[Bug-gnubg] Backpropagation in neural net.

[Øystein Johansen]

Gary or Joseph,

I'm studying the backpropagation implemetation in neuralnet.c (NeuralNetTrain). 
I notice that the weights are not updated according to derivative of the 
sigmoid function. (In fact the sigmoid function is not called at all in this 
function.) I have tried to find an explanation of this in litterature, but I've 
not found any. 

Can you please explain how this works and why it works, and/or suggest a 
reference to a book where this algoritm is explained?

And what's the difference of NeuralNetTrain and NeuralNetTrainS? Is the 
difference just that NeuralNetTrainS doesn't update the output weights? When is 
this used?

-Øystein