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This question already has an answer here:

In LaTeX the symbol for material implication is produced by $\to$, but for biconditional ?

marked as duplicate by karlkoeller, Werner, Ian Thompson, mafp, lockstep Aug 20 '13 at 21:23

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  • 9
    Isn't it \iff? – Ludovic C. Aug 20 '13 at 20:28
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    @LudovicC., no \iff is the symbol with two lines. I nedd the symbol with one line..!! – mle Aug 20 '13 at 20:35
  • @LudovicC. there is also a version without the think double arrow. It might seem redundant but for people working on mathematical logic its important to distinguish which ones are part of the formal language being developed and which ones are part of the meta-language proving the logic being developed in question. – Charlie Parker Nov 4 '18 at 3:51
39

LaTeX defines \to as \rightarrow:

\let\to\rightarrow % fontmath.ltx

The other direction is \gets:

\let\gets\leftarrow

For \leftrightarrow you can define your own command, e.g. \biconditional:

\documentclass{article}
\let\biconditional\leftrightarrow
\begin{document}
\[ A \to B \biconditional C \gets D \]

\[ A \rightarrow B \leftrightarrow C \leftarrow D \]
\[ A \longrightarrow B \longleftrightarrow C \longleftarrow D \]

\[ A \Rightarrow B \Leftrightarrow C \Leftarrow D \]
\[ A \Longrightarrow B \Longleftrightarrow C \Longleftarrow D \]
\[ B \iff C \]
\end{document}

Result

Remarks:

  • \iff adds some extra space (from fontmath.ltx):

    \DeclareRobustCommand\iff{\;\Longleftrightarrow\;}
    
  • The example also shows some other arrow variants.

  • thanks.. but I use \biconditional and I have "Undefined control sequence" – mle Aug 20 '13 at 20:41
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    @Soviet: Then you have missed the line \let\biconditional\leftrightarrow. – Heiko Oberdiek Aug 20 '13 at 20:44
  • if I use \leftrightarrow it is ok.. – mle Aug 20 '13 at 20:45
9

You can use \Leftrightarrow or \Longleftrightarrow

\documentclass{article}
\begin{document}
$ \Leftrightarrow \Longleftrightarrow $
\end{document}

enter image description here

You might want to also bookmark this: http://web.ift.uib.no/Teori/KURS/WRK/TeX/symALL.html

5

Double line:

  • \iff (= \Leftrightarrow) or
  • \longLeftrightarrow or
  • \xLeftrightarrow (extensible, load the mathtools package)

Single line:

  • \leftrightarrow or
  • \longleftrightarrow or
  • \xleftrightarrow (extensible, load the mathtools package)

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