Add the following line to the header in _layouts/post.html or anywhere else you want to use MathJax.

_layouts/post.html partial listing

Add the following line to the YAML front matter of a post to enable MathJax on a post by post basis.

For example, the front matter of this post looks like this.
Note that Disqus comments have been added with the same strategy.

If all goes well, you should be able to use MathJax inline and display modes.
Note that the MathJax dynamic preview can be useful when formatting complex equations.

In N-dimensional simplex noise, the squared kernel summation radius $r^2$ is $\frac 1 2$
for all values of N. This is because the edge length of the N-simplex $s = \sqrt {\frac {N} {N + 1}}$
divides out of the N-simplex height $h = s \sqrt {\frac {N + 1} {2N}}$.
The kerel summation radius $r$ is equal to the N-simplex height $h$.