2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/55958The fields nonlinear modes quantization scheme is discussed. New form of the perturbation theory achieved by unitary mapping the quantum dynamics in the space $W_G$ of (action, angle)-type collective variables. It is shown why the transformed perturbation theory contributions may accumulated exactly on the boundary $\pa W_G$. Abilities of the developed formalism are illustrated by examples from quantum mechanics and field theory.14 pp., LaTex, no figures. Talk given at XXXIII PINP Winter School, Gatchina, Febr. 8-12, 1999High Energy Physics - TheoryHigh Energy Physics - Phenomenologytext