On the structure of the tight-span of a totally split-decomposable metric

The tight-span of a finite metric space is a polytopal complex with a structure that reflects properties of the metric. In this paper we consider the tight-span of a totally split-decomposable metric. Such metrics are used in the field of phylogenetic analysis, and a better knowledge of the structure of their tight-spans should ultimately provide improved phylogenetic techniques. Here we prove that a totally split-decomposable metric is cell-decomposable. This allows us to break up the tight-span of a totally split-decomposable metric into smaller, easier to understand tight-spans. As a consequence we prove that the cells in the tight-span of a totally split-decomposable metric are zonotopes that are polytope isomorphic to either hypercubes or rhombic dodecahedra.