Two-sided estimates of energies and the “forgotten” exactly solvable potential V (r) = -a2r−2 + b2r−4

Abstract

We show how a pair of truncations of a Hamiltonian (here, a model-space dimension M < ∞ and certain “antivariational” t < ∞) may generate a quickly convergent double sequence of approximants E[M][t] which sandwiches its bound-state energies, E[ML][tL] ⩽ E ⩽ E[MU][tU].