Ego-munities: Exploring Socially Cohesive Person-based Communities

New Ressearch Report by Adrien Friggeri — Guillaume Chelius — Eric Fleury.

In the last few years, there has been a great interest in detecting overlap- ping communities in complex networks, which is understood as dense groups of nodes featuring a low outbound density. To date, most methods used to com- pute such communities stem from the field of disjoint community detection by either extending the concept of modularity to an overlapping context or by at- tempting to decompose the whole set of nodes into several possibly overlapping subsets. In this report we take an orthogonal approach by introducing a metric, the cohesion, rooted in sociological considerations. The cohesion quantifies the community-ness of one given set of nodes, based on the notions of triangles – triplets of connected nodes – and weak ties, instead of the classical view using only edge density. A set of nodes has a high cohesion if it features a high den- sity of triangles and intersects few triangles with the rest of the network. As such, we introduce a numerical characterization of communities: sets of nodes featuring a high cohesion. We then present a new approach to the problem of overlapping communities by introducing the concept of ego-munities, which are subjective communities centered around a given node, specifically inside its neighborhood. We build upon the cohesion to construct a heuristic algorithm which outputs a node’s ego-munities by attempting to maximize their cohesion. We illustrate the pertinence of our method with a detailed description of one person’s ego-munities among Facebook friends. We finally conclude by describ- ing promising applications of ego-munities such as information inference and interest recommendations, and present a possible extension to cohesion in the case of weighted networks.