2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/99740Motivated by issues in string theory and M-theory, we provide a pedestrian introduction to automorphic forms and theta series, emphasizing examples rather than generality.22 pages, to appear in the Proceedings of Les Houches Winter School ``Frontiers in Number Theory, Physics and Geometry'', March 9-21, 2003; v2: minor changes and clarifications, section 3.5 on pure spinors has been rewrittenHigh Energy Physics - Theorytext