# How to typeset longer $\Rightarrow$ with text under it using MathJax?

I don't know how to typeset the $\Rightarrow$ with the text under it as shown in this image? I know that \xrightarrow{\text{substitution}} gives rise to $\xrightarrow{\text{substitution}}$ but when changing the r with R I get an error. Furthermore, how to make the text appear under the arrow?

Thanks!

You could use the (non-standard) \Newextarrow command from the extpfeil extension to define \xRightarrow:

$\require{extpfeil}\Newextarrow{\xRightarrow}{5,5}{0x21D2}$
$\xRightarrow[\text{text below}]{}$


which produces $\require{extpfeil}\Newextarrow{\xRightarrow}{5,5}{0x21D2}$ $\xRightarrow[\text{text below}]{}$

Perhaps that will work for you.

• +1: Many thanks! $\text{}$ – user93957 Dec 6 '13 at 11:52
• For completeness, the "above" version: $\xRightarrow[\text{below}]{\text{above}}$. Testing: $\require{extpfeil}\Newextarrow{\xRightarrow}{5,5}{0x21D2}$ $\xRightarrow[\text{below}]{\text{above}}$ – user147263 Nov 6 '15 at 6:32

I didn't find a way to extend the double arrow in MathJax, but you can get text underneath as an optional argument

A\xrightarrow[\text{under the arrow}]{\text{above the arrow}}B $$A\xrightarrow[\text{under the arrow}]{\text{above the arrow}}B$$ I think that if MathJax had an option to use mathtools, we could use \xRightarrow.

• Thanks, that's very helpful @robjohn ! – user93957 Dec 5 '13 at 20:42

This won't extend the arrow, but you can use \overset and \underset on, say, \Longrightarrow. You might also want to look at this answer in tex.SE, but it probably won't work on SE itself.