Umbel convexity and the geometry of trees

Abstract

For every p ∈ (0, ∞), a new metric invariant called umbel p-convexity is introduced. The asymptotic notion of umbel convexity captures the geometry of countably branching trees, much in the same way as Markov convexity, the local invariant which inspired it, captures the geometry of bounded degree trees. Umbel convexity is used to provide a "Poincaré-type" metric characterization of the class of Banach spaces that admit an equivalent norm with Rolewicz's property (β). We explain how a relaxation of umbel p-convexity, called umbel cotype p, plays a role in obtaining compression rate bounds for coarse embeddings of countably branching trees. Local analogs of these invariants, fork q-convexity and fork cotype q, are introduced and their relationship to Markov q-convexity and relaxations of the q-tripod inequality is discussed. The metric invariants are estimated for a large class of Heisenberg groups. Finally, a new characterization of non-negative curvature is given. Contents 1. Introduction 1 2. Property (β) with power type p implies umbel p-convexity 6 3. Distortion and compression rate of embeddings of countably branching trees 13 4. Stability under nonlinear quotients 17 5. More examples of metric spaces with non-trivial umbel cotype 22 6. Relaxations of the tripod inequality and of Markov convexity 27 7. A characterization of non-negative curvature 39 8. Concluding remarks and open problems 40 References 41