The uniqueness property for networks with several origin-destination pairs

TL;DR

This paper investigates the conditions under which network congestion games with multiple origin-destination pairs have unique equilibrium flows, providing a complete characterization for bidirectional rings and necessary conditions for other networks.

Contribution

It introduces a necessary condition based on excluded minors for networks with multiple OD pairs to have the uniqueness property and fully characterizes bidirectional rings with this property.

Findings

Complete characterization of bidirectional rings with the uniqueness property.

Identification of nine specific networks where the property holds.

Construction of affine cost functions demonstrating non-uniqueness in other rings.

Abstract

We consider congestion games on networks with nonatomic users and user-specific costs. We are interested in the uniqueness property defined by Milchtaich [Milchtaich, I. 2005. Topological conditions for uniqueness of equilibrium in networks. Math. Oper. Res. 30 225-244] as the uniqueness of equilibrium flows for all assignments of strictly increasing cost functions. He settled the case with two-terminal networks. As a corollary of his result, it is possible to prove that some other networks have the uniqueness property as well by adding common fictitious origin and destination. In the present work, we find a necessary condition for networks with several origin-destination pairs to have the uniqueness property in terms of excluded minors or subgraphs. As a key result, we characterize completely bidirectional rings for which the uniqueness property holds: it holds precisely for nine networks and those obtained from them by elementary operations. For other bidirectional rings, we exhibit affine cost functions yielding to two distinct equilibrium flows. Related results are also proven. For instance, we characterize networks having the uniqueness property for any choice of origin-destination pairs.