Hi there I'm stuck on one of the normalization question, I'd be glad if you can help me out. The questions is, psi(r)=Ne^(-ar) where N is a normalization constant and a is a known real parameter. Solution reads, 1=int dr lpsi(r)l^2 = 4piN^2 int (from 0 to infinity) dr r^2 e^(-2ar) What I don't get is how to get integral from 0 to infinity?(instead of -infinity to +infinity) with extra factor 4pi r^2? Thanks for your help P.S. the solution also says, use the following integral, n larger than or equal to 0: int(0 to infinity)dx x^n e^(-x)=n factorial I don't get where you use above integral.