arXiv subjects
Publications and source records attributed to Carlos Uzcategui.
We study a property about Polish inverse semigroups similar to the classical theorem of Pettis about Polish groups. In contrast to what happens with Polish groups, not every Polish inverse semigroup have the Pettis property. We present several examples of Polish inverse subsemigroup of the symmetric inverse semigroup I(N) of all partial bijections between subsets of N. We also study whether our examples satisfy automatic continuity.
Let $X$ be a compact metric countable space, let $f:X\to X$ be a homeomorphism and let $E(X,f)$ be its Ellis semigroup. Among other results we show that the following statements are equivalent: (i) $(X,f)$ is equicontinuous, (ii) $(X,f)$ is distal and (iii) every point is periodic. We use this result to give a direct proof of a theorem of Ellis saying that $(X,f)$ is distal if, and only if, $E(X,f)$ is a group.
An ideal on a set $X$ is a collection of subsets of $X$ closed under the operations of taking finite unions and subsets of its elements. Ideals are a very useful notion in topology and set theory and have been studied for a long time. We present a survey of results about ideals on countable sets and include many open questions.
We show that the Nash-Williams theorem has a uniform version and that the Galvin theorem does not. We show that there is an $F_σ$ tall ideal on $\mathbb{N}$ without a Borel selector and also construct a $\mathbfΠ^1_2$ tall ideal without a tall closed subset.
We show that the following properties are preserved under inverse limits: countable fan-tightness, q+, discrete generation and selective separability. We also present several examples based on inverse limits of countable spaces.
We study Borel ideals $I$ on $\mathbb{N}$ with the Fréchet property such its orthogonal $I^\perp$ is also Borel (where $A\in I^\perp$ iff $A\cap B$ is finite for all $B\in I$ and $I$ is Fréchet if $I=I^{\perp\perp}$). Let $\mathcal{B}$ be the smallest collection of ideals on ${\mathbb{N}}$ containing the ideal of finite sets and closed under countable direct sums and orthogonal. All ideals in $\mathcal{B}$ are Fréchet, Borel and have Borel orthogonal. We show that $\mathcal{B}$ has exactly $\aleph_1$ non isomorphic members. The family $\mathcal{B}$ can be characterized as the collection of all Borel ideals which are isomorphic to an ideal of the form $I_{wf}\up A$, where $I_{wf}$ is the ideal on $\mathbb{N}^{<ω}$ generated by the wellfounded trees. Also, we show that $A\subseteq \mathbb{Q}$ is scattered iff $WO({\mathbb{Q}})\up A$ is isomorphic to an ideal in $\mathcal{B}$, where $WO(\mathbb{Q})$ is the ideal of well founded subset of $\mathbb{Q}$.
We show that the enveloping space $X_G$ of a partial action of a Polish group $G$ on a Polish space $X$ is a standard Borel space, that is to say, there is a topology $τ$ on $X_G$ such that $(X_G, τ)$ is Polish and the quotient Borel structure on $X_G$ is equal to $Borel(X_G,τ)$. To prove this result we show a generalization of a theorem of Burgess about Borel selectors for the orbit equivalence relation induced by a group action and also show that some properties of the Vaught's transform are valid for partial actions of groups.
Let $X$ be a separable metrizable space. We establish a criteria for the existence of a metrizable globalization for a given continuous partial action of a separable metrizable group $G$ on $X.$ If $G$ and $X$ are Polish spaces, we show that the globalization is also a Polish space. We also show the existence of an universal globalization for partial actions of Polish groups.
Given a continuum $X$, for each $A\subseteq X$, the Jones' set function $\mathcal{T}$ is defined by $\mathcal{T}(A)=\{x\in X : \text{for each subcontinuum }K\text{ such that }x\in \textrm{Int}(K), \text{ then }K\cap A\neq\emptyset\}.$ We show that $\mathcal{D}=\{\mathcal{T}(\{x\}):x\in X\}$ is decomposition of $X$ when $\mathcal{T}$ is continuous. We present a characterization of the continuity of $\mathcal{T}$ and answer several open questions posed by D. Bellamy.
It is shown that Matet's characterization of the Ramsey property relative to a selective co-ideal $\mathcal{H}$, in terms of games of Kastanas, still holds if we consider semiselectivity instead of selectivity. Moreover, we prove that a co-ideal $\mathcal{H}$ is semiselective if and only if Matet's game-theoretic characterization of the $\mathcal{H}$-Ramsey property holds. This lifts Kastanas's characterization of the classical Ramsey property to its optimal setting, from the point of view of the local Ramsey theory and gives a game-theoretic counterpart to a theorem of Farah \cite{far}, asserting that a co-ideal $\mathcal{H}$ is semiselective if and only if the family of $\mathcal{H}$-Ramsey subsets of $\N^{[\infty]}$ coincides with the family of those sets having the abstract $\mathcal{H}$-Baire property. Finally, we show that under suitable assumptions, for every semiselective co-ideal $\mathcal H$ all sets of real numbers are $\mathcal H$-Ramsey.