ABSTRACT
Often one meets a problem in quantum mechanics that can’t be solved exactly, but looks very similar to a problem that can be solved exactly. For example, suppose that we have to find the energies and eigenstates of a Hamiltonian of the form () https://www.w3.org/1998/Math/MathML"> H = H 0 + V https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429499265/ff9ae276-bf0f-4700-9154-eaa3ea82fbc6/content/eq1244.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>
