This is a sample of using latex in a markdown document.

To use MathJax with Jekyll, make sure to include the following tag in your head.html file.

<script 
    src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML" 
    type="text/javascript">
</script>
\[\begin{equation} \textbf{y} = \textbf{X} \beta + \textbf{Z} u + \boldsymbol{\epsilon} \end{equation}\]

With

\[\begin{equation} u_i \sim N_q(0,\boldsymbol{\Psi})\\ \end{equation}\] 
$$
\begin{equation}
\textbf{y} = \textbf{X} \beta + \textbf{Z} u + \boldsymbol{\epsilon}
\end{equation}
$$
  
With  

&&
\end{equation}
u_i \sim N_q(0,\boldsymbol{\Psi})
\end{equation}
&&
\[y = \begin{bmatrix} mobility\\ 2\\ 2\\ \vdots\\ 3 \end{bmatrix} \begin{matrix} n_{ij}\\ 1 \\ 2 \\ \vdots \\ 8525 \end{matrix} X = \begin{bmatrix} Intercept & Age & Married & Sex & WBC & RBC \\ 1 & 64.97 & 0 & 1 & 6087 & 4.87\\ 1 & 53.92 & 0 & 0 & 6700 & 4.68\\ \vdots & \vdots & \vdots & \vdots & \vdots & \vdots \\ 1 & 56.07 & 0 & 1 & 6430 & 4.73 \end{bmatrix}\] 
$$
y =
\begin{bmatrix}
mobility\\
2\\
2\\
\vdots\\
3
\end{bmatrix}
\begin{matrix}
n_{ij}\\
1 \\
2 \\ 
\vdots \\
8525
\end{matrix}
X =
\begin{bmatrix}
Intercept & Age & Married & Sex & WBC  & RBC \\
1 & 64.97 & 0 & 1 & 6087 & 4.87\\
1 & 53.92 & 0 & 0 & 6700 & 4.68\\
\vdots & \vdots & \vdots & \vdots & \vdots & \vdots \\
1 & 56.07 & 0 & 1 & 6430 & 4.73
\end{bmatrix}
$$
\[\small \text{Font size is small, eg. $\sum{x_i = 10}$}\] 
$$\small \text{Font size is small, eg. $\sum{x_i = 10}$}$$