2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/57997A remarkable extension of Rayleigh-Schroedinger perturbation method is found. Its (N+q) x (N+1) - dimensional Hamiltonians (as emerging, e.g., during quasi-exact constructions of bound states) are non-square matrices at q > 1. The role of an eigenvalue is played by an energy/coupling q-plet. In all orders, its perturbations are defined via a q-dimensional inversion.21 pagesMathematical Physics81Q05; 34L16; 34M25; 65F15; 15A90text