A Sharp Threshold for Spanning 2-Spheres in Random 2-Complexes

Abstract

A Hamiltonian cycle in a graph is a spanning subgraph that is homeomorphic to a circle. With this in mind, it is natural to define a Hamiltonian d-sphere in a d-dimensional simplicial complex as a spanning subcomplex that is homeomorphic to a d-dimensional sphere. We consider the Linial-Meshulam model for random simplicial complexes, and prove that there is a sharp threshold at $p=\sqrt{\frac{e}{γn}}$ for the appearance of a Hamiltonian $2$-sphere in a random $2$-complex, where $γ= 4^4/3^3$.

Main changes from last version: An explanation of the proof structure was added as a subsection in the introduction. A computational mistake in Lemma 4.5 was corrected, and a result some constants throughout the proof had to be modified (in the definition of good graph, in the final estimation) . Some other minor corrections