Traveling Salesmen in the Presence of Competition

TL;DR

This paper introduces the Competing Salesmen Problem, a two-player competitive variant of the Traveling Salesman Problem, analyzing its computational complexity and strategic properties on various graph structures.

Contribution

It formalizes the CSP, proves its PSPACE-completeness, and explores strategic outcomes and optimal strategies on bipartite graphs and trees.

Findings

CSP is PSPACE-complete even on bipartite graphs.

Starting player can lose even from the same vertex as the opponent.

On trees, the second player can avoid losing more than one customer.

Abstract

We propose the ``Competing Salesmen Problem'' (CSP), a 2-player competitive version of the classical Traveling Salesman Problem. This problem arises when considering two competing salesmen instead of just one. The concern for a shortest tour is replaced by the necessity to reach any of the customers before the opponent does. In particular, we consider the situation where players take turns, moving along one edge at a time within a graph G=(V,E). The set of customers is given by a subset V_C V of the vertices. At any given time, both players know of their opponent's position. A player wins if he is able to reach a majority of the vertices in V_C before the opponent does. We prove that the CSP is PSPACE-complete, even if the graph is bipartite, and both players start at distance 2 from each other. We show that the starting player may lose the game, even if both players start from the same vertex. For bipartite graphs, we show that the starting player always can avoid a loss. We also show that the second player can avoid to lose by more than one customer, when play takes place on a graph that is a tree T, and V_C consists of leaves of T. For the case where T is a star and V_C consists of n leaves of T, we give a simple and fast strategy which is optimal for both players. If V_C consists not only of leaves, the situation is more involved.