On the modularity of 3-regular random graphs and random graphs with given degree sequences

TL;DR

This paper investigates the modularity of random 3-regular graphs and graphs with given degree sequences, establishing asymptotic bounds and analyzing community structure in different regimes.

Contribution

It proves new asymptotic bounds for the modularity of 3-regular graphs and analyzes modularity behavior in graphs with specified degree sequences.

Findings

Modularity of 3-regular graphs is at least 0.667026 asymptotically almost surely.

Upper bound for modularity in 3-regular graphs is improved to 0.789998.

In graphs with given degree sequences, modularity behaves differently in subcritical and supercritical regimes.

Abstract

The modularity of a graph is a parameter that measures its community structure; the higher its value (between $0$ and $1$), the more clustered the graph is. In this paper we show that the modularity of a random $3$-regular graph is at least $0.667026$ asymptotically almost surely (a.a.s.), thereby proving a conjecture of McDiarmid and Skerman. We also improve the a.a.s. upper bound given therein to $0.789998$. For a uniformly chosen graph $G_n$ over a given bounded degree sequence with average degree $d(G_n)$ and with $|CC(G_n)|$ many connected components, we distinguish two regimes with respect to the existence of a giant component. In the subcritical regime, we compute the second term of the modularity. In the supercritical regime, we prove that there is $\varepsilon > 0$, for which the modularity is a.a.s. at least \begin{equation*} \dfrac{2\left(1 - \mu\right)}{d(G_n)}+\varepsilon, \end{equation*} where $\mu$ is the asymptotically almost sure limit of $\dfrac{|CC(G_n)|}{n}$.