[Tex/LaTex] How to get a equals sign under letters

math-modestacking-symbols

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

Best Answer

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{#1}}

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

rendered output

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).