**Start from the Lanczos tridiagonalization of your uperturbed Hamiltonian T.****In zero order, consider just the "QES" cases which terminate****(illustration: V = r^2 + a r^2/(1 + g r^2)).****Recall the****modified Rayleigh-Schrödinger perturbation recipe****where****the wavefunctions become easily defined (by recurrences) in all the higher orders, and****the current diagonalization of H_0 is replaced by the (much simpler) recurrent inversion.****Published in Phys. Lett. A 155 (1991) 87 - 93.**