Hitting-time conjecture for the giant shadow and homological giant

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Let Yd(n)\mathcal{Y}_d(n) be the stochastic Linial–Meshulam process in dimension d≥2d\geq2. Let LT(n)LT(n) denote its largest torsion group, and for a dd-complex YY with complete (d−1)(d-1)-skeleton over R\mathbb{R} let SHR(Y)SH_{\mathbb{R}}(Y) be its shadow. A core is a subcomplex in which every (d−1)(d-1)-face lies in at least two dd-faces. Hitting-time conjecture. There exists a constant δ\delta depending on dd such that, with high probability, there is an m0∈{1,…,(nd+1)}m_0\in\{1,\ldots,\binom{n}{d+1}\} for which: (1) the torsion part of Hd−1(Yd(n,m0))H_{d-1}(Y_d(n,m_0)) is isomorphic to LT(n)LT(n); (2) Yd(n,m0)Y_d(n,m_0) contains a spanning core Y′Y' with Hd−1(Y′)≅LT(n)H_{d-1}(Y')\cong LT(n), whereas Yd(n,m0−1)Y_d(n,m_0-1) contains no such core; and (3) ∣SHR(Yd(n,m0))∣≥δnd+1|SH_{\mathbb{R}}(Y_d(n,m_0))|\geq\delta n^{d+1} while ∣SHR(Yd(n,m0−1))∣≤O(n)|SH_{\mathbb{R}}(Y_d(n,m_0-1))|\leq O(n). This conjecture identifies the hitting time of the homological giant with that of the giant shadow and predicts a one-step jump from a small to a giant shadow; the source attributes the shadow-jump prediction to Linial and Peled and presents the stronger structural statement as conjectural.

References

Primary source

Matthew Kahle, Frank Lutz, Andrew Newman and Kyle Parsons, “Cohen–Lenstra heuristics for torsion in homology of random complexes”, arXiv:1710.05683 (2018).

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