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

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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.

References

Primary source

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

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