I recently had to do an integral like the one in the thread below: http://math.stackexchange.com/quest...-of-radial-function-without-bessel-and-neuman The problem I had was also evaluating the product and I am quite sure that the answer in the thread is the one I need. I just don't understand it fully. They say we fix our x-vector such that its angle with the z-axis is the same as its dot product with the other vector. But isn't x an everchanging vector? I meant we are integrating over it. What am I missing?