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Abstract: We provide novel lower bounds on the Betti numbers of Vietoris-Rips complexes of hypercube graphs of all dimensions, and at all scales. In more detail, let $Q_n$ be the vertex set of $2^n$ vertices in the $n$-dimensional hypercube graph, equipped with the shortest path metric. Let ${\rm VR}(Q_n;r)$ be its Vietoris-Rips complex at scale parameter $r \ge 0$, which has $Q_n$ as its vertex set, and all subsets of diameter at most $r$ as its simplices. For integers $r < r'$ the inclusion ${\rm VR}(Q_n;r) \hookrightarrow {\rm VR}(Q_n;r')$ is nullhomotopic, meaning no persistent homology bars have length longer than one, and we therefore focus attention on the individual spaces ${\rm VR}(Q_n;r)$. We provide lower bounds on the ranks of homology groups of ${\rm VR}(Q_n;r)$. For example, using cross-polytopal generators, we prove that the rank of $H_{2^r-1}({\rm VR}(Q_n;r))$ is at least $2^{n-(r+1)}\binom{n}{r+1}$. We also prove a version of homology propagation: if $q\ge 1$ and if $p$ is the smallest integer for which ${\rm rank} H_q({\rm VR}(Q_p;r)) \neq 0$, then ${\rm rank} H_q({\rm VR}(Q_n;r)) \ge \sum_{i=p}^n 2^{i-p} \binom{i-1}{p-1} \cdot {\rm rank} H_q({\rm VR}(Q_p;r))$ for all $n \ge p$. When $r \le 3$, this result and variants thereof provide tight lower bounds on the rank of $H_q({\rm VR}(Q_n;r))$ for all $n$, and for each $r \ge 4$ we produce novel lower bounds on the ranks of homology groups. Furthermore, we show that for each $r\ge 2$, the homology groups of ${\rm VR}(Q_n;r)$ for $n \ge 2r+1$ contain propagated homology not induced by the initial cross-polytopal generators.
Keywords: Vietoris–Rips complexes, clique complexes, hypercubes, Betti numbers
Published in DiRROS: 05.03.2024; Views: 136; Downloads: 42
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