Hitting-time conjecture for the giant shadow and homological giant

Let Yd(n)\mathcal{Y}_d(n) be the stochastic Linial–Meshulam process in dimension d2d\geq2. Let LT(n)LT(n) denote its largest torsion group, and for a dd-complex YY with complete (d1)(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 (d1)(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 Hd1(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 YY' with Hd1(Y)LT(n)H_{d-1}(Y')\cong LT(n), whereas Yd(n,m01)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,m01))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.

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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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