Tight Practical Bounds for Subgraph Densities in Ego-centric Networks

TL;DR

This paper introduces tighter bounds on subgraph densities in ego-centric networks and a new metric, the subgraph spread ratio, to compare network structures, with applications in social media analysis.

Contribution

It develops novel bounds on subgraph densities using flag algebras and topological data analysis, and introduces the subgraph spread ratio for network comparison.

Findings

Feasible regions are up to three times tighter than previous results.

Social networks have smaller subgraph spread ratios than other network types.

The subgraph spread ratio effectively quantifies network structural differences.

Abstract

Subgraph densities play a crucial role in network analysis, especially for the identification and interpretation of meaningful substructures in complex graphs. Localized subgraph densities, in particular, can provide valuable insights into graph structures. Distinguishing between mathematically-determined and domain-driven subgraph density features, however, poses challenges. For instance, the lack or presence of certain structures can be explained by graph density or degree distribution. These differences are especially meaningful in applied contexts as they allow us to identify instances where the data induces specific network structures, such as friendships in social networks. The goal of this paper is to measure these differences across various types of graphs, conducting social media analysis from a network perspective. To this end, we first provide tighter bounds on subgraph densities. We then introduce the subgraph spread ratio to quantify the realized subgraph densities of specific networks relative to the feasible bounds. Our novel approach combines techniques from flag algebras, motif-counting, and topological data analysis. Crucially, effective adoption of the state-of-the-art in the plain flag algebra method yields feasible regions up to three times tighter than prior best-known results, thereby enabling more accurate and direct comparisons across graphs. We additionally perform an empirical analysis of 11 real-world networks. We observe that social networks consistently have smaller subgraph spread ratios than other types of networks, such as linkage-mapping networks for Wikipedia pages. This aligns with our intuition about social relationships: such networks have meaningful structure that makes them distinct. The subgraph spread ratio enables the quantification of intuitive understandings of network structures and provides a metric for comparing types of networks.