81

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

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  • 16
    Isn't it \iff?
    – Ludovic C.
    Aug 20, 2013 at 20:28
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    – Mensch
    Aug 20, 2013 at 20:33
  • 2
    @LudovicC., no \iff is the symbol with two lines. I nedd the symbol with one line..!!
    – mle
    Aug 20, 2013 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, 2018 at 3:51

3 Answers 3

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72

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.

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  • thanks.. but I use \biconditional and I have "Undefined control sequence"
    – mle
    Aug 20, 2013 at 20:41
  • 4
    @Soviet: Then you have missed the line \let\biconditional\leftrightarrow.
    – Heiko Oberdiek
    Aug 20, 2013 at 20:44
  • if I use \leftrightarrow it is ok..
    – mle
    Aug 20, 2013 at 20:45
  • which one most closely resembles the style of \implies?
    – joseville
    Jan 22, 2022 at 3:06
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    @joseville Package amsmath defines \implies as \Longrightarrow with some additional horizontal space (\;) around the symbol: \newcommand{\implies}{\DOTSB\;\Longrightarrow\;}
    – Heiko Oberdiek
    Jan 22, 2022 at 7:37
19

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: https://oeis.org/wiki/List_of_LaTeX_mathematical_symbols

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  • The link you provided is now dead. If you have any other resource of the link then please update your answer.
    – Christos Lytras
    Jan 5, 2020 at 11:50
9

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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