Shared weights problem in CNN while trying a problem

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C#

artificial-intelligence

Hi,

I'm trying to learn CNN (convolutional neural network), but I have problem with shared weights.
I mean, when I have a single weight, I don't know how to obtain new value in backpropagation algorithm.

In regular multilayer network new value is easy to calculate, it is:

w(i,j) += LearningRate * MyNeuron.Error * DerivativeActivationFunction(MyNeuron.Sum) * Neurons[m-i,k,j].Output;

where :

MyNeuron - is a neuron class which you enter our weight
MyNeuron.Error - is an error value which we calculated from next layer 
DerivativeActivationFunction() - is Derivate of activation function
MyNeuron.Sum - is sum of multiplication weights with neurons outputs from early layer  
Neurons[m-i,k,j].Output - is a neuron output from earlier layer (here comes our weight) 

But when I have shared weight, I don't know what to do, should I calculate an average from every neurons?

some help:
http://piczasso.com/i/es81j.png

Please help!