2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/77137Techniques of gauge theory are used to define and compute an invariant of certain diffeomorphisms of 4-manifolds. The invariant vanishes for any diffeomorphism which is smoothly isotopic to the identity. As an application, we give the first example of a diffeomorphism of a simply-connected 4-manifold which is homotopic to the identity map, but not smoothly isotopic to the identity.Minor revisions (typos and one formula). AMS-LaTeX. 17 pages, 2 postscript figures. To appear in Mathematical Research LettersGeometric TopologyDifferential Geometrytext