Minimum degree $k$ and $k$-connectedness usually arrive together

TL;DR

This paper proves that in certain regular graphs, the time to reach minimum degree $k$ and $k$-connectedness in a random edge addition process usually coincide, confirming a conjecture and highlighting conditions for this equivalence.

Contribution

It establishes conditions under which the hitting times for minimum degree and $k$-connectedness align in the random graph process, confirming a conjecture and extending understanding to pseudo-random graphs.

Findings

Hitting times of minimum degree and $k$-connectedness typically coincide under mild expansion conditions.

The result applies to high-dimensional product graphs and pseudo-random graphs.

The paper shows the tightness of the result with specific counterexamples.

Abstract

Let $d,n\in \mathbb{N}$ be such that $d=\omega(1)$, and $d\le n^{1-a}$ for some constant $a>0$. Consider a $d$-regular graph $G=(V, E)$ and the random graph process that starts with the empty graph $G(0)$ and at each step $G(i)$ is obtained from $G(i-1)$ by adding uniformly at random a new edge from $E$. We show that if $G$ satisfies some (very) mild global edge-expansion, and an almost optimal edge-expansion of sets up to order $O(d\log n)$, then for any constant $k\in \mathbb{N}$ in the random graph process on $G$, typically the hitting times of minimum degree at least $k$ and of $k$-connectedness are equal. This, in particular, covers both $d$-regular high dimensional product graphs and pseudo-random graphs, and confirms a conjecture of Joos from 2015. We further demonstrate that this result is tight in the sense that there are $d$-regular $n$-vertex graphs with optimal edge-expansion of sets up to order $\Omega(d)$, for which the probability threshold of minimum degree at least one is different than the probability threshold of connectivity.