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This question already has an answer here:

How do you write the equal-to symbol with delta on top ("delta-equal to")? One similar to http://www.decodeunicode.org/de/u+225c. Are special packages required for that? It is supposed to be used for definition of equalities, similar to :=.

marked as duplicate by egreg, user11232, Werner, barbara beeton, Svend Tveskæg Mar 5 '14 at 15:25

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    Try $\overset{\Delta}{=}$. – azetina Mar 5 '14 at 14:57
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    \triangleq, after \usepackage{amssymb} – egreg Mar 5 '14 at 15:06
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How about \triangleq from the AMS fonts? Loading \usepackage{amssymb} will make the command available

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    actually, \usepackage{amssymb} is needed for this. while amsmath loads amsfonts, not all the symbols are "named" unless amssymb is loaded explicitly. – barbara beeton Mar 5 '14 at 15:22
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EDITED to ensure rendering of the = in math mode. For more flexibility in the vertical placement, than the \overset (which requires amsmath package), you could try

\documentclass{article}
\usepackage{stackengine}
\def\delequal{\mathrel{\ensurestackMath{\stackon[1pt]{=}{\scriptstyle\Delta}}}}
\begin{document}
$a \delequal b$
\end{document}

enter image description here

The optional argument to \stackon specifies the vertical gap between symbols. If you wanted the delta smaller, you could add \scriptscriptstyle in place of the \scriptstyle:

enter image description here

For comparison, here is $a \overset{\Delta}{=} b$. Note the vertical offset is significantly larger than what I proposed.

enter image description here

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    Maybe illustrate the \overset{}{} just to compare. – azetina Mar 5 '14 at 15:04
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    Thanks for the exhaustive answer, I really appreciate it, I'll keep this in mind when creating other stacked symbol monstrosities, but today the simpler \triangleq saves the day (and my latex refuses to download stackengine.sty for some reason). – the swine Mar 5 '14 at 15:12
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    Understood. Best wishes. FYI, the package is at ctan.org/pkg/stackengine – Steven B. Segletes Mar 5 '14 at 15:13
  • I needed $=$, not =, as in \def\delequal{\mathrel{\stackon[1pt]{$=$}{$\scriptscriptstyle\Delta$}}} to get a correct result, since the first argument of \stackon appears to be rendered in text mode just like the second. At least, with = the output looks wrong and different. – Blaisorblade Mar 24 '16 at 9:32
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    @Blaisorblade Thank you for pointing that out. For some fonts, there is no difference I think, but to guarantee math mode rendering, I have updated the answer. – Steven B. Segletes Mar 24 '16 at 10:06

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