I'm trying to solve problem 9.1 from griffiths book on quantum mechanics. I've written down the hydrogen wavefunctions for all combinations allowed of n, l and m. Now according to griffiths (and my memory) only one excitation is actually allowed between n=1 and n=2. This would minimize the amount of integration. Problem is, I can't find out wich wave functions have no business here. Any help would be welcome, spec. if you could give the correct set of n,l,m and motivation (so don't start TEX-ing in all the wave functions, there in the book).