//i = layer we are currently working in
for (j = 0; j < outputs[i].length; j++) {
    Sum = 0;
    for (k = 0; k < outputs[i + 1].length; k++) {
        Sum = Sum + weights[i + 1][k][j] * error_signals[i + 1][k];
    }
    error_signals[i][j] = derivatives[i][j] * Sum;
}