I'm standing here looking at this problems for 1 hour already and don't know how to handle them. The problem is that the exam is tomorrow. I thought I had it in hand because I pretty much knew how to work my way through the CG coefficients in quantum mechanics. There you have the recurence relations, but here it's just general conditions. Problem no.1: calculate the CG coefficient if S is an invariant operator. there is no coherent solution attempt from me. I start with Wigner-Eckart to get: [tex] C^{\star}(\alpha\beta\gamma,ijk)=\frac{<\tilde{\phi}^\gamma_k|S^\alpha_i|\phi^\beta_j>} {<\tilde{\phi}^\gamma||S^\alpha||\phi^\beta>} [/tex] as S is invariant [tex]\alpha=0[/tex] and there is no more i necessary. then what? introduce some [tex]T[/tex] and [tex]T^{-1}[/tex] and get what? Probably I'm not introducing these T's correctly because I have the feeling I'm running around in circles as I return to the same formula. I shoud get a damn number. What am I missing? Problem no.2: Having a parity invariant potential V prove that [tex]<\phi|V|\psi>=0[/tex] if [tex]\psi[/tex] is even and [tex]\phi[/tex] is odd using WE theorem. [tex] <\phi|V|\psi>=<\phi\pi^{-1}|\pi V \pi^{-1}|\pi\psi>=<-\phi|V|\psi>=-<\phi|V|\psi>=0 [/tex] where does WE come in? Please throw me a bone. Even a small one, but remember I'm cramming here.