1. The problem statement, all variables and given/known data |phi (n)> being eigen states of hermitian operator H ( H could be for example the hamiltonian of anyone physical system ). The states |phi (n)> form an orthonormal discrete basis. The operator U(m,n) is defined by: U(m,n)= |phi(m)><phi(n)| Calculate the commutator: [H,U(m,n)] ( this is part of the first problem in Cohen, Tannoudji, Diu, Laloe textbook in quantum mechanics.) 3. The attempt at a solution[itex]\[/itex] [HU-UH] (ψ) = H|phi(m)><phi (n)|ψ> - |phi (m)><phi(n)| H| ψ> = <phi(n)|ψ> H |phi (m>) - |phi (m><phi(n)| <ψ | H and then ? i did not find symbol phi.