2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/58225A resolvent formula, originally presented by Karner in his habilitation, is discussed. First the formula is considered abstractly and then it is demonstrated on an explicit example -- the so called simplified Fermi accelerator.LaTeX, to appear in Proceedings of the conference "Mathematical Results in Quantum Mechanics" (Prague, June 1998)Mathematical Physics81Q10; 47A10 ; 47B15text