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I'm trying to use this symbol in a LaTeX document I'm writing but I can't for the life of me, find how to write it in LaTeX:

enter image description here

The symbol means that two expressions are logicically equivalent (i.e. A |==| B)Can anybody identify the missing symbol?

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I am not sure which symbol you look for, but did you try detexify: detexify.kirelabs.org/classify.html –  Yossi Gil Jan 9 at 19:43
    
From mathabx package you can do a trick $\vDash\!\!\Dashv $ –  Sigur Jan 9 at 19:49
    
The standard symbol for logical equivalence is “⇔”. Are you sure you are looking for a different symbol (apparently, one that has not even been coded as a character in Unicode)? –  Jukka K. Korpela Jan 9 at 19:52
    
Sigur, that's right. I'll accept it as an answer if you write one! –  Todd Davies Jan 9 at 19:59
    
@JukkaK.Korpela -- this symbol is in unicode -- U+29E6, "gleich stark = tautological equivalent". see also the question Does the mirror image of the \models symbol exist in TeX?. also note that, since the symbol is in unicode, it should be found in the stix or xits fonts. –  barbara beeton Jan 9 at 20:00

3 Answers 3

up vote 5 down vote accepted

Without the use of packages, you could do this:

\documentclass{article}
\def\logequiv{\mathrel{\vert\mkern-3mu{=}\mkern-3mu{=}\mkern-3mu\vert}}
\begin{document}
$ A \logequiv B$
\end{document}

enter image description here

If you didn't like the height of the \verts, you could do this instead:

\documentclass{article}
\def\logequiv{\mathrel{%
  \rule{.3pt}{1.3ex}\mkern-1mu{=}\mkern-8mu{=}\mkern-2mu\rule{.3pt}{1.3ex}}}
\begin{document}
$ A \logequiv B$
\end{document}

enter image description here

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Nice. Could you explain us why -3mu? Is the length of some symbol? –  Sigur Jan 9 at 20:24
2  
@Sigur The \mkern with a negative argument removes the natural space between the two symbols. The mu is the unit of mathematical kerning. I just played with the actual numbers until I ate up all the space. My experience is that symbols have a natural space around them in integer units of mu, though I could be wrong. –  Steven B. Segletes Jan 9 at 20:26
    
@Sigur I should add that, in both math and text kerning, the kern has no glue, and will therefore not stretch or shrink depending on the line length, unlike (for example) \hspace. Therefore, the (m)kern is the proper way to combine symbols together. –  Steven B. Segletes Jan 9 at 20:37
    
I think this might be flawed; what happens when the text size of the document isn't the default size but is changed to say 14pt? Wouldn't the spacing change? –  Todd Davies Jan 9 at 21:15
    
@ToddDavies It is true that the solution is tailored to a particular font size. Different versions could be developed for different font sizes, which would be the easiest. With more calculation, a self-adjusting version could be developed, as well, I'm sure. –  Steven B. Segletes Jan 9 at 22:09

There is actually a dedicated package for typesetting symbols of this sort. They are called turnstiles and the package is called turnstile. It is included in TeX Live and can typeset turnstiles of essentially any kind. It is actually a big improvement on what was available before which was essentially just mathematics symbols. For very simple turnstiles, I sometimes find the maths symbols look better but it really depends what you need and what the context is (e.g. where you are pulling your connective symbols from etc.) In any case, for more complex turnstiles such as this one, it is definitely the best option I'm aware of:

\documentclass{article}
\usepackage{turnstile}
\newcommand*{\myequiv}{\sdststile{}{}}

\begin{document}

The symbol you need:
\[
  \myequiv
\]

A selection of other possibilities:
\[
    \sststile{T}{}\quad \sdtstile{a^+}{}\quad \dtdtstile{A}{B}\quad \nsttstile{}{C}
\]

\end{document}

produces (probably - I can't check):

selected turnstiles

Note that the sample image posted in the question seems to have a slight gap on the right. I'm pretty sure that shouldn't be there.

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If you don't find that symbol (I believe that there is one) as a unique character you can combine two others to obtain a very similar one.

From mathabx package you can define

\newcommand{\myequiv}{\vDash\!\!\Dashv}

to use as $A\myequiv B$.

enter image description here

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1  
but beware that the mathabx fonts change a lot of other symbols as well. there are several questions that address how to access just a few symbols from a font rather than loading the whole thing via a package. –  barbara beeton Jan 9 at 20:11
    
@barbarabeeton, well pointed. –  Sigur Jan 9 at 20:23
    
To extract just a particular symbol from mathabx you could follow an approach such as tex.stackexchange.com/questions/113437/stealth-arrow-in-math/… or tex.stackexchange.com/questions/14386/…. –  Steven B. Segletes Jan 9 at 20:40
    
If you didn't want to use mathabx, you could first define \Dashv by \newcommand{\Dashv}{\mathrel{\scalebox{-1}[1]{$\vDash$}}} (using package graphicx). –  dbmag9 Jan 9 at 23:31

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