On the geometry of the countably branching diamond graphs

In this article, the bi-Lipschitz embeddability of the sequence of countably branching diamond graphs (D_k ^ω)_k∈N is investigated. In particular it is shown that for every ε>0 and k∈N, D_k ^ω embeds bi-Lipschiztly with distortion at most 6(1+ε) into any reflexive Banach space with an unconditional asymptotic structure that does not admit an equivalent asymptotically uniformly convex norm. On the other hand it is shown that the sequence (D_k ^ω)_k∈N does not admit an equi-bi-Lipschitz embedding into any Banach space that has an equivalent asymptotically midpoint uniformly convex norm. Combining these two results one obtains a metric characterization in terms of graph preclusion of the class of asymptotically uniformly convexifiable spaces, within the class of reflexive Banach spaces with an unconditional asymptotic structure. Applications to bi-Lipschitz embeddability into L_p-spaces and to some problems in renorming theory are also discussed.