2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/111558This is a survey article with a limited list of references (as required by the publisher) which appears in the Encyclopedia of Mathematical Physics, eds. J.-P. Francoise, G.L. Naber and Tsou S.T. Oxford: Elsevier, 2006. vol.4, pp.94--104.19 pages + title pageDifferential Geometry53C50 primary; 22E25, 53B30, 53C30 secondarytext