nLab
separable metacompact spaces are Lindelöf

Statement

Recall that a topological space XX is metacompact if every open cover has a point-finite open refinement which covers XX. Further a space is called Lindelöf if every open cover has a countable sub-cover.

Theorem

Assuming the law of excluded middle:

Every separable metacompact topological space XX is Lindelöf .

Proof

If {U i} iI\{U_i\}_{i\in I} is an open cover with no countable subcover, let {U j} jJ\{U_j\}_{j\in J} be a point finite refinement. Then {U j} jJ\{U_j\}_{j\in J} is uncountable and so some point of the countable dense set is in U jU_j for uncountably many jJj \in J.

propertiesimplications
second-countable: there is a countable base of the topology.A second-countable space has a σ \sigma -locally finite base: take the the collection of singeltons of all elements of countable cover of XX.
σ\sigma-locally finite base, i.e. XX has a countably locally finite base, e.g. a metrisable topological space by Nagata-Smirnov metrization theorem.second-countable spaces are separable: choose a point in each set of countable cover.
separable: there is a countable dense subset.second-countable spaces are Lindelöf
Lindelöf: every open cover has a countable sub-cover.weakly Lindelöf spaces with countably locally finite base are second countable
weakly Lindelöf: every open cover has a countable subcollection the union of which is dense.separable metacompact spaces are Lindelöf
countable choice: the natural numbers is a projective object in Set.separable spaces are weakly Lindelöf: given a countable dense subset and an open cover choose for each point of the subset an open from the cover.
metacompact: every open cover has a point-finite open refinement.Lindelöf spaces are trivially also weakly Lindelöf.

Created on April 2, 2019 at 12:13:15. See the history of this page for a list of all contributions to it.