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A Cantor set in the plane that is not σ-monotone

Aleš Nekvinda, Ondřej Zindulka (2011)

Fundamenta Mathematicae

A metric space (X,d) is monotone if there is a linear order < on X and a constant c such that d(x,y) ≤ cd(x,z) for all x < y < z in X, and σ-monotone if it is a countable union of monotone subspaces. A planar set homeomorphic to the Cantor set that is not σ-monotone is constructed and investigated. It follows that there is a metric on a Cantor set that is not σ-monotone. This answers a question raised by the second author.

A categorical concept of completion of objects

Guillaume C. L. Brümmer, Eraldo Giuli (1992)

Commentationes Mathematicae Universitatis Carolinae

We introduce the concept of firm classes of morphisms as basis for the axiomatic study of completions of objects in arbitrary categories. Results on objects injective with respect to given morphism classes are included. In a finitely well-complete category, firm classes are precisely the coessential first factors of morphism factorization structures.

A -compactifications and A -weight of Alexandroff spaces

A. Caterino, G. Dimov, M. C. Vipera (2002)

Bollettino dell'Unione Matematica Italiana

The paper is devoted to the study of the ordered set A K X , α of all, up to equivalence, A -compactifications of an Alexandroff space X , α . The notion of A -weight (denoted by a w X , α ) of an Alexandroff space X , α is introduced and investigated. Using results in ([7]) and ([5]), lattice properties of A K X , α and A K α w X , α are studied, where A K α w X , α is the set of all, up to equivalence, A -compactifications Y of X , α for which w Y = a w X , α . A characterization of the families of bounded functions generating an A -compactification of X , α is obtained. The notion...

A continuous operator extending fuzzy ultrametrics

I. Stasyuk, Edward D. Tymchatyn (2011)

Commentationes Mathematicae Universitatis Carolinae

We consider the problem of simultaneous extension of fuzzy ultrametrics defined on closed subsets of a complete fuzzy ultrametric space. We construct an extension operator that preserves the operation of pointwise minimum of fuzzy ultrametrics with common domain and an operation which is an analogue of multiplication by a constant defined for fuzzy ultrametrics. We prove that the restriction of the extension operator onto the set of continuous, partial fuzzy ultrametrics is continuous with respect...

A continuous operator extending ultrametrics

I. Stasyuk, Edward D. Tymchatyn (2009)

Commentationes Mathematicae Universitatis Carolinae

The problem of continuous simultaneous extension of all continuous partial ultrametrics defined on closed subsets of a compact zero-dimensional metric space was recently solved by E.D. Tymchatyn and M. Zarichnyi and improvements to their result were made by I. Stasyuk. In the current paper we extend these results to complete, bounded, zero-dimensional metric spaces and to both continuous and uniformly continuous partial ultrametrics.

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