On the tree conjecture for the network creation game

Selfish Network Creation focuses on modeling real world networks from a game-theoretic point of view. One of the classic models by Fabrikant et al. [PODC†03] is the network creation game, where agents correspond to nodes in a network which buy incident edges for the price of a per edge to minimize their total distance to all other nodes. The model is well-studied but still has intriguing open problems. The most famous conjectures state that the price of anarchy is constant for all a and that for a = n all equilibrium networks are trees. We introduce a novel technique for analyzing stable networks for high edge-price a and employ it to improve on the best known bounds for both conjectures. In particular we show that for a > 4n - 13 all equilibrium networks must be trees, which implies a constant price of anarchy for this range of a. Moreover, we also improve the constant upper bound on the price of anarchy for equilibrium trees.