# Are \choose and \align incompatible?

I ran across a strange warning when trying to use the math command \choose inside the amsmath package's \align* environment:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
{n \choose k}
\end{align*}
\end{document}


Package amsmath Warning: Foreign command \atopwidthdelims; \frac or \genfrac should be used instead

The document still compiles and produces perfectly fine output. But what is causing this warning? And is there a way to get rid of it?

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It seems amsmath doesn't recognize her own grandfather: \choose is short for \atopwithdelims(), and \atopwithdelims is a TeX primitive. So in that light the word ‘foreign‘ seems quite strange; after all, amsmath, like any other TeX macro package, is built on TeX primitives. – morbusg Mar 23 '12 at 6:33
@morbusg "Foreign" refers to the syntax of \choose that doesn't fit with the standard syntax of LaTeX commands. – egreg Mar 23 '12 at 9:23
@egreg: if it means that, it'd be nice if it read like that, but as it stands, it reads (emphasis mine): “Warning: Foreign command \atopwithdelims…”. – morbusg Mar 23 '12 at 13:15
@morbusg Unfortunately it's not \choose that triggers the warning, but only \atopwithdelims. All the part dealing with this warning should be rewritten if it wanted to cope with a variety of foreign commands. – egreg Mar 23 '12 at 13:32
@egreg: you just said “foreign commands” on purpose, didn't you? :-D even though you just said it in perfect form in the previous comment: “doesn't fit with the standard syntax of LaTeX commands”. – morbusg Mar 23 '12 at 16:01

Do not use {n \choose k} in LaTeX (it is a plain TeX command). The recommended way in LaTeX is \usepackage{amsmath} in the preamble and then \binom{n}{k}. (You have then also \tbinom and \dbinom for text and display styles also.)
Simple answer: Don't use \choose, or don't use amsmath, or live with the warnings.