Whats the command for this symbol: ⇅
I've found ⇈ \upuparrows but I can't find the other one.
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The package mathabx provides all arrows of this kind. The one you're looking for is \updownarrows.
\documentclass{article}
\usepackage{mathabx}
\begin{document}
$\upuparrows$ - $\downdownarrows$ - $\updownarrows$ - $\downuparrows$
\end{document}
For such questions, a good place to look at is the comprehensive LaTeX symbols list.
Note that mathabx redefines many symbols. An alternative is the following code, which produces updownarrows in a pure-latex fashion, but as you can see, the result is still not fully identical to amssymb's \upuparrows.
\documentclass{article}
\usepackage{amsmath,amssymb}
\newcommand{\updownarrows}{\mathbin\uparrow\hspace{-.5em}\downarrow}
\newcommand{\downuparrows}{\mathbin\downarrow\hspace{-.5em}\uparrow}
\begin{document}
$\upuparrows$ - $\downdownarrows$ - $\updownarrows$ - $\downuparrows$
\end{document}
A possible solution would be to redefine \upuparrows and \downdownarrows as well :
\documentclass{article}
\usepackage{amsmath,amssymb}
\newcommand{\updownarrows}{\mathbin\uparrow\hspace{-.3em}\downarrow}
\newcommand{\downuparrows}{\mathbin\downarrow\hspace{-.3em}\uparrow}
\renewcommand{\upuparrows}{\mathbin\uparrow\hspace{-.3em}\uparrow}
\renewcommand{\downdownarrows}{\mathbin\downarrow\hspace{-.3em}\downarrow}
\begin{document}
$\upuparrows$ - $\downdownarrows$ - $\updownarrows$ - $\downuparrows$
\end{document}
In comments, egreg proposed the following answer, for the same result (well, not technically the same), but with a cleaner code. It will probably give better spacing results if used within longer formulas :
\documentclass{article}
\usepackage{amsmath,amssymb}
\newcommand{\updownarrows}{\uparrow\mathrel{\mspace{-1mu}}\downarrow}
\newcommand{\downuparrows}{\downarrow\mathrel{\mspace{-1mu}}\uparrow}
\renewcommand{\upuparrows}{\uparrow\uparrow}
\renewcommand{\downdownarrows}{\downarrow\downarrow}
\begin{document}
$\upuparrows$ - $\downdownarrows$ - $\updownarrows$ - $\downuparrows$
\end{document}
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1@egreg : Indeed, and even those arrows don't have the same heads as the usual arrows. I will edit with a pure-latex solution, but the result isn't exactly similar to amssymb's double arrows either. – T. Verron Dec 9 '12 at 14:17
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Why those
\mathbintokens in the last solution? They act only on the following token and affect the global spacing. – egreg Dec 9 '12 at 14:50 -
@egreg : If I don't add them, the arrows will be stuck to the dash, unlike the regular
\upuparrows. About the command affecting only the first token, that's what I thought as well, but since the result seems quite symmetrically spaced, I did not change it further. – T. Verron Dec 9 '12 at 14:56 -
1I'd simply go for
\uparrow\uparrowand\uparrow\mathrel{\mspace{-1mu}}\downarrow, using the fact that those arrows are defined as relation symbols. – egreg Dec 9 '12 at 16:04
\updownarrows– Jesper Ipsen Dec 9 '12 at 13:29