latex symbol for "if and only if"

Question

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

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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 04 '18 at 03:51

Heiko Oberdiek · Accepted Answer · 2013-08-20T20:43:46.760

74

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}

Remarks:

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

\DeclareRobustCommand\iff{\;\Longleftrightarrow\;}

The example also shows some other arrow variants.

edited Aug 20 '13 at 20:43

answered Aug 20 '13 at 20:33

Heiko Oberdiek

271,626

thanks.. but I use \biconditional and I have "Undefined control sequence" – mle Aug 20 '13 at 20:41
4

@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
which one most closely resembles the style of \implies? – joseville Jan 22 '22 at 03:06
1

@joseville Package amsmath defines \implies as \Longrightarrow with some additional horizontal space (\;) around the symbol: \newcommand{\implies}{\DOTSB\;\Longrightarrow\;} – Heiko Oberdiek Jan 22 '22 at 07:37

mvarela · Answer 2 · 2020-01-27T18:33:52.897

21

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

edited Jan 27 '20 at 18:33

answered Aug 20 '13 at 20:38

mvarela

734

@ChristosLytras The link works for me. Please update your comment. – john Jun 05 '23 at 02:15

score 10 · Answer 3 · edited Aug 20 '13 at 22:52

10

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)

edited Aug 20 '13 at 22:52

ChrisS

13,529

answered Aug 20 '13 at 20:43

Bernard

136
2

Isn't it \Longleftrightarrow? – Steffo Oct 05 '23 at 13:30

latex symbol for "if and only if"

3 Answers3