[Tex/LaTex] Can one break stackrel text into multiple lines

stacking-symbols

I'm writing something up where I want to be explicit about some elementary operations I'm performing. I have an arrow between matrices with some text above the arrow showing what I'm doing. The text above the arrow is quite long, is it possible to cause a line break of some sort in stackrel?

My code is the following:

\stackrel{R2:= R2-2R1,\ R3:= R3+2R1}{\longrightarrow}

Best Answer

\substack or subarray are your friends.

I suggest two commands, one for centered alignment of the elementary operation denotations, one for left alignment, that you might prefer.

\documentclass{article}
\usepackage{amsmath}

\newcommand{\elem}[1]{% elementary operations
  \xrightarrow{\substack{#1}}%
}

\newcommand{\lelem}[1]{% elementary operations (left alignment)
  \xrightarrow{\begin{subarray}{l}#1\end{subarray}}%
}

\begin{document}

\[
\elem{R2:= R2-2R1,\\ R3:= R3+2R1}\quad
\lelem{R2:= R2-2R1,\\ R3:= R3+2R1}
\]

\end{document}

enter image description here

There's no need to have multiple lines: \elem{R2:=R2-2R1} would work seamlessly.

Related Solutions

[Tex/LaTex] How to adjust the fine positioning of stackrel

For the vertical spacing you can use \raisebox{<amount>}{<content>} to raise or lower (negative amount) the content. The horizontal centering was already shown for the similar question Overlay symbol with another, which might need a little adjustment here:

Here my suggestion. It works in the shown tests in all the different math modes.

\documentclass{article}

\usepackage{amsmath}
\usepackage{stmaryrd}

\newcommand\qMapsfrom{\mathrel{%
    \mathchoice{\qmf}{\qmf}{\scriptsize\qmf}{\tiny\qmf}%
}}
\newcommand\qmf{{%
    \setbox0\hbox{$\Mapsfrom$}%
    \rlap{\hbox to \wd0{\hss\hss\hss\raisebox{-.15\height}{?}\hss}}\box0
}}

\textwidth=2cm
\begin{document}

$ A \qMapsfrom B $

$ A \Mapsfrom B $

\[ A \qMapsfrom B \]
\[ A \Mapsfrom B \]

$ S_{ A \qMapsfrom B } $

$ S_{ A \Mapsfrom B } $

$ S_{S_{ A \qMapsfrom B }} $

$ S_{S_{ A \Mapsfrom B }} $

\end{document}

Update:

I just remembered that amsmaths \text can be used for math style depending scaling. It already uses similar code as the one above. This simplifies the macro to:

\newcommand\qMapsfrom{\mathrel{\text{%
    \setbox0\hbox{$\Mapsfrom$}%
    \rlap{\hbox to \wd0{\hss\hss\hss\raisebox{-.15\height}{?}\hss}}\box0
}}}

Result

[Tex/LaTex] How to i shrink (or expand) the vertical spacing in \stackrel

\stackrel exploits the same TeX's internal mechanism used for setting limits over \sum:

\newcommand{\stackrel}[2]{\mathrel{\mathop{#2}\limits^{#1}}

Therefore lowering the "superscript" won't have any effect (raising would). In any case it's better to use \overset from amsmath. And, in this particular case, to use the predefined symbol.

How to solve your problem in other situations, without measuring?

Alignments are what we need: but array would leave too much space and we'd need measuring again. Here's a way out; I'll use the "triangle over equals" just by way of example: always use an already available symbol.

\newcommand{\defeq}{%
  \mathrel{\vbox{\offinterlineskip\ialign{%
    \hfil##\hfil\cr
    $\scriptscriptstyle\triangle$\cr
    %\noalign{\kern0ex}
    $=$\cr
}}}}

I've left a (commented) line where one could adjust the separation between the two parts; it's better to specify ex rather than pt units, so that the symbol can be used in various font sizes.

We say that the symbol is a relation.
We enclose the building into a \vbox, so that the baseline of the bottom element will be used for the whole thing.
In the \vbox we suppress the usual interline skip mechanism (\offinterlineskip).
We start an alignment; \ialign is just a way to use \halign with \tabskip set to zero.
The alignment consists of one column, with centered items (\hfil#\hfil\cr, the # needs to be doubled because we're in a definition).
The two rows, possibly adjusting the separation with a positive or negative kern.

Here are some examples