Location Routing Problems on Trees

This paper addresses combined location/routing problems defined on trees, where not all vertices have to be necessarily visited. A mathematical programming formulation is presented, which has the integrality property. The formulation models a directed forest where each connected component hosts at least one open facility, which becomes the root of the component. The problems considered can also be optimally solved with ah-hoc solution algorithms. Greedy type optimal algorithms are presented for the cases when all vertices have to be visited and facilities have no setup costs. Facilities set-up costs can be handled with low-order interchanges, whose optimality check is a re-statement of the complementary slackness conditions of the proposed formulation. The general problems where not all vertices have to be necessarily visited can also be optimally solved with low-order optimal algorithms based on recursions.