Ultrametric approximation conjecture for compact semimetric spaces

Let XX be a compact nonempty semimetric space and let ε>0\varepsilon>0. A compact semimetric approximation conjecture asserts that there exist continuous mappings

F ⁣:YX,Φ ⁣:WYF\colon Y \to X,\qquad \Phi\colon W \to Y

such that YY is compact ultrametric, WUCW\in\mathfrak U_C, Φ\Phi is an ε\varepsilon-isometry, and FF is ball-preserving. Moreover,

BYBF(B)BX\mathbf{B}_Y\ni B\mapsto F(B)\in\mathbf{B}_X

is an arc-surjective homomorphism from (BY,AY)(\mathbf{B}_Y,A_Y) to (BX,AX)(\mathbf{B}_X,A_X). Here UC\mathfrak U_C denotes the class of compact ultrametric spaces all of whose finite subspaces belong to U\mathfrak U, the class of finite ultrametric spaces extremal for the Gomory–Hu inequality. This conjecture proposes that every compact nonempty semimetric space admits arbitrarily accurate structural approximation by a compact ultrametric space; the supplied text gives no resolution, so its status remains open.

Sources & referencesView supporting material

Primary source

O. Dovgoshey, E. Petrov and H. -M. Teichert, “On spaces extremal for the Gomory-Hu inequality”, arXiv:1412.1979 (2014).

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