Boundaries of cocompact proper CAT(0) spaces

A proper CAT(0) metric space X 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 ∂ ∞ X ; indeed, up to homeomorphism, this boundary is arbitrary. However, cocompactness...

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Published in:Topology (Oxford) 2007-03, Vol.46 (2), p.129-137
Main Authors: Geoghegan, Ross, Ontaneda, Pedro
Format: Article
Language:English
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Summary:A proper CAT(0) metric space X 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 ∂ ∞ X ; 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 X has to be finite-dimensional. Here we show more: the dimension of ∂ ∞ X has to be equal to the global Čech cohomological dimension of ∂ ∞ X . For example: a compact manifold with non-empty boundary cannot be ∂ ∞ X with X 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 X can “almost” be extended to geodesic rays, i.e. X is almost geodesically complete.
ISSN:0040-9383
1879-3215
DOI:10.1016/j.top.2006.12.002