To enable the display of math equations in GitHub pages, we need to include the corresponding Javascript in our page header. Taking my current blog as an example (one can go to MORE LINKS \(\rightarrow\) BLOG REPO to visit the GitHub repo for current blog), in the header part of each post, it is specified the layout is post. We then can find post.html in the _layouts directory which includes the definition for the outlook of a post. There, in the header part, one can find it is further pointing to the base layout which then refers to the base.html file under _layouts. Opening the base.html file, we can find that it includes the head.html file (which fundamentally defines the <header> section in the rendered HTML file). The head.html file can be found under _includes directory.
We then need to include the following codes in the head.html file,

<script type="text/javascript" async

Then for both inline and display equation, we can just use ‘$$’ symbol to claim an equation environment to write out equations.