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OP-00864 Open

A strict gap between face and cycle percolation thresholds

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Fix integers $D \ge 3$ and $2 \le d < D$, and let $X_\lambda$ be a homogeneous Poisson point process of intensity $\lambda > 0$ in $\mathbb{R}^D$. Consider the Vietoris–Rips complex on $X_\lambda$ whose simplices are the finite subsets with all pairwise Euclidean distances at most $1$, and call two $d$-simplices adjacent when they share a $(d-1)$-face. Let $F_d$ be the event that this adjacency graph on $d$-simplices has an infinite connected component, and $C_d$ the event that there is an infinite, face-connected collection $M$ of $d$-simplices in which every $(d-1)$-simplex is a face of finitely many, and an even number, of members of $M$. Such an $M$ need not represent a nonzero homology class. Put $$\lambda_d^{\mathrm{face}} = \inf\{\lambda > 0 : \mathbb{P}(F_d) > 0\}, \qquad \lambda_d^{\mathrm{cycle}} = \inf\{\lambda > 0 : \mathbb{P}(C_d) > 0\}.$$ Hirsch and Valesin proved $$\lambda_1^{\mathrm{face}} = \lambda_1^{\mathrm{cycle}} \le \lambda_2^{\mathrm{face}} \le \lambda_2^{\mathrm{cycle}} \le \cdots \le \lambda_{D-1}^{\mathrm{face}} \le \lambda_{D-1}^{\mathrm{cycle}} \le \lambda_D^{\mathrm{face}},$$ and that the phase transition for face percolation is sharp. They also proved $\lambda^*_{d,r_0} < \lambda_d^{\mathrm{face}}$ for every $r_0 > 2$, where $\lambda^*_{d,r_0}$ is the threshold of a variant in which two $d$-simplices count as adjacent when their Euclidean distance is at most $r_0$.

Problem. Is $\lambda_d^{\mathrm{face}} < \lambda_d^{\mathrm{cycle}}$ for every $D \ge 3$ and every $2 \le d < D$?

Source: Hirsch and Valesin (2025), the conjecture following their Theorem 2 (arXiv 2022).

Imported from github.com/MColbrook/AIM.

Classification

Primary
PR — Probability
Secondary
AT
Keywords
AIM Open Applied Problems, continuum percolation, random simplicial complexes

References

@article{hirsch2025,
  author        = {Hirsch, Christian and Valesin, Daniel},
  title         = {Face and cycle percolation},
  journal       = {J. Appl. Comput. Topol.},
  volume        = {9},
  number        = {1},
  year          = {2025},
  pages         = {6},
  doi           = {10.1007/s41468-025-00202-2},
  eprint        = {2212.06243},
  archivePrefix = {arXiv},
}
@misc{aim-face-cycle-percolation-gap,
  author       = {Brunton, Steve and Colbrook, Matthew J. and de Hoop, Maarten V. and Stepaniants, George and Townsend, Alex and Ward, Rachel},
  title        = {{A strict gap between face and cycle percolation thresholds}},
  howpublished = {AIM --- Open Applied Problems, GitHub repository},
  year         = {2026},
  url          = {https://github.com/MColbrook/AIM/blob/24edaadc6bb272737d8da0745c662c992db07b22/problems/502-face-cycle-percolation-gap.md},
  note         = {Entry 502 at commit 24edaad, 2026-10-15},
}

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OP-00864, "A strict gap between face and cycle percolation thresholds", clopen.solutions.