106

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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7 Answers 7

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123

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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5
  • 1
    Is there any reason to use the ams solution over the plain TeX version?
    – Seamus
    Dec 17, 2010 at 18:05
  • 16
    »amsmath« is always preferable.
    – Thorsten Donig
    Dec 17, 2010 at 22:18
  • 3
    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, 2010 at 6:32
  • 18
    @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, 2011 at 13:07
  • 2
    @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 "="?
    – Leo
    Aug 22, 2012 at 7:02
26 
\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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16

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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14

just to add another one....

\mathop{=}\limits^!
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2
  • Also fine! Just the spacing is too tight for a relation symbol, but you could add \mathrel around the expression.
    – Stefan Kottwitz ♦
    Dec 17, 2010 at 18:28
  • This is perfect. A quick explanation of what is going on would be nice though. E.g. what does the limits do?
    – lucidbrot
    Apr 29, 2021 at 7:16
5

Here's an oldie goldie

\documentclass{article}
\begin{document}
\[ a \buildrel!\over= b \]
\end{document}
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2

To reduce typing and opportunity for syntax errors, it is better to place a frequently used typesetting algorithm inside a macro. Rather than type

    \[
        a\overset{!}{=} b
    \]

for each equation, one might utilize a macro in the preamble

    \newcommand{\XoverE}{\ensuremath{\overset{!}{=}}}

and use it thusly

    $a\XoverE{}b$

For myself, I use the following macro for an overset question mark:

    \newcommand{\?}{\ensuremath{\overset{?}{-}}}

then

    $a\?{}b$

Edited

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1
  • 3
    Try to use it in text mode and you'll have a nasty surprise :-) This works in math mode because of a happy coincidence, since \ensuremath basically becomes \@firstofone. However, in text mode this becomes $relax\overset${!}{=} and goes quite wrong. Either you enclose the whole \overset construction in braces or (IMNSHO better) you eliminate \ensuremath altogether.
    – campa
    Mar 6, 2018 at 16:12
0

Here is how you can define custom commands using special characters for the \stackrel approach.

\catcode`\!=11
\newcommand{\eq!}{\stackrel{?}{=}}
% ...
$f(x) \eq! 4$

It is also nice to define \leq?, \mid? etc.

This solution is significantly better than writing \stackrel{?}{=} and friends frequently.

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3
  • 4
    You'll regret doing this when $x/\!y$ will throw you a strange error.
    – egreg
    Oct 9, 2019 at 9:07
  • @egreg Why? Just write $x/!y$
    – okovko
    Oct 9, 2019 at 10:48
  • That's a different thing.
    – egreg
    Oct 9, 2019 at 12:37

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