TeX capacity exceeded, when flexisym scrreport, hyperref, and section heading with math all used together

Question

I'm getting the error:

TeX capacity exceeded, sorry [input stack size=5000]

Under the following very specific combination of packages and document class:

\documentclass{scrreprt}
\usepackage{hyperref}
\usepackage{flexisym}
\begin{document}
\chapter{When $\ne$.}
\end{document}

Removing either the hyperref, or flexisym package references, or changing the document class to article from scrreport, or removing the math content from the \chapter (or \section or \subsection) heading, appears to eliminate the issue.

This looks pretty similar to "TeX capacity exceeded" because of a couple of raiseboxes in a \section?

but changing the chapter reference to:

\chapter{When \protect$\ne$.}

doesn't seem to help. Is this a bug in one of the packages above? If not, what's the proper way to deal with this?

Answer 1

This is because of the toc

The \chapter command has the syntax

\chapter[optional argument for the toc]{name of the chapter}

If you don't use the <optional argument for the toc> then LaTeX will try and use name of the chapter, which causes issues when you have mathematical content (as in your example) because the hyperref package can not make mathematical symbols into hyperlinks.

You can fix it by using (something like)

\documentclass{scrreprt}
\usepackage{flexisym}
\usepackage{hyperref}

\begin{document}
\chapter[for the toc]{When $\ne$.}
\end{document}

Note that hyperref should be loaded after most packages (exceptions are discussed in Which packages should be loaded after hyperref instead of before?)

As mentioned in the comments, and in Equations in section heading/title you can use \texorpdfstring{<TeX>}{<PDF>}