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

Lyuben Lichev, Dieter Mitsche

Published 2020-07-30Version 1

The modularity of a graph is a parameter introduced by Newman and Girvan measuring its community structure; the higher its value (between $0$ and $1$), the more clustered a 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 stating that a random $3-$regular graph has modularity strictly larger than $\frac{2}{3}$ a.a.s. We also improve the upper bound given therein by showing that the modularity of such a graph is a.a.s. at most $0.789998$. 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 more detail, we precisely compute the second term of the modularity in the subcritical regime. In the supercritical regime, we further prove that there is $\varepsilon > 0$ depending on the degree sequence, 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 $\dfrac{|CC(G_n)|}{n}$.