Point-finite collection

In mathematics, a collection or family $𝒰$ of subsets of a topological space $X$ is said to be point-finite if every point of $X$ lies in only finitely many members of $𝒰.$

A metacompact space is a topological space in which every open cover admits a point-finite open refinement. Every locally finite collection of subsets of a topological space is also point-finite. A topological space in which every open cover admits a locally finite open refinement is called a paracompact space. Every paracompact space is therefore metacompact.

Dieudonné's theorem

Theorem— A topological space $X$ is normal if and only if each point-finite open cover of $X$ has a shrinking; that is, if $\{U_i\mid i\in I\}$ is an open cover indexed by a set $I$, there is an open cover $\{V_i\mid i\in I\}$ indexed by the same set $I$ such that $\overline{V_i}\subset U_i$ for each $i\in I$.

The original proof uses Zorn's lemma, while Willard uses transfinite recursion.

References

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