$\mathsf{PFA}$ and $ω_1$-free compact spaces

非林德洛夫空间中的最终常数和停滞函数

Eventually Constant and stagnating functions in non-Lindelöf spaces

研究非林德洛夫空间中函数性质，引入推广林德洛夫性的四个性质$\mathsf{P}(X,Y)$，研究其与其他拓扑性质关系，给出如特定树子空间、流形等相关结果示例，部分结果是对前人工作的重新表述。

The completion of the hyperspace of finite subsets, endowed with the $\ell^1$-metric

For a metric space $X$, let $\mathsf FX$ be the space of all nonempty finite subsets of $X$ endowed with the largest metric $d^1_{\mathsf FX}$ such that for every $n\in\mathbb N$ the map $X^n\to\mathsf FX$, $(x_1,\dots,x_n)\mapsto \{x_1,\dots,x_n\}$, is non-expanding with respect to the $\ell^1$-metric on $X^n$. We study the completion of the metric space $\mathsf F^1\!X=(\mathsf FX,d^1_{\mathsf FX})$ and prove that it coincides with the space $\mathsf Z^1\!X$ of nonempty compact subsets of $X$ that have zero length (defined with the help of graphs). We prove that each subset of zero length in…

Spaces of countable free set number and PFA

The main result of this paper is that, under PFA, for every {\em regular} space $X$ with $F(X) = ω$ we have $|X| \le w(X)^ω$; in particular, $w(X) \le \mathfrak{c}$ implies $|X| \le \mathfrak{c}$. This complements numerous prior results that yield consistent examples of even compact Hausdorff spaces $X$ with $F(X) = ω$ such that $w(X) = \mathfrak{c}$ and $|X| = 2^\mathfrak{c}$. We also show that regularity cannot be weakened to Hausdorff in this result because we can find in ZFC a Hausdorff space $X$ with $F(X) = ω$ such that $w(X) = \mathfrak{c}$ and $|X| = 2^\mathfrak{c}$. In fact, this spac…

Compactly generated spaces and quasi-spaces in topology

The notions of compactness and Hausdorff separation for generalized enriched categories allow us, as classically done for the category $\mathsf{Top}$ of topological spaces and continuous functions, to study $\textit{compactly generated spaces}$ and $\textit{quasi-spaces}$ in this setting. Moreover, for a class $\mathcal{C}$ of objects we generalize the notion of $\textit{$\mathcal{C}$-generated spaces}$, from which we derive, for instance, a general concept of $\textit{Alexandroff spaces}$. Furthermore, as done for $\mathsf{Top}$, we also study, in our level of generality, the relationship bet…

On Scott power spaces

In this paper, we mainly discuss some basic properties of Scott power spaces. For a $T_0$ space $X$, let $\mathsf{K}(X)$ be the poset of all nonempty compact saturated subsets of $X$ endowed with the Smyth order. It is proved that the Scott power space $Σ\mathsf{K}(X)$ of a well-filtered space $X$ is still well-filtered, and a $T_0$ space $Y$ is well-filtered iff $Σ\mathsf{K}(Y)$ is well-filtered and the upper Vietoris topology is coarser than the Scott topology on $\mathsf{K}(Y)$. A sober space is constructed for which its Scott power space is not sober. A few sufficient conditions are given …

关于正规第一可数线性H-闭空间的三个小结果

Three small results on normal first countable linearly H-closed spaces

本研究借助PFA、MA$_{ω_1}$(S)[S]与PFA(S)[S]的已知拓扑推论，证明在对应模型下，带多种附加性质的正规第一可数线性H-闭空间均为紧空间，补充了相关拓扑空间的紧性结论。

Countably compact groups without non-trivial convergent sequences

We construct, in $\mathsf{ZFC}$, a countably compact subgroup of $2^{\mathfrak{c}}$ without non-trivial convergent sequences, answering an old problem of van Douwen. As a consequence we also prove the existence of two countably compact groups $\mathbb{G}_{0}$ and $\mathbb{G}_{1}$ such that the product $\mathbb{G}_{0} \times \mathbb{G}_{1}$ is not countably compact, thus answering a classical problem of Comfort.

The cometrizability of generalized metric spaces

A topological space $X$ is cometrizable if it admits a weaker metrizable topology such that each point $x\in X$ has a (not necessarily open) neighborhood base consisting of metrically closed sets. We study the relation of cometrizable spaces to other generalized metric spaces and prove that all $\mathsf{as}$-cosmic spaces are cometrizable. Also, we present an example of a regular countable space of weight $ω_1$, which is not cometrizable. Under $ω_1=\mathfrak c$ this space contains no infinite compact subsets and hence is $\mathsf{cs}$-cosmic. Under $ω_1<\mathfrak p$ this countable space is Fr…

Countable dense homogeneity and $λ$-sets

We show that all sufficiently nice $λ$-sets are countable dense homogeneous ($\mathsf{CDH}$). From this fact we conclude that for every uncountable cardinal $κ\le \mathfrak{b}$ there is a countable dense homogeneous metric space of size $κ$. Moreover, the existence of a meager in itself countable dense homogeneous metric space of size $κ$ is equivalent to the existence of a $λ$-set of size $κ$. On the other hand, it is consistent with the continuum arbitrarily large that every $\mathsf{CDH}$ metric space has size either $ω_1$ or size $\mathfrak c$. An example of a Baire $\mathsf{CDH}$ metric s…

Locally compact, $ω_1$-compact spaces

An $ω_1$-compact space is a space in which every closed discrete subspace is countable. We give various general conditions under which a locally compact, $ω_1$-compact space is $σ$-countably compact, i.e., the union of countably many countably compact spaces. These conditions involve very elementary properties. Many results shown here are independent of the usual (ZFC) axioms of set theory, and the consistency of some may involve large cardinals. For example, it is independent of the ZFC axioms whether every locally compact, $ω_1$-compact space of cardinality $\aleph_1$ is $σ$-countably compac…

可数空间、实紧性与伪交集数

Countable spaces, realcompactness, and the pseudointersection number

本文研究可数空间的实紧嵌入维数 Exp(X)，证明存在可数拥挤空间 X 使 Exp(X)=κ 当且仅当伪交集数 p≤κ≤连续统 c，并借助第一可数空间乘积的可数子集伪特征分析及散射空间紧化结论，给出任意可数空间 Exp(X) 的计算。

Pseudoradial spaces and copies of $ω_1+1$

In this paper we compare the concepts of pseudoradial spaces and the recently defined strongly pseudoradial spaces in the realm of compact spaces. We show that $\mathrm{MA}+\mathfrak{c}=ω_2$ implies that there is a compact pseudoradial space that is not strongly pseudoradial. We essentially construct a compact, sequentially compact space $X$ and a continuous function $f:X\toω_1+1$ in such a way that there is no copy of $ω_1+1$ in $X$ that maps cofinally under $f$. We also give some conditions that imply the existence of copies of $ω_1$ in spaces. In particular, $\mathrm{PFA}$ implies that comp…

On the spectrum of the hierarchical Laplacian

Let $(X,d)$ be a locally compact separable ultrametric space. We assume that $(X,d)$ is proper, that is, any closed ball $B$ in $X$ is a compact set. Given a measure $m$ on $X$ and a function $C(B)$ defined on the set of balls (the choice function), we define the hierarchical Laplacian $L_C$ which is closely related to the concept of the hierarchical lattice of F.J. Dyson. $L_C$ is a non-negative definite, self-adjoint operator in $L^2(X,m)$. We address in this paper to the following question: How general can be the spectrum $\mathsf{Spec}(L_C)$ as a subset of the non-negative reals? When $(X,…

Notes on linearly H-closed spaces and od-selection principles

A space is called linearly H-closed iff any chain cover possesses a dense member. This property lies strictly between feeble compactness and H-closedness. While regular H-closed spaces are compact, there are linearly H-closed spaces which are even collectionwise normal and Fréchet-Urysohn. We give examples in other classes, and ask whether there is a first countable normal linearly H-closed non-compact space in ZFC. We show that PFA implies a negative answer if the space is moreover either locally separable or locally compact and locally ccc. Ostaszewski space (built with $\diamondsuit$) is an…

可分紧致线的函数空间

Function spaces on separable compact lines

在鲍姆加特纳公理BA假设下，对权重为ω₁的可分紧致线K赋予点态收敛拓扑的实值连续函数空间C_p(K)进行同构分类，表明此类K恰有两个线性同胚的函数空间C_p(K)，还构造了权重为2^ω的可分紧致线K的例子。

到一般子流形的距离函数的临界点

Critical points of the distance function to a generic submanifold

本文研究紧子流形距离函数临界点的正则性，证明一般子流形的临界点与μ-临界点满足正则性条件且C²扰动稳定，对应嵌入在Whitney C²拓扑中开稠，此时距离函数满足类Morse条件，还举例说明缺条件时的病态情形。