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To show that a letter is a matrix my math book puts a equals sign under the letter. How do I do this?

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3  
It's a very old fashioned notation, from the typewriter times. –  egreg Dec 30 '11 at 14:04
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3 Answers

That notation is quite old fashioned. If you really want to use it, probably the best way is to define

\newcommand{\matr}[1]{\underline{\underline{A}}

Here, for comparison, the result of \matr{A} and of \underset{=}{A}:

enter image description here

In any case, define your own command, so that you can easily change your mind later; with \underset it would be

\newcommand{\matr}[1]{\underset{=}{#1}}

According to ISO norms, variables denoting matrices should be in boldface italic:

\usepackage{bm}
\newcommand{\matr}[1]{\bm{#1}}

(or, simply, \newcommand{\matr}{\bm} or, even more simply, \let\matr\bm).

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I think \uuline from ulem package will give nicer results then a double underline –  someonr Dec 30 '11 at 14:30
    
The lines are nearer to each other with uuline; the result is ugly nonetheless. :) ISO norms say that variables denoting matrices should be in boldface italics. –  egreg Dec 30 '11 at 14:42
    
I never said I like this typesetting ;) I also prefer bold matrix variables. –  someonr Dec 30 '11 at 14:46
    
I'd plus one this again for the advice "define your own command (anyway)", if I could. This is somewhere high up on the list of good ideas, and saves significant time when the journal I'm sending a paper to changes. –  qubyte Dec 30 '11 at 15:11
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\documentclass{article}
\usepackage{amsmath}
\begin{document}
equal sign under letter: $\underset{=}{a}$
\[ \underset{=}{a} \]
\end{document}
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You can use the amsmath package

\documentclass{article}

\usepackage{amsmath} 
\begin{document}
   \[\underset{=}{A}=
     \begin{pmatrix}
        1&2\\3&4
     \end{pmatrix}
   \]
\end{document}

result will be:

result

or use ulem package for double underlining. I think this will result in a nicer spacing.

\documentclass{article}
\usepackage{ulem}

\usepackage{amsmath} 
\begin{document}
\[\uuline{A}=
\begin{pmatrix}
1&2\\3&4
\end{pmatrix}\]
\end{document}

(amsmath is only used for pmatrix environment) result is:

result

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