Combinatorial approach to Modularity

Filippo Radicchi1, Andrea Lancichinetti1,2 and José J. Ramasco1
1Complex Networks Lagrange Laboratory (CNLL), ISI Foundation, Turin I-10133, Italy.
2Physics Department, Politecnico di Torino, Turin, Italy.

(April 2010)

Communities are clusters of nodes with a higher than average density of internal connections. Their detection is of great relevance to better understand the structure and hierarchies present in a network. Modularity has become a golden standard in the area of community detection, providing at the same time a tool to evaluate partitions and, by maximizing it, a method to find communities. In this work, we study the modularity from a combinatorial point of view. Our analysis (as the modularity definition) relies on the use of the configurational model, a technique that given a graph produces a series of randomized copies keeping the degree sequence invariant. We develop an approach that enumerates the null model partitions and can be used to calculate the probability distribution function of the modularity. Our theory allows for a deep inquiry of several interesting features characterizing modularity such as its resolution limit and the statistics of the partitions that maximize it. Additionally, the study of the probability of extremes of the modularity in the random graph partitions opens the way for a definition of the statistical significance of network partitions.