Let $f : \mathbb{R}^{N \times 1} \to \mathbb{R}^{N \times 1}$ be a D5657: Real column matrix function.
The neuron layer function with respect to $f$, $W \in \mathbb{R}^{N \times K}$, and $b \in \mathbb{R}^{N \times 1}$ is the D5657: Real column matrix function \begin{equation} \mathbb{R}^{K \times 1} \to \mathbb{R}^{N \times 1}, \quad x \mapsto f \left( W x + b \right) \end{equation}