Thresholds for vanishing of `Isolated' faces in random \v{C}ech and Vietoris-Rips complexes

TL;DR

This paper investigates the precise thresholds at which isolated faces in random cech and Vietoris-Rips complexes vanish, revealing detailed bounds and differences in phase transitions for these models as the number of points grows large.

Contribution

It provides tighter bounds on the threshold constants for face vanishing and highlights differences between cech and Vietoris-Rips complexes, including second-order effects.

Findings

Threshold radius for face vanishing is  ((rac{\u2212 extlog n}{n})^{1/d})

Differences in phase transition behavior between cech and Vietoris-Rips complexes

Non-monotonicity of isolated face counts affects phase transition analysis.

Abstract

We study combinatorial connectivity for two models of random geometric complexes. These two models - \v{C}ech and Vietoris-Rips complexes - are built on a homogeneous Poisson point process of intensity $n$ on a $d$-dimensional torus using balls of radius $r_n$. In the former, the $k$-simplices/faces are formed by subsets of $(k+1)$ Poisson points such that the balls of radius $r_n$ centred at these points have a mutual interesection and in the latter, we require only a pairwise intersection of the balls. Given a (simplicial) complex (i.e., a collection of $k$-simplices for all $k \geq 1$), we can connect $k$-simplices via $(k+1)$-simplices (`up-connectivity') or via $(k-1)$-simplices (`down-connectivity). Our interest is to understand these two combinatorial notions of connectivity for the random \v{C}ech and Vietoris-Rips complexes asymptically as $n \to \infty$. In particular, we analyse in detail the threshold radius for vanishing of isolated $k$-faces for up and down connectivity of both types of random geometric complexes. Though it is expected that the threshold radius $r_n = \Theta((\frac{\log n}{n})^{1/d})$ in coarse scale, our results give tighter bounds on the constants in the logarithmic scale as well as shed light on the possible second-order correction factors. Further, they also reveal interesting differences between the phase transition in the \v{C}ech and Vietoris-Rips cases. The analysis is interesting due to the non-monotonicity of the number of isolated $k$-faces (as a function of the radius) and leads one to consider `monotonic' vanishing of isolated $k$-faces. The latter coincides with the vanishing threshold mentioned above at a coarse scale (i.e., $\log n$ scale) but differs in the $\log \log n$ scale for the \v{C}ech complex with $k = 1$ in the up-connected case.