1. The problem statement, all variables and given/known data Hi. For the question given (attachment) where does the factor of 2PI in the function for sigma come from in the first step because I don't get it in my working for the first step. 2. Relevant equations see attachment 3. The attempt at a solution I got sigma (without the 2PI) by changing variable to t =cos theta, thus integrating between minus and plus 1 and using m=n in the legendre polynomial, so I am wondering if the 2PI came from integrating over the phi dependence as we are dealing with scattering from a radial potential? I also need prodding in the right direction for the next step!