Vertex Percolation on Expander Graphs

TL;DR

This paper investigates how randomly deleting vertices from expander graphs affects their structure, showing that under certain conditions, a large expander component persists with high probability.

Contribution

It provides a detailed analysis of vertex percolation on expander graphs, including bounds for the existence of a giant expander component after random deletions.

Findings

A giant expander component remains after vertex deletion in bounded degree expanders.

Conditions under which the graph stays connected after random vertex removal.

Extension of results to random regular graphs and graphs with unbounded expansion ratios.

Abstract

We say that a graph $G=(V,E)$ on $n$ vertices is a $\beta$-expander for some constant $\beta>0$ if every $U\subseteq V$ of cardinality $|U|\leq \frac{n}{2}$ satisfies $|N_G(U)|\geq \beta|U|$ where $N_G(U)$ denotes the neighborhood of $U$. In this work we explore the process of deleting vertices of a $\beta$-expander independently at random with probability $n^{-\alpha}$ for some constant $\alpha>0$, and study the properties of the resulting graph. Our main result states that as $n$ tends to infinity, the deletion process performed on a $\beta$-expander graph of bounded degree will result with high probability in a graph composed of a giant component containing $n-o(n)$ vertices that is in itself an expander graph, and constant size components. We proceed by applying the main result to expander graphs with a positive spectral gap. In the particular case of $(n,d,\lambda)$-graphs, that are such expanders, we compute the values of $\alpha$, under additional constraints on the graph, for which with high probability the resulting graph will stay connected, or will be composed of a giant component and isolated vertices. As a graph sampled from the uniform probability space of $d$-regular graphs with high probability is an expander and meets the additional constraints, this result strengthens a recent result due to Greenhill, Holt and Wormald about vertex percolation on random $d$-regular graphs. We conclude by showing that performing the above described deletion process on graphs that expand sub-linear sets by an unbounded expansion ratio, with high probability results in a connected expander graph.