How do I get a equals sign under letters?

Question

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

Answer 1

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}:

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).

Answer 2

\documentclass{article}
\usepackage{amsmath}
\begin{document}
equal sign under letter: $\underset{=}{a}$
\[ \underset{=}{a} \]
\end{document}

Answer 3

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:

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: