Testing latex rendering:

$$

\left[ \begin{array}{a} a^l_1 \\ ⋮ \\ a^l_{d_l} \end{array}\right]

= \sigma(

\left[ \begin{matrix}

w^l_{1,1} & ⋯ & w^l_{1,d_{l-1}} \\

⋮ & ⋱ & ⋮ \\

w^l_{d_l,1} & ⋯ & w^l_{d_l,d_{l-1}} \\

\end{matrix}\right] ·

\left[ \begin{array}{x} a^{l-1}_1 \\ ⋮ \\ ⋮ \\ a^{l-1}_{d_{l-1}} \end{array}\right] +

\left[ \begin{array}{b} b^l_1 \\ ⋮ \\ b^l_{d_l} \end{array}\right])

$$