Function: qfrep
Section: linear_algebra
C-Name: qfrep0
Prototype: GGD0,L,
Help: qfrep(q,B,{flag=0}): vector of (half) the number of vectors of norms
 from 1 to B for the integral and definite quadratic form q. If flag is 1,
 count vectors of even norm from 1 to 2B.
Doc:
 $q$ being a square and symmetric matrix with integer entries representing a
 positive definite quadratic form, count the vectors representing successive
 integers.

 \item If $\fl = 0$, count all vectors. Outputs the vector whose $i$-th
 entry, $1 \leq i \leq B$ is half the number of vectors $v$ such that $q(v)=i$.

 \item If $\fl = 1$, count vectors of even norm. Outputs the vector
 whose $i$-th entry, $1 \leq i \leq B$ is half the number of vectors such
 that $q(v) = 2i$.

 \bprog
 ? q = [2, 1; 1, 3];
 ? qfrep(q, 5)
 %2 = Vecsmall([0, 1, 2, 0, 0]) \\ 1 vector of norm 2, 2 of norm 3, etc.
 ? qfrep(q, 5, 1)
 %3 = Vecsmall([1, 0, 0, 1, 0]) \\ 1 vector of norm 2, 0 of norm 4, etc.
 @eprog\noindent
 This routine uses a naive algorithm based on \tet{qfminim}, and
 will fail if any entry becomes larger than $2^{31}$ (or $2^{63}$).
