"TeX capacity exceeded" because of a couple of raiseboxes in a \section?

Question

In a package, I have the following command defined:

\newcommand\fkm{
    \ensuremath{\mathrm{
        \raisebox{-0.1ex}{\rotatebox{22}{f}}\!
        \raisebox{0.2ex}{\rotatebox{-15} {k}}\!
        m\!^{*}
    }}
}

It works just as I'd expect in text, but if I say \section{\fkm} I get an error saying

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

Why doesn't this work within sections? What can I do to make it work?

Answer 1

The argument of \section could be used for the table of contents. In that process, commands within this argument may be expanded and written to the toc file for later use. To prevent expansion, you could use the \protect command:

\section{\protect\fkm}

Or, use \DeclareRobustCommand to define the command such that it's working in such moving arguments. You might also consider to use the optional argument of \section for the toc entry. See: Fragile and Robust commands.

Other commands with such moving arguments are \chapter, \subsection etc. and \caption.

Answer 2

This is not the error that I get when I try it. However, the solution is to use \DeclareRobustCommand instead of \newcommand.