If you want to test it quickly, copy+paste the following code in the live version (https://josephernest.github.io/writing/):

    #LINEAR ALGEBRA  
    ##1. Introduction  
    **Definition :** A matrix is ... a matrix.

    Examples:  
    $$A = \begin{pmatrix}0&1&-2&3\\5&-3&1&-2\\5&-2&-1&1  \end{pmatrix},\  
     B = \begin{pmatrix}0&8&-5\\0&0&0\\-1&-9&4\\2&19&-8\\1&1&1\end{pmatrix}$$

(paste it without the 4-spaces indentation)