [ [ 2, 1 ], [ 1, 3 ] ],
[ [ 2, 1 ], [ 1, 3 ] ],
[ [ 2, 1 ], [ 1, 3 ] ] ],
(1,4)(2,6,3) ] 

## 74.167 kbs

kbs( M )

Let M be a square matrix of degree n. kbs (konjugierte Blockstruktur = conjugated block structure) returns the partition kbs(M) = {1,...,n}/R* where R is the reflexive, symmetric, transitive closure of the relation R defined by (i,j)∈ R⇔ M[i][j]\neq 0.

For an investigation of the kbs of a matrix see Egn97.

    gap> M := [[1,0,1,0], [0,2,0,3], [1,0,3,0], [0,4,0,1]];
[ [ 1, 0, 1, 0 ], [ 0, 2, 0, 3 ], [ 1, 0, 3, 0 ], [ 0, 4, 0, 1 ] ]
gap> PrintArray(M);
[ [  1,  0,  1,  0 ],
[  0,  2,  0,  3 ],
[  1,  0,  3,  0 ],
[  0,  4,  0,  1 ] ]
gap> kbs(M);
[ [ 1, 3 ], [ 2, 4 ] ] 

kbs( list )

kbs returns the joined kbs of the matrices in list. The matrices in list must have common size otherwise an error is signaled.

## 74.168 DirectSumPerm

DirectSumPerm( list1, list2 )

Let list2 contain permutations and list1 be of the same length and contain degrees equal or larger than the corresponding largest moved points. DirectSumPerm returns the direct sum of the permutations defined via the direct sum of the corresponding matrices.

    gap> DirectSumPerm( [3, 3], [(1,2), (1,2,3)] );
(1,2)(4,5,6) 

## 74.169 TensorProductPerm

TensorProductPerm( list1, list2 )

Let list2 contain permutations and list1 be of the same length and contain degrees equal or larger than the corresponding largest moved points. TensorProductPerm returns the tensor product (Kronecker product) of the permutations defined via the tensor product of the corresponding matrices.

    gap> TensorProductPerm( [3, 3], [(1,2), (1,2,3)] );
(1,5,3,4,2,6)(7,8,9) 

gap3-jm
19 Feb 2018