Function: sumdiv
Section: sums
C-Name: divsum
Prototype: GVE
Help: sumdiv(n,X,expr): sum of expression expr, X running over the divisors
 of n.
Doc: sum of expression \var{expr} over the positive divisors of $n$.
 This function is a trivial wrapper essentially equivalent to
 \bprog
   D = divisors(n);
   for (i = 1, #D, X = D[i]; eval(expr))
 @eprog\noindent (except that \kbd{X} is lexically scoped to the \kbd{sumdiv}
 loop).
 Arithmetic functions like \tet{sigma} use the multiplicativity of the
 underlying expression to speed up the computation. Since there is no way to
 indicate that \var{expr} is multiplicative in $n$, specialized functions
 should always be preferred.
 %\syn{NO}
