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Topology and Computing

Computing Session Talk #4.2

Subevent of Computing Session #4

HUMB 142

Times: 2025 Aug 13 from 09:00AM to 09:25AM (Central Time (US & Canada))

Webinar link: Icon: video Webinar

Homotopy connectivity of Cech complexes of spheres.

Sucharita Mallick ⟨sucharitamallick@ufl.edu⟩

Abstract:

Let $S^n$ be the $n$-sphere with the geodesic metric and of diameter $\pi$. The intrinsic \v{C}ech complex of $S^n$ at scale $r$ is the nerve of all open balls of radius $r$ in $S^n$. In this talk, we will show how to control the homotopy connectivity of \v{C}ech complexes of spheres at each scale between $0$ and $\pi$ in terms of coverings of spheres. Our upper bound on the connectivity, which is sharp in the case $n=1$, comes from the chromatic numbers of Borsuk graphs of spheres. Our lower bound is obtained using the conicity (in the sense of Barmak) of \v{C}ech complexes of the sufficiently dense, finite subsets of $S^n$. Our bounds imply the new result that for $n\ge 1$, the homotopy type of the \v{C}ech complex of $S^n$ at scale $r$ changes infinitely many times as $r$ varies over $(0,\pi)$. Additionally, we lower bound the homological dimension of \v{C}ech complexes of finite subsets of $S^n$ in terms of their packings. This is joint work with Henry Adams and Ekansh Jauhari.

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