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On the dimension of the space of ℝ-places of certain rational function fields

Taras Banakh, Yaroslav Kholyavka, Oles Potyatynyk, Michał Machura, Katarzyna Kuhlmann (2014)

Open Mathematics

We prove that for every n ∈ ℕ the space M(K(x 1, …, x n) of ℝ-places of the field K(x 1, …, x n) of rational functions of n variables with coefficients in a totally Archimedean field K has the topological covering dimension dimM(K(x 1, …, x n)) ≤ n. For n = 2 the space M(K(x 1, x 2)) has covering and integral dimensions dimM(K(x 1, x 2)) = dimℤ M(K(x 1, x 2)) = 2 and the cohomological dimension dimG M(K(x 1, x 2)) = 1 for any Abelian 2-divisible coefficient group G.

On the disjoint (0,N)-cells property for homogeneous ANR's

Paweł Krupski (1993)

Colloquium Mathematicae

A metric space (X,ϱ) satisfies the disjoint (0,n)-cells property provided for each point x ∈ X, any map f of the n-cell B n into X and for each ε > 0 there exist a point y ∈ X and a map g : B n X such that ϱ(x,y) < ε, ϱ ^ ( f , g ) < ε and y g ( B n ) . It is proved that each homogeneous locally compact ANR of dimension >2 has the disjoint (0,2)-cells property. If dimX = n > 0, X has the disjoint (0,n-1)-cells property and X is a locally compact L C n - 1 -space then local homologies satisfy H k ( X , X - x ) = 0 for k < n and Hn(X,X-x) ≠ 0.

On the entropy of Darboux functions

Ryszard J. Pawlak (2009)

Colloquium Mathematicae

We prove some results concerning the entropy of Darboux (and almost continuous) functions. We first generalize some theorems valid for continuous functions, and then we study properties which are specific to Darboux functions. Finally, we give theorems on approximating almost continuous functions by functions with infinite entropy.

On the existence of true uniform ultrafilters

Petr Simon (2004)

Commentationes Mathematicae Universitatis Carolinae

We shall show that there is an ultrafilter on singular κ with countable cofinality, which cannot be reached from the set of all subuniform ultrafilters by iterating the closure of sets of size < κ .

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