Structural Properties of Minimum Multi-source Multi-Sink Steiner Networks in the Euclidean Plane

Abstract

Given two finite sets A and B of points in the Euclidean plane, a minimum multi-source multi-sink Steiner network in the plane, or a minimum (A, B)-network, is a directed graph embedded in the plane with a dipath from every node in A to every node in B such that the total length of all arcs in the network is minimised. Such a network may contain Steiner points—nodes appearing in the solution that are neither in A nor B. We show that for any finite point sets A, B in the plane, there exists a minimum (A, B)-network that is constructible by straightedge and compass (this was claimed in a paper by Maxwell and Swanepoel, but their argument is incorrect). We use this property to formulate an algorithmic framework for exactly finding minimum (A, B)-networks in the Euclidean plane. We also present several new structural and geometric properties of minimum (A, B)-networks. In particular, we resolve a conjecture of Alfaro by proving that, for any terminal set A, adding an appropriate orientation to the edges of a minimum 2-edge-connected Steiner network on A yields a minimum (A, A)-network.