10 Time-Independent Perturbation Theory Content: • Non-degenerate case. • Degenerate case.

Only a few quantum mechanical problems can be solved exactly. However, if the system Hamil-tonian can be separated into two parts: an unperturbed part (we know how to solve it) and a perturbation (it is small as compared to the first term) we may get an approximate solution using a perturbation approach. Suppose that we search for eigenenergies and eigenstates of a Hamiltonian H, H|k 〉 = Ek|k〉, (1) and the Hamiltonian can be written in a for