I want to add equations.

How about this?

\(\prod_{i=1}^{N}(x-x_i) = \sum_{k=0}^{N} (-1)^{N-k} e_{N-k}(x_1, x_2, \dots, x_N) x^{k}\).

The coefficients \(e_k(x_1, x_2, \dots, x_N)\) are called the elementary symmetric functions. For example, we have

\[\begin{equation} \begin{split} e_0(x_1, \dots, x_N) &=1\\ e_1(x_1, \dots, x_N) &= x_1 +x_2 + \cdots + x_N \end{split} \end{equation}\]

How I added math

This was failry confusing process. I tried several things. In the end, I created a file _layouts/bease.html, where I copied the file from the minima GitHub repository click here and then I added the following scritp that I found here:

<script type="text/javascript" async src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js?config=TeX-MML-AM_CHTML" async></script>

Also, I changed the _layouts/bease.html to the file given in this repository for the math-minima Jekyll theme. For some reason, I wasn’t able to get the math-minima theme to work. Instead, I just copied the file from their repo.