Topology & Geometric Group Theory Seminar

Fall 2009

1:30 – 2:30, Malott 203

Tuesday, September 22

Tim Riley, Cornell University

Hydra groups

I will describe a family of groups that are CAT(0), 1-relator, and free-by-cyclic, and yet have free subgroups of extraordinary (Ackermannian) distortion. This wild geometry, in an apparently benign setting, has its origins in a simple computational game—a realisation of Hercules' battle with the hydra, played out in string rewriting. This is work with Will Dison.

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