Repeating certain signs after linebreak in inline math expressions

Question

I use LaTeX to type my University Maths notes. In that, I often find myself faced with linebreaks in inline math expressions. In such a situation, I usually want to double the sign just before the break, putting one copy at line end and the other copy at the start of the following line. Unfortunately, TeX won't do that. Up till now, I had it do this manually (e.g. $d=$ $=\dim M$, $f:M\to$ $\to\mathbb{R}$ or even $x\mapsto$ \linebreak $\mapsto(x,f(x))$, because sometimes TeX breaks the line after the second copy). It is pretty boring to inspect the document for all such linebreaks, so I was wondering if there was a way of telling TeX to do this automatically. The ideal thing would be a macro \doubleatbreaks which, with the syntax \doubleatbreaks{<sign>} makes sure <sign> is doubled at linebreaks in inline math. E.g., \doubleatbreaks{=} would make =s double at breaks, \doubleatbreaks{\times} would make \times double at breaks, etc. Is there a way to do this?

Edit

As @egreg noted, rmathbr is the way. Just one thing: When I have a minus at a line break, I usually put a plus on the new line, because -+=- and --=+. rmathbr doubles the -. How can I get - (linebreak) + instead?

Answer 1

A few steps to get this done:

1. Go to CTAN;
2. Find the package rmathbr;
3. Download it;
4. Locate the downloaded file;
5. Open rmathbr.ins;
6. Typeset it (i.e. compile it);
7. This produces a .sty; locate it and place it in a directory TeX will find it in;
8. Add \usepackage{rmathbr} to your document's preamble.

This makes a lot of symbols repeat. Opening the .sty now also opens the .pdf of the documentation. The package seems to have few known issues, and the documentation says it should be loaded after other packages.

If someone wants, like me, to break a minus with a plus (i.e. $a-$ $+b$ with a linebreak in between), because -+=- and --=+, I hacked into the code of the package and produced this:

\makeatletter
\begingroup
  \catcode`\-\active
  \ifnum\mathcode`\-<32768
    \xdef-{\noexpand\xbrokenbin{\mathchar\number\mathcode`\-}}
  \else
    \gdef-{\xbrokenbin{\mathchar8704}}
  \fi
\endgroup
\DeclareRobustCommand{\xbrokenbin}[1]{\rmathbr@xbrokenbin{#1}}
\def\rmathbr@xbrokenbin#1%
{
  \relax
  \def\rmathbr@arg{#1}
  \def\rmathbr@this{\rmathbr@xbrokenbin}
  \def\rmathbr@output{\rmathbr@@xbrokenbin}
  \futurelet\rmathbr@let@token\rmathbr@brokenop%
}
\def\rmathbr@@xbrokenbin#1%
{
  \ifnum\lastpenalty=\relpenalty
    \mathbin{#1}
  \else
    \mathbin{#1}
    \ifx$\rmathbr@let@token
    \else
      \rmathbr@selector{+}
    \fi
  \fi
  \rmathbr@setbrokens
  \penalty\binoppenalty %
}
\def\rmathbr@selector#1%
{
  \mathchoice
    {\discretionary{}{\hbox{$\m@th\displaystyle#1$}}{}}
    {\discretionary{}{\hbox{$\m@th\textstyle#1$}}{}}
    {\discretionary{}{\hbox{$\m@th\scriptstyle#1$}}{}}
    {\discretionary{}{\hbox{$\m@th\scriptscriptstyle#1$}}{}}%
}
\makeatother

I created a file handleminus.tex containing the above code and with that, I got the following very simple MWE:

\documentclass[a4paper]{report}
\usepackage{amsmath,rmathbr}
\input handleminus.tex

\begin{document}
Foobar Foobar Baz Baz Tom Dick Harry Tizio Caio Sempronio sorrata $d=\dim M$. wojgaoperjgojefgiwejafoiwjofipjaweoifjwaeoifjowipjfowiepjf pqppp $a\times b$ qoifjoqiwjfoiwqjfoiqwjfoiqwjfoiqwjfoiwqjfo qpogkpoqwkgpoqwkpowq pqi $a-b$
\end{document}

Compiling to:

Remember, of course, to \input after loading the package, or the code above will be overwritten by the package's code and you will be back with repeated minuses.

Thanks egreg for pointing that package out.

Working a bit it is certainly possible to make a version of \xbrokenbin that takes two arguments and creates a command that prints the first argument, breaks the line and then gives the second argument, i.e. one such that \xbrokenbin{-}{+} gives what the above code produces. However, I don't need such contortions, so I will not take the time to do that. Also, I would discourage such contortions: having a \subseteq repeat like a \cdot would be very odd.