Function: sigma
Section: number_theoretical
C-Name: sumdivk
Prototype: GD1,L,
Help: sigma(x,{k=1}): sum of the k-th powers of the divisors of x. k is
 optional and if omitted is assumed to be equal to 1.
Description:
 (gen, ?1):int           sumdiv($1)
 (gen, 0):int            numdiv($1)
Doc: sum of the $k^{\text{th}}$ powers of the positive divisors of $|x|$. $x$
 and $k$ must be of type integer.
Variant: Also available is \fun{GEN}{sumdiv}{GEN n}, for $k = 1$.
