106 Cyclotomic polynomials

Cyclotomic numbers, and cyclotomic polynomials over the rationals or some cyclotomic field, play an important role in the study of reductive groups, so they do in CHEVIE. Special facilities are provided to deal with them. The most prominent is the type CycPol which represents the product of a polynomial with a rational fraction in one variable with all poles or zeroes equal to 0 or roots of unity.

The advantages of representing as CycPol objects which can be so represented are: nice display (factorized), less storage, faster multiplication, division and evaluation. The big drawback is that addition and subtraction are not implemented!

    gap> q:=X(Cyclotomics);;q.name:="q";;
    gap> p:=CycPol(q^18 + q^16 + 2*q^12 + q^8 + q^6);
    (1+q^2-q^4+q^6+q^8)q^6P8
    gap> p/CycPol(q^2+q+1);
    (1+q^2-q^4+q^6+q^8)q^6P3^-1P8

The variable in a CycPol will be denoted by q. It is usually printed as q but it is possible to change its name, see Format in Functions for CycPols.

CycPols are represented internally by a record with fields:

.coeff:
a coefficient, usually a cyclotomic number, but it can also be a polynomial and actually can be any GAP3 object which can be multiplied by cyclotomic polynomials.

.valuation:
the valuation, positive or negative.

.vcyc:
a list of pairs [ei,mi] representing a root of unity and a multiplicity mi. Actually ei should be a fraction p/d with p< d representing E(d)p. The pair represents (q-E(d)p)mi.

So if we let mu(e):=e->E(Denominator(e))^Numerator(e), a record r represents

r.coeff*q^r.valuation*Product(r.vcyc,p->(q-mu(p[1]))^p[2]).

Subsections

  1. AsRootOfUnity
  2. CycPol
  3. IsCycPol
  4. Functions for CycPols

106.1 AsRootOfUnity

AsRootOfUnity( c )

c should be a cyclotomic number. AsRootOfUnity returns the rational e/n with 0 ≤ e<n (that is, e/n∈ℚ/ℤ) if c=E(n)^e, and false if c is not a root of unity. The code for this function has been provided by Thomas Breuer; we thank him for his help.

    gap> AsRootOfUnity(-E(9)^2-E(9)^5);
    8/9
    gap> AsRootOfUnity(-E(9)^4-E(9)^5);
    false
    gap> AsRootOfUnity(1);
    0

106.2 CycPol

CycPol( p )

In the first form CycPol( p ) the argument is a polynomial:

    gap> CycPol(3*q^3-3);
    3P1P3

Special code makes the conversion fast if p has not more than two nonzero coefficients.

The second form is a fast and efficient way of specifying a CycPol with only positive multiplicities: p should be a vector. The first element is taken as a the .coeff of the CycPol, the second as the .valuation. Subsequent elements are rationals i/d (with i< d) representing (q-E(d)^i) or are integers d representing Φd(q).

    gap> CycPol([3,-5,6,3/7]);
    3q^-5P6(q-E7^3)

106.3 IsCycPol

IsCycPol( p )

This function returns true if p is a CycPol and false otherwise.

    gap> IsCycPol(CycPol(1));
    true
    gap> IsCycPol(1);
    false

106.4 Functions for CycPols

Multiplication * division / and exponentiation ^ work as usual, and the functions Degree, Valuation and Value work as for polynomials:

    gap> p:=CycPol(q^18 + q^16 + 2*q^12 + q^8 + q^6);
    (1+q^2-q^4+q^6+q^8)q^6P8
    gap> Value(p,q);
    q^18 + q^16 + 2*q^12 + q^8 + q^6
    gap> p:=p/CycPol(q^2+q+1);
    (1+q^2-q^4+q^6+q^8)q^6P3^-1P8
    gap> Value(p,q);
    Error, Cannot evaluate the non-Laurent polynomial CycPol (1+q^2-q^4+q^\ 
    6+q^8)q^6P3^-1P8 in
    f.operations.Value( f, x ) called from
    Value( p, q ) called from
    main loop
    brk>
    gap> Degree(p);
    16
    gap> Value(p,3);
    431537382/13

The function ComplexConjugate conjugates .coeff as well as all the roots of unity making up the CycPol.

Functions String and Print display the d-th cyclotomic polynomial Φd over the rationals as Pd. They also display as P'd, P"d, P"'d, P""d factors of cyclotomic polynomials over extensions of the rationals:

    gap> List(SchurElements(Hecke(ComplexReflectionGroup(4),q)),CycPol);
    [ P2^2P3P4P6, 2ER(-3)q^-4P2^2P'3P'6, -2ER(-3)q^-4P2^2P"3P"6,
      2q^-4P3P4, (3-ER(-3))/2q^-1P2^2P'3P"6, (3+ER(-3))/2q^-1P2^2P"3P'6,
      q^-2P2^2P4 ]

If Φd factors in only two pieces, the one which has root E(d) is denoted P'd and the other one P"d . The list of commonly occuring factors is as follows (note that the conventions in Car85, pages 489--490 are different):

    P'3=q-E(3)
    P"3=q-E(3)^2
    P'4=q-E(4)
    P"4=q+E(4)
    P'5=q^2+(1-ER(5))/2*q+1
    P"5=q^2+(1+ER(5))/2*q+1
    P'6=q+E(3)^2
    P"6=q+E(3)
    P'7=q^3+(1-ER(-7))/2*q^2+(-1-ER(-7))/2*q-1
    P"7=q^3+(1+ER(-7))/2*q^2+(-1+ER(-7))/2*q-1
    P'8=q^2-E(4)
    P"8=q^2+E(4)
    P"'8=q^2-ER(2)*q+1
    P""8=q^2+ER(2)*q+1
    P""'8=q^2-ER(-2)*q-1
    P"""8=q^2+ER(-2)*q-1
    P'9=q^3-E(3)
    P"9=q^3-E(3)^2
    P'10=q^2+(-1-ER(5))/2*q+1
    P"10=q^2+(-1+ER(5))/2*q+1
    P'11=q^5+(1-ER(-11))/2*q^4-q^3+q^2+(-1-ER(-11))/2*q-1
    P"11=q^5+(1+ER(-11))/2*q^4-q^3+q^2+(-1+ER(-11))/2*q-1
    P'12=q^2-E(4)*q-1
    P"12=q^2+E(4)*q-1
    P"'12=q^2+E(3)^2
    P""12=q^2+E(3)
    P""'12=q^2-ER(3)*q+1
    P"""12=q^2+ER(3)*q+1
    P(7)12=q+E(12)^7
    P(8)12=q+E(12)^11
    P(9)12=q+E(12)
    P(10)12=q+E(12)^5
    P'13=q^6+(1-ER(13))/2*q^5+2*q^4+(-1-ER(13))/2*q^3+2*q^2+(1-ER(13))/2*q+1
    P"13=q^6+(1+ER(13))/2*q^5+2*q^4+(-1+ER(13))/2*q^3+2*q^2+(1+ER(13))/2*q+1
    P'14=q^3+(-1+ER(-7))/2*q^2+(-1-ER(-7))/2*q+1
    P"14=q^3+(-1-ER(-7))/2*q^2+(-1+ER(-7))/2*q+1
    P'15=q^4+(-1-ER(5))/2*q^3+(1+ER(5))/2*q^2+(-1-ER(5))/2*q+1
    P"15=q^4+(-1+ER(5))/2*q^3+(1-ER(5))/2*q^2+(-1+ER(5))/2*q+1
    P"'15=q^4+E(3)^2*q^3+E(3)*q^2+q+E(3)^2
    P""15=q^4+E(3)*q^3+E(3)^2*q^2+q+E(3)
    P""'15=q^2+((1+ER(5))*E(3)^2)/2*q+E(3)
    P"""15=q^2+((1-ER(5))*E(3)^2)/2*q+E(3)
    P(7)15=q^2+((1+ER(5))*E(3))/2*q+E(3)^2
    P(8)15=q^2+((1-ER(5))*E(3))/2*q+E(3)^2
    P'16=q^4-ER(2)*q^2+1
    P"16=q^4+ER(2)*q^2+1
    P'18=q^3+E(3)^2
    P"18=q^3+E(3)
    P'20=q^4+(-1-ER(5))/2*q^2+1
    P"20=q^4+(-1+ER(5))/2*q^2+1
    P"'20=q^4+E(4)*q^3-q^2-E(4)*q+1
    P""20=q^4-E(4)*q^3-q^2+E(4)*q+1
    P'21=q^6+E(3)*q^5+E(3)^2*q^4+q^3+E(3)*q^2+E(3)^2*q+1
    P"21=q^6+E(3)^2*q^5+E(3)*q^4+q^3+E(3)^2*q^2+E(3)*q+1
    P'22=q^5+(-1-ER(-11))/2*q^4-q^3-q^2+(-1+ER(-11))/2*q+1
    P"22=q^5+(-1+ER(-11))/2*q^4-q^3-q^2+(-1-ER(-11))/2*q+1
    P'24=q^4+E(3)^2
    P"24=q^4+E(3)
    P"'24=q^4-ER(2)*q^3+q^2-ER(2)*q+1
    P""24=q^4+ER(2)*q^3+q^2+ER(2)*q+1
    P""'24=q^4-ER(6)*q^3+3*q^2-ER(6)*q+1
    P"""24=q^4+ER(6)*q^3+3*q^2+ER(6)*q+1
    P(7)24=q^4+ER(-2)*q^3-q^2-ER(-2)*q+1
    P(8)24=q^4-ER(-2)*q^3-q^2+ER(-2)*q+1
    P(9)24=q^2+ER(-2)*E(3)^2*q-E(3)
    P(10)24=q^2-ER(-2)*E(3)^2*q-E(3)
    P(11)24=q^2+ER(-2)*E(3)*q-E(3)^2
    P(12)24=q^2-ER(-2)*E(3)*q-E(3)^2
    P'25=q^10+(1-ER(5))/2*q^5+1
    P"25=q^10+(1+ER(5))/2*q^5+1
    P'26=q^6+(-1-ER(13))/2*q^5+2*q^4+(1-ER(13))/2*q^3+2*q^2+(-1-ER(13))/2*q+1
    P"26=q^6+(-1+ER(13))/2*q^5+2*q^4+(1+ER(13))/2*q^3+2*q^2+(-1+ER(13))/2*q+1
    P'27=q^9-E(3)
    P"27=q^9-E(3)^2
    P'30=q^4+(1-ER(5))/2*q^3+(1-ER(5))/2*q^2+(1-ER(5))/2*q+1
    P"30=q^4+(1+ER(5))/2*q^3+(1+ER(5))/2*q^2+(1+ER(5))/2*q+1
    P"'30=q^4-E(3)*q^3+E(3)^2*q^2-q+E(3)
    P""30=q^4-E(3)^2*q^3+E(3)*q^2-q+E(3)^2
    P""'30=q^2+((-1+ER(5))*E(3)^2)/2*q+E(3)
    P"""30=q^2+((-1-ER(5))*E(3)^2)/2*q+E(3)
    P(7)30=q^2+((-1+ER(5))*E(3))/2*q+E(3)^2
    P(8)30=q^2+((-1-ER(5))*E(3))/2*q+E(3)^2
    P'42=q^6-E(3)^2*q^5+E(3)*q^4-q^3+E(3)^2*q^2-E(3)*q+1
    P"42=q^6-E(3)*q^5+E(3)^2*q^4-q^3+E(3)*q^2-E(3)^2*q+1

Finally the function Format(c,options) takes the options:

.vname:
a string, the name to use for printing the variable of the CycPol instead of q.

.expand:
if set to true, each cyclotomic polynomial is replaced by its value before being printed.

    gap> p:=CycPol(q^6-1);
    P1P2P3P6
    gap> Format(p,rec(expand:=true));
    "(q-1)(q+1)(q^2+q+1)(q^2-q+1)"
    gap> Format(p,rec(expand:=true,vname:="x"));
    "(x-1)(x+1)(x^2+x+1)(x^2-x+1)"

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gap3-jm
18 Jun 2018