Torsion in negative curvature

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Roman Sauer
Karlsruhe Institute of Technology

A classical theorem of Gromov states that the Betti numbers, i.e. the size of the free part of the homology groups, of negatively curved manifolds are bounded by the volume. We extend this theorem to the torsion part of the homology in all dimensions d > 3. From Gromov’s work it is known that in dimension 3 the size of torsion homology cannot be bounded in terms of the volume. In dimension 4 we give a somewhat precise estimate for the number of negatively curved manifolds of finite volume, up to homotopy, and in dimension d > 4 up to homeomorphism. These results are based on an effective simplicial thick-thin decomposition which is of independent interest.

Based on joint work with Uri Bader and Tsachik Gelander.