Noetherian type in topological products

Abstract

The cardinal invariant "Noetherian type" of a topological space $X$ (Nt(X)) was introduced by Peregudov in 1997 to deal with base properties that were studied by the Russian School as early as 1976. We study its behavior in products and box-products of topological spaces. We prove in Section 2: 1) There are spaces $X$ and $Y$ such that $Nt(X \times Y) < \min\{Nt(X), Nt(Y)\}$. 2) In several classes of compact spaces, the Noetherian type is preserved by the operations of forming a square and of passing to a dense subspace. The Noetherian type of the Cantor Cube of weight $\aleph_ω$ with the countable box topology, $(2^{\aleph_ω})_δ$, is shown in Section 3 to be closely related to the combinatorics of covering collections of countable subsets of $\aleph_ω$. We discuss the influence of principles like $\square_{\aleph_ω}$ and Chang's conjecture for $\aleph_ω$ on this number and prove that it is not decidable in ZFC (relative to the consistency of ZFC with large cardinal axioms). Within PCF theory we establish the existence of an $(\aleph_4,\aleph_1)$-sparse covering family of countable subsets of $\aleph_ω$. From this follows an absolute upper bound of $\aleph_4$ on the Noetherian type of $(2^{\aleph_ω})_δ$. The proof uses ideas from Shelah's proof that if $κ^+ <λ$ then his ideal $I[λ]$ contains a stationary set consisting of points of cofinality $κ$.