The Dugundji extension property can fail in ωµ -metrizable spaces

Ian Stares; Jerry Vaughan

Displaying similar documents to “The Dugundji extension property can fail in ωµ -metrizable spaces”

A classification of definable forcings on ω1

Jindřich Zapletal (1997)

Fundamenta Mathematicae

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Under the assumption of the existence of sharps for reals all simply definable posets on $ω_{1}$ are classified up to forcing equivalence.

If it looks and smells like the reals...

Franklin Tall (2000)

Fundamenta Mathematicae

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Given a topological space ⟨X,T⟩ ∈ M, an elementary submodel of set theory, we define $X_{M}$ to be X ∩ M with topology generated by U ∩ M:U ∈ T ∩ M. We prove that if $X_{M}$ is homeomorphic to ℝ, then $X = X_{M}$ . The same holds for arbitrary locally compact uncountable separable metric spaces, but is independent of ZFC if “local compactness” is omitted.

Standardness of sequences of σ-fields given by certain endomorphisms

Jacob Feldman, Daniel Rudolph (1998)

Fundamenta Mathematicae

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Let E be an ergodic endomorphism of the Lebesgue probability space X, ℱ, μ. It gives rise to a decreasing sequence of σ-fields $ℱ, E^{- 1} ℱ, E^{- 2} ℱ, . . .$ A central example is the one-sided shift σ on $X = {0, 1}^{ℕ}$ with $\frac{1}{2}, \frac{1}{2}$ product measure. Now let T be an ergodic automorphism of zero entropy on (Y, ν). The [I|T] endomorphismis defined on (X× Y, μ× ν) by $(x, y) \to (σ (x), T^{x (1)} (y))$ . Here ℱ is the σ-field of μ× ν-measurable sets. Each field is a two-point extension of the one beneath it. Vershik has defined as “standard” any decreasing sequence of...

Each nowhere dense nonvoid closed set in Rn is a σ-limit set

Andrei Sivak (1996)

Fundamenta Mathematicae

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We discuss main properties of the dynamics on minimal attraction centers (σ-limit sets) of single trajectories for continuous maps of a compact metric space into itself. We prove that each nowhere dense nonvoid closed set in $ℝ^{n}$ , n ≥ 1, is a σ-limit set for some continuous map.

Gδ -sets in topological spaces and games

Winfried Just, Marion Scheepers, Juris Steprans, Paul Szeptycki (1997)

Fundamenta Mathematicae

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Players ONE and TWO play the following game: In the nth inning ONE chooses a set $O_{n}$ from a prescribed family ℱ of subsets of a space X; TWO responds by choosing an open subset $T_{n}$ of X. The players must obey the rule that $O_{n} \subseteq O_{n + 1} \subseteq T_{n + 1} \subseteq T_{n}$ for each n. TWO wins if the intersection of TWO’s sets is equal to the union of ONE’s sets. If ONE has no winning strategy, then each element of ℱ is a $G_{δ}$ -set. To what extent is the converse true? We show that: (A) For ℱ the collection of countable subsets of X: 1....

Convexity ranks in higher dimensions

Menachem Kojman (2000)

Fundamenta Mathematicae

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A subset of a vector space is called countably convex if it is a countable union of convex sets. Classification of countably convex subsets of topological vector spaces is addressed in this paper. An ordinal-valued rank function ϱ is introduced to measure the complexity of local nonconvexity points in subsets of topological vector spaces. Then ϱ is used to give a necessary and sufficient condition for countable convexity of closed sets. Theorem. Suppose that S is a closed subset of a...

Every reasonably sized matrix group is a subgroup of S ∞

Robert Kallman (2000)

Fundamenta Mathematicae

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Every reasonably sized matrix group has an injective homomorphism into the group $S_{\infty}$ of all bijections of the natural numbers. However, not every reasonably sized simple group has an injective homomorphism into $S_{\infty}$ .

Are initially $ω_{1}$ -compact separable regular spaces compact?

Alan Dow, Istvan Juhász (1997)

Fundamenta Mathematicae

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We investigate the question of the title. While it is immediate that CH yields a positive answer we discover that the situation under the negation of CH holds some surprises.

Nonseparable Radon measures and small compact spaces

Grzegorz Plebanek (1997)

Fundamenta Mathematicae

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We investigate the problem if every compact space K carrying a Radon measure of Maharam type κ can be continuously mapped onto the Tikhonov cube ${[0, 1]}^{κ}$ (κ being an uncountable cardinal). We show that for κ ≥ cf(κ) ≥ κ this holds if and only if κ is a precaliber of measure algebras. Assuming that there is a family of $ω_{1}$ null sets in $2^{ω 1}$ such that every perfect set meets one of them, we construct a compact space showing that the answer to the above problem is “no” for κ = ω. We also give alternative...

On the equation $a^{p} + 2^{α} b^{p} + c^{p} = 0$

Kenneth A. Ribet (1997)

Acta Arithmetica

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We discuss the equation $a^{p} + 2^{α} b^{p} + c^{p} = 0$ in which a, b, and c are non-zero relatively prime integers, p is an odd prime number, and α is a positive integer. The technique used to prove Fermat’s Last Theorem shows that the equation has no solutions with α < 1 or b even. When α=1 and b is odd, there are the two trivial solutions (±1, ∓ 1, ±1). In 1952, Dénes conjectured that these are the only ones. Using methods of Darmon, we prove this conjecture for p≡ 1 mod 4.

Topological entropy of nonautonomous piecewise monotone dynamical systems on the interval

Sergiĭ Kolyada, Michał Misiurewicz, L’ubomír Snoha (1999)

Fundamenta Mathematicae

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The topological entropy of a nonautonomous dynamical system given by a sequence of compact metric spaces ${(X_{i})}_{i = 1}^{\infty}$ and a sequence of continuous maps ${(f_{i})}_{i = 1}^{\infty}$ , $f_{i} : X_{i} \to X_{i + 1}$ , is defined. If all the spaces are compact real intervals and all the maps are piecewise monotone then, under some additional assumptions, a formula for the entropy of the system is obtained in terms of the number of pieces of monotonicity of $f_{n} ○ . . . ○ f_{2} ○ f_{1}$ . As an application we construct a large class of smooth triangular maps of the square of type $2^{\infty}$ and...

The sequential topology on complete Boolean algebras

Wiesław Główczyński, Bohuslav Balcar, Thomas Jech (1998)

Fundamenta Mathematicae

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We investigate the sequential topology $τ_{s}$ on a complete Boolean algebra B determined by algebraically convergent sequences in B. We show the role of weak distributivity of B in separation axioms for the sequential topology. The main result is that a necessary and sufficient condition for B to carry a strictly positive Maharam submeasure is that B is ccc and that the space $(B, τ_{s})$ is Hausdorff. We also characterize sequential cardinals.

Raising dimension under all projections

John Cobb (1994)

Fundamenta Mathematicae

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As a special case of the general question - “What information can be obtained about the dimension of a subset of $ℝ^{n}$ by looking at its orthogonal projections into hyperplanes?” - we construct a Cantor set in $ℝ^{3}$ each of whose projections into 2-planes is 1-dimensional. We also consider projections of Cantor sets in $ℝ^{n}$ whose images contain open sets, expanding on a result of Borsuk.

Entropy and growth of expanding periodic orbits for one-dimensional maps

A. Katok, A. Mezhirov (1998)

Fundamenta Mathematicae

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Let f be a continuous map of the circle $S^{1}$ or the interval I into itself, piecewise $C^{1}$ , piecewise monotone with finitely many intervals of monotonicity and having positive entropy h. For any ε > 0 we prove the existence of at least $e^{(h - ε) n_{k}}$ periodic points of period $n_{k}$ with large derivative along the period, $| {(f^{n_{k}})}^{'} | > e^{(h - ε) n_{k}}$ for some subsequence $n_{k}$ of natural numbers. For a strictly monotone map f without critical points we show the existence of at least $(1 - ε) e^{h n}$ such points.

Misiurewicz maps unfold generically (even if they are critically non-finite)

Sebastian van Strien (2000)

Fundamenta Mathematicae

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We show that in normalized families of polynomial or rational maps, Misiurewicz maps (critically finite or infinite) unfold generically. For example, if $f_{λ_{0}}$ is critically finite with non-degenerate critical point $c_{1} (λ_{0}), . . ., c_{n} (λ_{0})$ such that $f_{λ_{0}}^{k_{i}} (c_{i} (λ_{0})) = p_{i} (λ_{0})$ are hyperbolic periodic points for i = 1,...,n, then IV-1. Age impartible......................................................................................................................................................................... 31 $λ \mapsto (f_{λ}^{k_{1}} (c_{1} (λ)) - p_{1} (λ), . . ., f_{λ}^{k_{d - 2}} (c_{d - 2} (λ)) - p_{d - 2} (λ))$ is a local diffeomorphism...

A Ramsey theorem for polyadic spaces

Murray Bell (1996)

Fundamenta Mathematicae

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A polyadic space is a Hausdorff continuous image of some power of the one-point compactification of a discrete space. We prove a Ramsey-like property for polyadic spaces which for Boolean spaces can be stated as follows: every uncountable clopen collection contains an uncountable subcollection which is either linked or disjoint. One corollary is that ${(α κ)}^{ω}$ is not a universal preimage for uniform Eberlein compact spaces of weight at most κ, thus answering a question of Y. Benyamini, M. Rudin...

Ordered spaces with special bases

Harold Bennett, David Lutzer (1998)

Fundamenta Mathematicae

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We study the roles played by four special types of bases (weakly uniform bases, ω-in-ω bases, open-in-finite bases, and sharp bases) in the classes of linearly ordered and generalized ordered spaces. For example, we show that a generalized ordered space has a weakly uniform base if and only if it is quasi-developable and has a $G_{δ}$ -diagonal, that a linearly ordered space has a point-countable base if and only if it is first-countable and has an ω-in-ω base, and that metrizability in a generalized...

On the diophantine equation $(\binom{n}{k}) = x^{l}$

K. Győry (1997)

Acta Arithmetica

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P. 294, line 14: For “Satz 8” read “Satz 7”, and for “equation (10)” read “equation (13)”.