2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/40630In this series of lectures we present the universal method based on the use of the functional integrals to derive the bound state equations for different two-body systems in elementary particle physics as well as in condensed matter theory: the relativistic Nambu-Jona-Lasinio equation and the relativistic Bethe-Salpeter equation in elementary particle physics, the Bethe-Salpeter equation for the bipolaritons. The bound state equation for the three-body systems is also established.25 pages, no figure. Lectures at the 7-th International School in Theoretical Physics, Hanoi 23 December 2000-05 January 2001High Energy Physics - Phenomenologytext