# MathJax v3 in Jekyll

This is an update to my two-year-old post “MathJax in Jekyll” (2018-08-05).

Last week, as I proofread my post “Concepts can’t do quantifiers” (2020-08-10), I noticed that its inline math wasn’t rendering correctly. I’d write $$\neg\exists x\in X:P(x)$$ in the Markdown, but instead of rendering on the live page as “$$\neg\exists x\in X:P(x)$$,” it would render as “$$\neg\exists x\in X:P(x)$$” (just without the teletype font).

I never figured out precisely what broke — it could have been some internal change to GitHub Pages’ Kramdown implementation, or some change on the CDN from which my blog includes mathjax.js — but anyway, I solved the problem by upgrading from MathJax v2 to MathJax v3.

MathJax v3 has a radically different configuration story from v2, although it starts the same way. You still place a snippet someplace where Jekyll will pick it up:

<script type="text/javascript" id="MathJax-script" async
src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js">
</script>


Step two is to specify the config, right above (not below!) that script tag. The config object in MathJax v2 was called MathJax.Hub.Config; but in v3 the config goes directly into the MathJax object itself, and all the other stuff that used to hang off of the MathJax object (e.g. MathJax.InputJax) now hangs off of MathJax._ instead (e.g. MathJax._.input).

My MathJax v3 config looks like this:

<script type="text/javascript">
window.MathJax = {
tex: {
packages: ['base', 'ams']
},
}
};
</script>


Step three hasn’t changed from my previous post “MathJax in Jekyll” (2018-08-05). Having followed steps 1 and 2 above, you can now mix TeX code into your Markdown, delimiting the TeX with double-dollar-sign escapes:

... a given wire happens to be carrying "$$\lvert 0\rangle$$."
By that we mean that it's carrying the linear combination
$$\begin{psmallmatrix} 1 \\ 0 \end{psmallmatrix}$$ ...


This renders as:

… a given wire happens to be carrying “$$\lvert 0\rangle$$.” By that we mean that it’s carrying the linear combination $$\begin{psmallmatrix} 1 \\ 0 \end{psmallmatrix}$$ …

And if you make a paragraph that contains nothing but a $$-escaped chunk of math, it will be rendered using MathJax’s mode=display, i.e., TeX display mode. There is one cheat here: MathJax doesn’t come with the \begin{psmallmatrix} environment set up out of the box. So I had to fake it, in both my old v2 install and my new v3 install. You can see how that was done in the commits below. I did the v2 one myself, by cargo-culting some JavaScript code from MathJax’s implementation of the AMSmath package. For the v3 one, I got help from Davide Cervone, a maintainer of the MathJax project; many thanks to him! Considering the negligible (zero?) rendering difference between $$\left(\begin{smallmatrix} 1 \\ 0 \end{smallmatrix}\right)$$ $$\left(\begin{smallmatrix} 1 \\ 0 \end{smallmatrix}\right)$$ and $$\begin{psmallmatrix} 1 \\ 0 \end{psmallmatrix} $$\begin{psmallmatrix} 1 \\ 0 \end{psmallmatrix}$$, if I had to do my quantum-computing post over again, I wouldn’t even try to implement \begin{psmallmatrix}. But I’m extremely susceptible to nerdsniping.

To see how to enable MathJax for your own Jekyll blog, click through to the relevant commits in this blog’s GitHub repository:

Posted 2020-08-19