10

By contradiction symbol I mean this

enter image description here

This is what I tried

\documentclass{article}
\begin{document}
\[
\Rightarrow\Leftarrow
\]
\end{document}

But there is some space between the head of two arrows.

  • Whatever you do, define a command \contradiction that outputs what you want. That way not only you can easily change the definition in case you want to tweak it a bit, but also your code will be much more clean. – Manuel Mar 12 '17 at 20:49
12

You might add a negativ space \! which results in no space between the two symbols:

\documentclass{article}
\begin{document}
\[
\Rightarrow\!\Leftarrow
\]
\end{document}

As @Rethliopuks noted in the comments, the package tipa disables the usage of \! in mathmode as negative space. If you use this package (or any other package which disables \!) you might use

\Rightarrow\mskip-\thinmuskip\Leftarrow

instead for the same results (\mskip-\thinmuskip is the default definition of \!).

  • 1
    Seems like I was too slow :( – Skillmon Mar 12 '17 at 10:36
  • But at least you explained what the \! is for. – TeXnician Mar 12 '17 at 10:46
  • 2
    @TeXnician "Presto e bene non vanno insieme" = "Quick and well don't go together" (Italian proverb). :):):) – CarLaTeX Mar 12 '17 at 11:05
  • 1
    @TeXnician Oooh, you're polyglot, then! However, the translation is for the others... – CarLaTeX Mar 12 '17 at 11:14
  • 1
    @Rethliopuks if you use the package tipa the following gives good results (works without tipa, too): \Rightarrow\kern-0.5ex\Leftarrow – Skillmon Nov 17 '17 at 9:51
3

That gives what you want (added \!):

\documentclass{article}
\begin{document}
\[
\Rightarrow\!\Leftarrow
\]
\end{document}
2

Math symbols usually have sidebearings and, at least with Computer Modern fonts. The sidebearings turn out to be 1mu, in the case of \Leftarrow and \Rightarrow.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\setlength{\fboxsep}{0pt}\setlength{\fboxrule}{0.1pt} % just for the example

\fbox{$\Rightarrow$} \fbox{$\Leftarrow$}
$\Rightarrow\Leftarrow$

\fbox{$\Rightarrow\mspace{-1mu}$} \fbox{$\mspace{-1mu}\Leftarrow$}
$\Rightarrow\mspace{-2mu}\Leftarrow$

\fbox{$\Rightarrow\mspace{-1.5mu}$} \fbox{$\mspace{-1.5mu}\Leftarrow$}
$\Rightarrow\mspace{-3mu}\Leftarrow$

\fbox{$\Rightarrow\mspace{-2mu}$} \fbox{$\mspace{-2mu}\Leftarrow$}
$\Rightarrow\mspace{-4mu}\Leftarrow$

\end{document}

enter image description here

Judging from the above pictures, I guess you want

\newcommand{\contradiction}{%
  \ensuremath{{\Rightarrow\mspace{-2mu}\Leftarrow}}%
}

Yes, with \ensuremath, because \contradiction could be used both in displays (during a proof) or in text.

Using a macro is better because it can be tailored based on the actual fonts used. For instance, if fourier is used, the pictures are

enter image description here

so probably \mspace{-3mu} should be used for this case.

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