Expansion, long cycles, and complete minors in supercritical random subgraphs of the hypercube

TL;DR

This paper investigates the complex structural properties of supercritical random subgraphs of hypercubes, revealing the presence of long cycles, large minors, and high genus in the phase where a giant component emerges.

Contribution

It establishes the existence of long cycles, large complete minors, and high genus in supercritical hypercube subgraphs, extending known results from random graphs to hypercubes.

Findings

Existence of cycles of length proportional to 2^d/d^3(log d)^3

Presence of complete minors of size proportional to 2^{d/2}/d^3(log d)^3

Largest component exhibits good edge-expansion properties

Abstract

Analogous to the case of the binomial random graph $G(d+1,p)$, it is known that the behaviour of a random subgraph of a $d$-dimensional hypercube, where we include each edge independently with probability $p$, which we denote by $Q^d_p$, undergoes a phase transition around the critical value of $p=\frac{1}{d}$. More precisely, standard arguments show that significantly below this value of $p$, with probability tending to one as $d \to \infty$ (whp for short) all components of this graph have order $O(d)$, whereas Ajtai, Koml\'{o}s and Szemer\'{e}di showed that significantly above this value, in the \emph{supercritical regime}, whp there is a unique `giant' component of order $\Theta\left(2^d\right)$. In $G(d+1,p)$ much more is known about the complex structure of the random graph which emerges in this supercritical regime. For example, it is known that in this regime whp $G(d+1,p)$ contains paths and cycles of length $\Omega(d)$, as well as complete minors of order $\Omega\left(\sqrt{d}\right)$. In this paper we obtain analogous results in $Q^d_p$. In particular, we show that for supercritical $p$, i.e., when $p=\frac{1+\epsilon}{d}$ for a positive constant $\epsilon$, whp $Q^d_p$ contains a cycle of length $\Omega\left(\frac{2^d}{d^3(\log d)^3} \right)$ and a complete minor of order $\Omega\left(\frac{2^{\frac{d}{2}}}{d^3(\log d)^3 }\right)$. In order to prove these results, we show that whp the largest component of $Q^d_p$ has good edge-expansion properties, a result of independent interest. We also consider the genus of $Q^d_p$ and show that, in this regime of $p$, whp the genus is $\Omega\left(2^d\right)$.