Extension of mappings on metric spaces
J. de Groot, R. McDowell (1960)
Fundamenta Mathematicae
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J. de Groot, R. McDowell (1960)
Fundamenta Mathematicae
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Bernard Brunet (1995)
Annales mathématiques Blaise Pascal
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K. Van Doren (1973)
Fundamenta Mathematicae
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Preiss, D.
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David Bryant, André Nies, Paul Tupper (2017)
Analysis and Geometry in Metric Spaces
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The Urysohn space is a separable complete metric space with two fundamental properties: (a) universality: every separable metric space can be isometrically embedded in it; (b) ultrahomogeneity: every finite isometry between two finite subspaces can be extended to an auto-isometry of the whole space. The Urysohn space is uniquely determined up to isometry within separable metric spaces by these two properties. We introduce an analogue of the Urysohn space for diversities, a recently developed...
G. Rangan (1970)
Fundamenta Mathematicae
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Julien Melleray (2010)
Fundamenta Mathematicae
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Using classical results of infinite-dimensional geometry, we show that the isometry group of the Urysohn space, endowed with its usual Polish group topology, is homeomorphic to the separable Hilbert space ℓ²(ℕ). The proof is based on a lemma about extensions of metric spaces by finite metric spaces, which we also use to investigate (answering a question of I. Goldbring) the relationship, when A,B are finite subsets of the Urysohn space, between the group of isometries fixing A pointwise,...
J. de Groot (1958)
Colloquium Mathematicae
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Julien Melleray (2006)
Fundamenta Mathematicae
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Building on earlier work of Katětov, Uspenskij proved in [8] that the group of isometries of Urysohn's universal metric space 𝕌, endowed with the pointwise convergence topology, is a universal Polish group (i.e. it contains an isomorphic copy of any Polish group). Answering a question of Gao and Kechris, we prove here the following, more precise result: for any Polish group G, there exists a closed subset F of 𝕌 such that G is topologically isomorphic to the group of isometries of...