Automatically typeset math in section headings in bold-face

Question

This question is motivated by a question asked me by a friend who is working on his thesis in mathematics. His section headings occasionally contain math, which:

• he would like to be in boldface to match the rest of the section title; but
• he would like to be medium-weight when it occurs in the table of contents, as with the rest of the section title.

He was using \mathbf to make the math boldface in his section headings; this of course also made them bold in the table of contents. I gave him a simple solution which inserted code at the beginning of the table of contents, which would make \mathbf pass its arguments unchanged. Of course, \mathbf also makes its arguments upright rather than math-italics, which I would also consider less than desirable; and it has no effect on e.g. greek characters, for which you would have to use something like the \bm command of bm.sty instead. (You could play the same trick with \bm as with \mathbf in the table of contents, but having to repeatedly use different math macros for all math in section headings to me seems inelegant.)

What would be the most elegant way to automatically render all math (including non-latin characters) in bold-face for section headings, but the normal medium-weight everywhere else (including the entries of the table of contents)?

Answer 1

The simplest thing is to patch \@startsection and \@chapter (assuming book or report class):

\usepackage{etoolbox}
\makeatletter
% \tracingpatches
\patchcmd{\@sect}{#8}{\boldmath #8}{}{}
\let\ori@chapter\@chapter
\def\@chapter[#1]#2{\ori@chapter[\boldmath#1]{\boldmath#2}}
\makeatother

#8 is the argument to \@sect that contains the title; but \boldmath will not leak to the table of contents nor to the headers (for headers it's sufficient to use fancyhdr and issue \boldmath, if bold headers are desired).

It doesn't work with \paragraph and \subparagraph, nor with \chapter*. It shouldn't be a problem to issue \boldmath in the argument of an occasional \chapter*.

Answer 2

Here's a solution that I've found to work very well -- add the following to your document preamble

\makeatletter
\g@addto@macro\bfseries{\boldmath}
\makeatother

(or just add the middle line to a .sty/.cls file, where the at-symbol escaping isn't needed.) This just modifies that \bfseries macro to include \boldmath. You also need to make sure that your font family contains bold math symbols, of course: not all do.

I'm not really sure why this isn't done by default... it seems an obviously desirable behaviour. Credit where it's due: I think this was supplied to me some years ago by Donald Arseneau on comp.text.tex.

(PS. Before finding out about this "trick" I wrote the hacky maybemath package, which is a far less elegant way to do this sort of thing but does also allow context-sensitive adaptation of math to sans-serif/roman fonts, and italic/upright styles. The latter may seem a bit strange given that math mode is already italicised by default, but is useful for when the math mode is only being used for access to sub/superscripting, in particular in the hepparticles package (see the testhepparticles.pdf example. A more elegant way to do this would be greatly appreciated!)

Answer 3

Well, it is possible to do this automatically, but here is a simpler "semi-automatic" solution. Note that \section has two arguments: the mandatory one sets section title, and the optional one (if present) sets the table of contents entry. Here we use \boldmath in the mandatory argument, but not in the optional one. Note also that bold math is not upright, as you want:

\documentclass{article}
\begin{document}
\pagestyle{empty}
\tableofcontents
\section[First: $a=b$]{First: \boldmath{$a=b$}} 
\end{document}

Actually, \boldmath does not look bad in TOC either:

\documentclass{article}
\begin{document}
\pagestyle{empty}
\tableofcontents
\section{First: \boldmath{$a=b$}} 
\end{document}

I personally like the second solution better: here math blends with the text both in the section title and TOC.

Answer 4

\usepackage{sectsty}
\allsectionsfont{\boldmath} % documentclass: not memoir; works also with KOMA-script
\setsecheadstyle{\boldmath} % documentclass: memoir

But be aware about @barbara-beeton's comment at a similiar question:

in some contexts it's not considered good practice to bold math just to follow the style of a title. if there's any chance that bold vs. non-bold has a distinct difference in meaning, introducing possible ambiguity isn't a good idea.

Answer 5

to expand on this answer by @andybuckley, the problem with amsthm discussed in the comments (I can't comment myself due to a lack of reputation) can be fixed by the following code:

\makeatletter
\g@addto@macro\bfseries{\boldmath}
\def\thmhead@plain#1#2#3{%
  \thmname{#1}\thmnumber{\@ifnotempty{#1}{ }\@upn{#2}}%
  \thmnote{ {\the\thm@notefont\unboldmath(#3)}}}
\let\thmhead\thmhead@plain
\makeatother

The reason why the problem appears is that amsthm roughly does the following:

\begin{theorem}[extra note $a$]
is compiled to
{\bfseries Theorem 1 {\fontseries\mddefault\upshape extra note $a$}.}

This is why extra note appears normal while $a$ appears in boldface if the fix is not applied.