Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / Section F / 47 / a

Let $z_{ij}$ be the input to output weight $w_j$ for datum $i$, including $z_{i0}=1$ for the bias. With identity output activation, $h_i=a_i={\sum }_j{w}_j{z}_{ij}$. The chain rule givesThe $z_{ij}$ are held fixed when differentiating an output-layer weight; they may be hidden-layer activations rather than raw descriptors. Hidden-weight derivatives require backpropagation through the preceding layers.