2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/54386The mathematical description of stable particle-like systems appearing in relativistic quantum field theory at large, respectively small scales or non-zero temperatures is discussed.28 pages, latex; This paper has already appeared in "Mathematical Physics Towards the 21st Century", R. Sen, A. Gersten eds. Beer-Sheva, Ben Gurion University Press 1994High Energy Physics - Theorytext