The Ultrametric Gromov–Wasserstein Distance

We investigate compact ultrametric measure spaces which form a subset U w of the collection of all metric measure spaces M w . In analogy with the notion of the ultrametric Gromov–Hausdorff distance on the collection of ultrametric spaces U , we define ultrametric versions of two metrics on U w , namely of Sturm’s Gromov–Wasserstein distance of order p and of the Gromov–Wasserstein distance of order p . We study the basic topological and geometric properties of these distances as well as their relation and derive for p = ∞ a polynomial time algorithm for their calculation. Further, several lower bounds for both distances are derived and some of our results are generalized to the case of finite ultra-dissimilarity spaces. Finally, we study the relation between the Gromov–Wasserstein distance and its ultrametric version (as well as the relation between the corresponding lower bounds) in simulations and apply our findings for phylogenetic tree shape comparisons.