Function: mfmatatkin
Section: modular_forms
C-Name: mfmatatkin
Prototype: GLD&
Help: mfmatatkin(mf,Q,{&A}): returns a matrix M with rational coefficients
 attached to the Atkin-Lehner operator W_Q on the basis formed by mfbasis(mf).
 The result is M / sqrt(A).
Doc:  matrix of the Atkin-Lehner operator $W_Q$ on the basis formed by
 \kbd{mfbasis(mf)}. Implemented only for a split newspace
 and real characters defined modulo $N/Q$.

 The function returns a matrix $M$ with rational entries which is
 a scalar multiple of the true Arkin-Lehner matrix: the latter is
 $M / \sqrt{A}$. The integer $A$ is a positive or negative divisor $A$ of
 $N$, always $1$ in even weight.
 \bprog
 ? mf=mfsplit([32,4]); mfmatatkin(mf, 32, &A)
 %1 =
 [5/16 11/2  55/8]

 [ 1/8    0  -5/4]

 [1/32 -1/4 11/16]

 ? A
 %2 = 1
 @eprog
