I found this to be the simplest way to integrate mathjax into Jekyll. This assumes you are using Kramdown as your markdown renderer, and your _config.yml contains the line markdown: kramdown

Here is the process:

1. Create an _includes directory in your root Jekyll directory
   
   
2. Create a file called mathjax.html inside it, and put the following inside the file
   
   

    <script type="text/x-mathjax-config">
      MathJax.Hub.Config({
        extensions: [
          "MathMenu.js",
          "MathZoom.js",
          "AssistiveMML.js",
          "a11y/accessibility-menu.js"
        ],
        jax: ["input/TeX", "output/CommonHTML"],
        TeX: {
          extensions: [
            "AMSmath.js",
            "AMSsymbols.js",
            "noErrors.js",
            "noUndefined.js",
          ]
        }
      });
    </script>
    <script type="text/javascript" 
    src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js">
    </script>
   

3. That’s it! When you’re creating a new page, just put this template tag at the top of your page
   
   

    {% include mathjax.html %}
   

4. You can now use the kramdown $$ syntax
   
   

    $$
    \begin{align*}
      & \phi(x,y) = \phi \left(\sum_{i=1}^n x_ie_i, \sum_{j=1}^n y_je_j \right)
      = \sum_{i=1}^n \sum_{j=1}^n x_i y_j \phi(e_i, e_j) = \\
      & (x_1, \ldots, x_n) \left( \begin{array}{ccc}
          \phi(e_1, e_1) & \cdots & \phi(e_1, e_n) \\
          \vdots & \ddots & \vdots \\
          \phi(e_n, e_1) & \cdots & \phi(e_n, e_n)
        \end{array} \right)
      \left( \begin{array}{c}
          y_1 \\
          \vdots \\
          y_n
        \end{array} \right)
    \end{align*}
    $$
   

   \(\begin{align*} & \phi(x,y) = \phi \left(\sum_{i=1}^n x_ie_i, \sum_{j=1}^n y_je_j \right) = \sum_{i=1}^n \sum_{j=1}^n x_i y_j \phi(e_i, e_j) = \\ & (x_1, \ldots, x_n) \left( \begin{array}{ccc} \phi(e_1, e_1) & \cdots & \phi(e_1, e_n) \\ \vdots & \ddots & \vdots \\ \phi(e_n, e_1) & \cdots & \phi(e_n, e_n) \end{array} \right) \left( \begin{array}{c} y_1 \\ \vdots \\ y_n \end{array} \right) \end{align*}\)