On tight spans and tropical polytopes for directed distances

Abstract: An extension $(V,d)$ of a metric space $(S,\mu)$ is a metric space with $S \subseteq V$ and $d|_S = \mu$, and is said to be tight if there is no other extension $(V,d')$ of $(S,\mu)$ with $d' \leq d$. Isbell and Dress independently found that every tight extension is isometrically embedded into a certain metrized polyhedral complex associated with $(S,\mu)$, called the tight span. This paper develops an analogous theory for directed metrics, which are "not necessarily symmetric" distance functions satisfying the triangle inequality. We introduce a directed version of the tight span and show that it has such a universal embedding property for tight extensions. Also we newly introduce another natural class of extensions, called cyclically tight extensions, and show that (a fiber of) the tropical polytope, introduced by Develin and Sturmfels, has a universal embedding property for cyclically tight extensions. As an application, we prove the following directed version of tree metric theorem: directed metric $\mu$ is a directed tree metric if and only if the tropical rank of $-\mu$ is at most two. Also we describe how tight spans and tropical polytopes are applied to the study in multicommodity flows in directed networks.