Resumen de Boundaries of cocompact proper CAT(0) spaces

Ross Geoghegan, Pedro Ontaneda

• A proper CAT(0) metric space is cocompact if it has a compact generating domain with respect to its full isometry group. Any proper CAT(0) space, cocompact or not, has a compact metrizable boundary at infinity ; indeed, up to homeomorphism, this boundary is arbitrary. However, cocompactness imposes restrictions on what the boundary can be. Swenson showed that the boundary of a cocompact has to be finite-dimensional. Here we show more: the dimension of has to be equal to the global Cech cohomological dimension of . For example: a compact manifold with non-empty boundary cannot be with cocompact. We include two consequences of this topological/geometric fact: (1) The dimension of the boundary is a quasi-isometry invariant of CAT(0) groups. (2) Geodesic segments in a cocompact can ¿almost¿ be extended to geodesic rays, i.e. is almost geodesically complete