Paragraph

To specify the rule for a function with small domain, use two-line notation by writing a matrix with each output directly below its corresponding input, as in:

\begin{equation*} f = \begin{pmatrix}1 \amp 2 \amp 3 \amp 4 \\ 2 \amp 1 \amp 3 \amp 1 \end{pmatrix}\text{.} \end{equation*}

in-context