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I've seen an equals sign with a small exclamation mark on top being used to say "equals because of data that was given in the problem", for example for a boundary condition or initial value in a differential equation.

A rough picture of what I mean:

     !
f(x) = 4

How do I typeset this in LaTeX? I've tried $=^!$ (which doesn't produce the desired result) and looked through the "relations" section of the Comprehensive List of Symbols without success.

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5 Answers 5

up vote 43 down vote accepted

That's very easy. And there are at least two identical solutions.

\documentclass[11pt,a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}

\begin{document}
 \[
   a\stackrel{!}{=} b   % from vanilla LaTeX
 \]

 \[
   a\overset{!}{=} b    % from »amsmath«
 \]
\end{document}

Both give identical output in this case but \overset should be preferred because it takes care of operator spacing.

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1  
Is there any reason to use the ams solution over the plain TeX version? –  Seamus Dec 17 '10 at 18:05
6  
»amsmath« is always preferable. –  Thorsten Donig Dec 17 '10 at 22:18
2  
in most cases, it's preferable for some reason! For my part I'm not skeptical that it is better here, but I would be interested to know in what ways it's better :-) –  Peter LeFanu Lumsdaine Dec 21 '10 at 6:32
14  
@Peter: when you use amsmath, \stackrel can be considered obsolete as \overset does the same thing better, because it gives correct spacing with binary operators (like +, \oplus, etc.) and ordinary symbols. –  Philippe Goutet Jan 20 '11 at 13:07
1  
@ThorstenDonig: The output is ugly when I want to put a questionmark over equals sign, because "?" is too high above "=". How can I make it appear lower, right above "="? –  Leon Aug 22 '12 at 7:02
\stackrel#1#2

is the command you search.

\stackrel{!}{=}

will lead to the sign you want to typeset. The first argument sits on top of the second argument, where the first one is smaller than the second one.

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just to add another one....

\mathop{=}\limits^!
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Also fine! Just the spacing is too tight for a relation symbol, but you could add \mathrel around the expression. –  Stefan Kottwitz Dec 17 '10 at 18:28

If you needed to customize the separation between the overset, here's a way it could be done. The default is 3pt, halfway between the two examples shown.

\documentclass[11pt]{article}
\usepackage{stackengine}
\begin{document}
\[
 a \mathrel{\stackon[5pt]{$=$}{$\scriptstyle!$}} b
\quad
 a \mathrel{\stackon[1pt]{$=$}{$\scriptstyle!$}} b
\]
\end{document}

enter image description here

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Here's an oldie goldie

\documentclass{article}
\begin{document}
\[ a \buildrel!\over= b \]
\end{document}
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