Here is a question that I have found in my quantum text which I have been thinking about for a few days and am unable to make sense of. If there is an operator A whose commutator with the Hamiltonian H is the constant c. [H,A]=c Find <A> at t>0, given that the system is in a normalized eigenstate of A at t=0 corresponding to the eigenvalue of a. -So here is what I am thinking. Please tell me what I am doing wrong. I know that where p is momentum operator [H,p]=ihdV/dx, and V is some potential in the Hamiltonian and d<p>/dt=-<dV/dx> So where [H,A]=c, wouldn't d<A>/dt=0? And then there would be no change in the eigenstate of A with time, and <A>=a which is its initial value at 0, which would be a(Psi) where Psi is my wave function or eigenvector or whatever I need it to be. Am I understanding the question wrong?