LaTeX symbol for "\leftrightarrow" with "\triangleq"

Question

In LaTeX, does it exist any symbol for

Answer 1

Here are two possible alternatives. In both cases (\symA and \symB), I use both a left and right arrow joined in order to get a wider arrow. The \mkern can be increased negatively, if you want the width narrower. The \symA uses the \triangleq component from amssymb, but the vertical gap can be controlled with the optional argument to \stackon. The \symB does not require amssymb, but tries to mimic the tapered equal sign you showed in your figure, by using stacked \rules. Again, the vertical spacings are adjustable.

\documentclass{article}
\usepackage[usestackEOL]{stackengine}
\usepackage{amssymb}
\def\symA{\mathrel{\stackon[-0.8pt]{\leftarrow\mkern-6mu\rightarrow}%
  {\scriptstyle\triangleq}}}
\def\symB{\mathrel{\stackon[.7pt]{\leftarrow\mkern-6mu\rightarrow}%
  {\setstackgap{S}{0pt}\Shortstack{%
  \scriptscriptstyle\triangle\\\rule[.8pt]{6pt}{0.25pt}\\\rule{8pt}{.25pt}}}}}
\stackMath
\begin{document}
$A \symA B \symB C$
\end{document}

Answer 2

There is no ready made symbol per se, but you can definitely create a new one.

Simply define a new macro using either \mathbin or \mathrel. I think that you will need the later one, since an arrow usually shows a relation.

Please see here two understand the subtle differences between the two solutions.

You use the \overset command to stack up your operators.

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Anyway here is your solution.

\documentclass{article}

\usepackage{amsmath}

\usepackage{amssymb}

\newcommand{\leftrightarrowtriangleqbin}{\mathbin{\overset\triangleq\leftrightarrow}}

\newcommand{\leftrightarrowtriangleqrel}{\mathrel{\overset\triangleq\leftrightarrow}}


\begin{document}

$S \leftrightarrowtriangleqbin T$

$S \leftrightarrowtriangleqrel T$

\end{document}

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And here is a sample output.

See the subtle spacing difference between the two?

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Actually, for the second one \mathrel around the construction is redundant. As pointed out by egreg below, \overset will keep the binary relation nature of its second argument.