Backpropagation, Neural Networks.
Similarly, for b and db, and z and dz, and so on. To make sure that the dimensions of these all match up, we have that dz¹ is equal to w² transpose, times dz². Then, this is an element-wise product times g¹ prime of z¹. Mashing the dimensions from above, this is going to be n¹ by 1, is equal to w²