1. The problem statement, all variables and given/known data Show that the mean value of a time-independent operator over an energy eigenstate is constant in time. 2. Relevant equations Ehrenfest theorem 3. The attempt at a solution I get most of it, I'm just wondering how to say/show that this operator will commute with the Hamiltonian operator. Is it correct to say that because they both act on energy eigenstates, they must commute? If so, is there a way to show this?