Why does amsmath print a warning about a foreign command?

Question

I was wondering what was the cause of the following warning message with the specific \usepackage used below:

Using the following:

Preamble

\documentclass[amssymb,twocolumn,aps]{revtex4}
 \usepackage{times,amsmath,latexsym}

Warning message

Package amsmath Warning: Foreign command \atopwithdelims;
(amsmath)                \frac or \genfrac should be used instead
(amsmath)                 on input line 56.

Line 56

\[ \frac{gcd(m,n)}{n} {n\choose m} \] is an integer for all pairs of integers $n\geq m\geq 1$.

Answer 1

\atopwithdelims is a TeX math primitive command. If you use amsmath, you don't need \atopwithdelims or \atop, use amsmath macros such as \frac, \binom, \genfrac etc. instead.

If a class or package uses this command, or a command such as \choose which uses it, you would get this warning. I would ignore the warning then, otherwise I would have to redefine the concerned macros of the package or class.

Alternatively, I suggest that you use \binom{a}{n}.