Q: punctuation mark should be placed after the end of math mode When writing there are $n!$ possible permutations of $n$ objects, lacheck complains that "punctuation mark should be placed after the end of math mode".
Why is this, and how should I do this properly?

A: If you want to get rid of this warning for the factorial sign, then you can hide it via a macro or by adding a space or a pair of curly braces:
\documentclass{article}
\newcommand*{\fac}{!}
\begin{document}
there are $n\fac$ or $n! $ or $n!{}$ possible permutations of $n$ objects
\end{document}

Then lacheck does not complain.
