2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/35669In this lecture we address some topological questions connected with the existence on a general spacetime manifold of diffeomorphisms connected to the identity which reverse the time-orientation.24 pages LaTeX, short version of an extended paper still under construction, to appear in the Proc. of the Erice School on ``String gravity and physics at the Planck scale'' (Erice, 8-19 September 1995)General Relativity and Quantum Cosmologytext