From this video by Andrew Ng around 5:00
How are $\delta_3$ and $\delta_2$ derived? In fact, what does $\delta_3$ even mean? $\delta_4$ is got by comparing to y, no such comparison is possible for the output of a hidden layer, right?
From this video by Andrew Ng around 5:00
How are $\delta_3$ and $\delta_2$ derived? In fact, what does $\delta_3$ even mean? $\delta_4$ is got by comparing to y, no such comparison is possible for the output of a hidden layer, right?