Renormalization of exponential matrix cocycles:
   To study the Kronig-Penney electron in a constant electric field, Buslaev proposed
   a heuristic method to reduce the study of
   
   for
   
   to the study of the same equation for the matrix
   
   where 
   We propose a renormalization scheme for such cocycles. 
   As for exponential sums, in the case of quadratic exponentials i.e. for matrices of
   the form
   
   the renormalization is exact.