Symbol for logical equivelence

Question

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:

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

Answer 1

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}

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}

Answer 2

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):

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.

Answer 3

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 $A\myequiv B$.