Summary: The math plugin uses MathJax (default) or KaTeX to render maths.
GitHub: GitHub  discourse/discoursemath: Official MathJax support for Discourse
Install: Follow the plugin installation guide.
Features
You can render blocks of maths by wrapping with $$
$$
\hat{H}\Psi=E\Psi
$$
You can render inline math by wrapping with a single $
like so: $E=mc^2$
.
Discourse math uses a heuristic to ensure that sentences such as: “I spent 20$ and another 100$” do not get converted.
When using MathJax, you can also enable asciimath processing which uses %
as a delimiter: %E=mc^2%
Configuration
This plugin is disabled by default, after installing be sure to enable it via site settings.
CHANGELOG
TODO
 We would love to support serverside rendering, there is already an online sample.
Extras
What about the old plugin?
There is a plugin maintained by @Kasper/ @masda70 called Discourse MathJax. This plugin is a ground up rewrite to support the new markdown it engine. The main differences are:

It is much stricter about block and inline syntax, only $ and $$ blocks are supported

It embeds MathJax or KaTeX renderer so you do not need to use a CDN

It only loads the math renderer if it detects equations on the page.

It surgically applies renderer as opposed to performing full page scans.

It is well specified using specs