Completion-determination conjecture for the isolated closed balls of ultrametric spaces

Let (X,d)(X,d) and (Y,ρ)(Y,\rho) be ultrametric spaces, with completions (X~,d~)(\tilde{X},\tilde{d}) and (Y~,ρ~)(\tilde{Y},\tilde{\rho}). Let BˉX0\bar{\mathbf{B}}_X^0 and BˉY0\bar{\mathbf{B}}_Y^0 denote the subspaces of isolated points in the balleans of closed balls, equipped with the Hausdorff metrics dHd_H and ρH\rho_H, respectively. Completion-determination conjecture. The following statements are equivalent: 1. The completions (X~,d~)(\tilde{X},\tilde{d}) and (Y~,ρ~)(\tilde{Y},\tilde{\rho}) are isometric. 2. The ultrametric spaces (BˉX0,dH)(\bar{\mathbf{B}}_X^0,d_H) and (BˉY0,ρH)(\bar{\mathbf{B}}_Y^0,\rho_H) are isometric. This proposed characterization says that the isometry type of the completion is exactly captured by the isolated closed balls, but the paper provides no resolution.

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Primary source

Oleksiy Dovgoshey, “Hausdorff distance between ultrametric balls”, arXiv:2509.00205 (2025).

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