Matrix dimensions are not matching after differentiation

[sakian]

I'm doing some work with neural networks lately and I'm having trouble with this seemingly simple equation.

The equation describing the network is:

y = \psi(W3 x \psi(W2 x \psi(W1 x I)))​

Where:

y (scalar) is the output value​

W1 (2x2 matrix) are the 1st layer weights​

W2 (2x2 matrix) are the 2nd layer weights​

W3 (1x2 matrix) are the output layer weight​

I (2x1 vector) is the input vector​

\psi is the activation function (log sigmoid)​

I'm trying to differentiate the equation by the weight matrices (using the chain rule) but I'm getting equations that don't work. When I try to differentiate by W1 I get:

dy/dW1 = \psi' (W3 x \psi(W2 x \psi(W1 x I))) x W3 x \psi' (W2 x \psi(W1 x I)) x W2 x \psi' (W1 x I) x I​

When I try to calculate I'm getting matrix dimension mismatches. Am I doing something wrong?

 

[Stephen Tashi]

I don't know the answer to your question, but I'm curious what definition you are using for the derivative of a function with respect to a matrix. Also, do you know a link that gives a rule for the derivative of a product of matrices with respect to a matrix?

I find it interesting that the current Wikipedia has a discussion page that brings up some of these issues ( http://en.wikipedia.org/wiki/Talk:Matrix_calculus ) but there is no article to go with it!