Randomly Generated Subgroups of the Symmetric Group and Random Lifts of Graphs

TL;DR

This paper analyzes the probability that random lifts of graphs are connected and expand well, using group theory to derive exact probabilities and extend results to iterated lifts, advancing understanding of random graph coverings.

Contribution

It introduces new group-theoretic methods to precisely estimate connectivity and expansion probabilities in random graph lifts, including iterated lifts.

Findings

Probability that a random lift of a connected graph is connected is equal to the transitivity probability of a subgroup generated by Betti number elements.

Derived exact probabilities for transitivity of subgroups in wreath products of symmetric groups.

Extended results to iterated random lifts, showing similar properties hold repeatedly.

Abstract

Amit and Linial showed that a random lift of a graph with minimum degree $\delta\ge3$ is asymptotically almost surely $\delta$-connected, and mentioned the problem of estimating this probability as a function of the degree of the lift. We relate a randomly generated subgroup of the symmetric group on $n$ elements to random $n$-lifts of a graph and use it to provide such an estimate along with related results. We also improve their later result showing a lower bound on the edge expansion on random lifts. Our proofs rely on new ideas from group theory which make several improvements possible. We exactly calculate the probability that a random lift of a connected graph with first Betti number $l$ is connected by showing that it is equal to the probability that a subgroup of the symmetric group generated by $l$ random elements is transitive. We also calculate the probability that a subgroup of a wreath product of symmetric groups generated by $l$ random generators is transitive. We show the existence of homotopy invariants in random covering graphs which reduces some of their properties to those of random regular multigraphs, and in particular makes it possible to compute the exact probability with which random regular multigraphs are connected. All our results about random lifts easily extend to iterated random lifts.