Cs224n lecture 3,4 neural nets, backprop
$\underbrace{\frac{\partial s}{\partial W}}{[n \times m]} = \overbrace{\underbrace{\delta ^T}{[n \times 1]}\underbrace{x^T}_{[1 \times m]}}^{\text{This is an outer product.}} $
$\underbrace{\frac{\partial s}{\partial W}}{[n \times m]} = \overbrace{\underbrace{\delta ^T}{[n \times 1]}\underbrace{x^T}_{[1 \times m]}}^{\text{This is an outer product.}} $