### University of Arkansas Topology Seminar: 11/19/2020

### Speaker: Nima Hoda

### Title: Hierarchical hyperbolicity, coarse Helly spaces and the strong shortcut property

This talk concerns three classes of metric spaces defined by
nonpositive curvature-like properties: hierarchically hyperbolic
spaces, coarse Helly spaces and strongly shortcut spaces. Roughly
speaking, hierarchically hyperbolic spaces are metric spaces whose
geometry can be understood through a structured family of projections
onto Gromov-hyperbolic spaces; coarse Helly spaces are metric spaces
whose balls satisfy a coarse version of the classical 1-dimensional
Helly property; and strongly shortcut spaces are metric spaces whose
subspaces cannot approximate long circles with arbitrary precision.

I will discuss recent joint work with Thomas Haettel and Harry Petyt
in which we prove that hierarchically hyperbolic spaces are coarsely
Helly and that coarse Helly spaces of uniformly bounded geometry are
strongly shortcut. This work has important consequences for
hierarchically hyperbolic groups: it shows that they are
semihyperbolic, have solvable conjugacy problem, are of type
FP_{∞}, have finitely many conjugacy classes of finite subgroups
and that their finitely generated abelian subgroups are undistorted.

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