Greed is Good for Deterministic Scale-Free Networks

TL;DR

This paper demonstrates that certain random graph models exhibit deterministic power-law properties, and shows that these properties enable constant-factor approximations for classical NP-hard problems.

Contribution

It extends prior work by proving that well-studied random graph models satisfy power-law bounds, and that these bounds allow for improved approximation algorithms for key problems.

Findings

Random graph models exhibit power-law degree bounds with high probability.

Power-law properties enable constant-factor approximation algorithms.

NP-hard problems remain APX-complete even with power-law properties.

Abstract

Large real-world networks typically follow a power-law degree distribution. To study such networks, numerous random graph models have been proposed. However, real-world networks are not drawn at random. Therefore, Brach, Cygan, {\L}acki, and Sankowski [SODA 2016] introduced two natural deterministic conditions: (1) a power-law upper bound on the degree distribution (PLB-U) and (2) power-law neighborhoods, that is, the degree distribution of neighbors of each vertex is also upper bounded by a power law (PLB-N). They showed that many real-world networks satisfy both deterministic properties and exploit them to design faster algorithms for a number of classical graph problems. We complement the work of Brach et al. by showing that some well-studied random graph models exhibit both the mentioned PLB properties and additionally also a power-law lower bound on the degree distribution (PLB-L). All three properties hold with high probability for Chung-Lu Random Graphs and Geometric Inhomogeneous Random Graphs and almost surely for Hyperbolic Random Graphs. As a consequence, all results of Brach et al. also hold with high probability or almost surely for those random graph classes. In the second part of this work we study three classical NP-hard combinatorial optimization problems on PLB networks. It is known that on general graphs with maximum degree {\Delta}, a greedy algorithm, which chooses nodes in the order of their degree, only achieves an {\Omega}(ln {\Delta})-approximation for Minimum Vertex Cover and Minimum Dominating Set, and an {\Omega}({\Delta})-approximation for Maximum Independent Set. We prove that the PLB-U property suffices for the greedy approach to achieve a constant-factor approximation for all three problems. We also show that all three combinatorial optimization problems are APX-complete, even if all PLB-properties hold.