2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/66953This is a survey on the subject of the title corresponding to three lectures I gave in June 2001 at the Workshop on Fourier Analysis and Convexity, at the Universita di Milano-Biccoca.52 pages, 15 figures 2 references added (Leptin and Mueller, Mackey) from first versionClassical Analysis and ODEsMathematical Physics42 52text