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Čech complete nearness spaces

H. L. Bentley, Worthen N. Hunsaker (1992)

Commentationes Mathematicae Universitatis Carolinae

We study Čech complete and strongly Čech complete topological spaces, as well as extensions of topological spaces having these properties. Since these two types of completeness are defined by means of covering properties, it is quite natural that they should have a convenient formulation in the setting of nearness spaces and that in that setting these formulations should lead to new insights and results. Our objective here is to give an internal characterization of (and to study) those nearness...

Cell-like resolutions of polyhedra by special ones

Dušan Repovš, Arkady Skopenkov (2000)

Colloquium Mathematicae

Suppose that P is a finite 2-polyhedron. We prove that there exists a PL surjective map f:Q → P from a fake surface Q with preimages of f either points or arcs or 2-disks. This yields a reduction of the Whitehead asphericity conjecture (which asserts that every subpolyhedron of an aspherical 2-polyhedron is also aspherical) to the case of fake surfaces. Moreover, if the set of points of P having a neighbourhood homeomorphic to the 2-disk is a disjoint union of open 2-disks, and every point of P...

Central subsets of Urysohn universal spaces

Piotr Niemiec (2009)

Commentationes Mathematicae Universitatis Carolinae

A subset A of a metric space ( X , d ) is central iff for every Katětov map f : X upper bounded by the diameter of X and any finite subset B of X there is x X such that f ( a ) = d ( x , a ) for each a A B . Central subsets of the Urysohn universal space 𝕌 (see introduction) are studied. It is proved that a metric space X is isometrically embeddable into 𝕌 as a central set iff X has the collinearity property. The Katětov maps of the real line are characterized.

Chaotic continua of (continuum-wise) expansive homeomorphisms and chaos in the sense of Li and Yorke

Hisao Kato (1994)

Fundamenta Mathematicae

A homeomorphism f : X → X of a compactum X is expansive (resp. continuum-wise expansive) if there is c > 0 such that if x, y ∈ X and x ≠ y (resp. if A is a nondegenerate subcontinuum of X), then there is n ∈ ℤ such that d ( f n ( x ) , f n ( y ) ) > c (resp. d i a m f n ( A ) > c ). We prove the following theorem: If f is a continuum-wise expansive homeomorphism of a compactum X and the covering dimension of X is positive (dim X > 0), then there exists a σ-chaotic continuum Z = Z(σ) of f (σ = s or σ = u), i.e. Z is a nondegenerate subcontinuum...

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