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The Arkhangel’skij-Tall problem: A consistent counterexample. (English) Zbl 0862.54020

The problem from the title is well-known and has been around since 1971: is every normal, locally compact, metacompact space paracompact? Arkhangel’skij proved that ‘perfectly normal’ suffices. In this paper the authors produce a model of ZFC in which there is a counterexample, thus laying the problem to rest. The problem is tucked in even tighter in another paper [the authors, ibid. 149, 275-285 (1996; Zbl 0855.54006)], where is shown that \(\text{MA}_{\sigma \text{-centered}} (\omega_1)\) implies a positive answer to the problem and that under the full \(\text{MA} (\omega_1)\) metacompactness may be relaxed to meta-Lindelöfness.
A brief description of the space follows: the underlying set is \(\omega_1\cup \bigcup_{n\in\omega} \omega_1^{\leq n}\). Each \(\omega_1^{\leq n}\) is given a compact Hausdorff topology by taking the family of sets of the form \(V(\sigma)= \{\tau: \sigma\subseteq \tau\}\) and their complements as a subbase. The sets \(\omega_1^{\leq n}\) will be clopen in the space. To get a local base at \(\alpha\in \omega_1\) one takes \(X_\alpha \subseteq \omega\) and, for each \(n\in X_\alpha\) an element \(\sigma_{\alpha,n}\) of \(\omega_1^{\leq n}\). A typical basic neighbourhood of \(\alpha\) then is \(\{\alpha\}\cup \bigcup_{n\in X_\alpha\smallsetminus k}V(\sigma_{\alpha,n})\). The demands are that (0) the \(X_\alpha\) form an almost disjoint family (to make the space Hausdorff); (1) for each \(A\subseteq \omega_1\) there must be \(u\subseteq \omega\) such that \(X_\alpha \subseteq^* u\) if \(\alpha\in A\) and \(X_\alpha\cap u=^*\emptyset\) if \(\alpha\not\in A\) (a step towards normality); (2) if \(\sigma\in \omega_1^{\leq n}\) then \(\{\alpha: \sigma_{\alpha,n}= \sigma\}\) is finite (towards metacompactness) and (3) if \(A\subseteq \omega_1\) then there are a countable subset \(B\) of \(A\), and a finite-to-one function \(f: B\to\omega\) such that for all \(\alpha\), for almost all \(n\in X_\alpha\) there is a \(\beta\in B\) such that \(n\in X_\beta\smallsetminus f(\beta)\) and \(\sigma_{\alpha,n}\supseteq \sigma_{\beta,n}\) (another step towards normality).
The bulk of the paper is devoted to showing that, consistently, such a choice is possible. The \(X_\alpha\) and \(\sigma_{\alpha,n}\) are obtained in a single-step forcing; the rest of the forcing is a proper countable-support iteration of length \(\omega_2\) in which condition 1 is made true, while retaining 3.
Reviewer: K.P.Hart (Delft)

MSC:

54D20 Noncompact covering properties (paracompact, Lindelöf, etc.)
54A35 Consistency and independence results in general topology
03E35 Consistency and independence results
54D15 Higher separation axioms (completely regular, normal, perfectly or collectionwise normal, etc.)
54G20 Counterexamples in general topology

Citations:

Zbl 0855.54006