A finite collection of subsets of a topological space is locally finite. Infinite collections can also be locally finite: for example, the collection of all subsets of R of the form with integern. A countable collection of subsets need not be locally finite, as shown by the collection of all subsets of R of the form with natural n. If a collection of sets is locally finite, the collection of all closures of these sets is also locally finite. The reason for this is that if an open set containing a point intersects the closure of a set, it necessarily intersects the set itself, hence a neighborhood can intersect at most the same number of closures. The converse, however, can fail if the closures of the sets are not distinct. For example, in the finite complement topology on R the collection of all open sets is not locally finite, but the collection of all closures of these sets is locally finite.
Compact spaces
No infinite collection of a compact space can be locally finite. Indeed, let be an infinite family of subsets of a space and suppose this collection is locally finite. For each point x of this space, choose a neighbourhood Ux that intersects the collection at only finitely many values of a. Clearly: and hence has a finite subcover, Ua1 ∪...... ∪ Uan. Since each Uai intersects for only finitely many values of a, the union of all such Uai intersects the collection for only finitely many values of a. It follows that X intersects the collection at only finitely many values of a, contradicting the infinite cardinality of the collection. A topological space in which every open cover admits a locally finite open refinement is called paracompact. Every locally finite collection of subsets of a topological space X is also point-finite. A topological space in which every open cover admits a point-finite open refinement is called metacompact.
No uncountable cover of a Lindelöf space can be locally finite, by essentially the same argument as in the case of compact spaces. In particular, no uncountable cover of a second-countable space is locally finite.
Closed sets
It is clear from the definition of a topology that a finite union of closed sets is closed. One can readily give an example of an infinite union of closed sets that is not closed. However, if we consider a locally finite collection of closed sets, the union is closed. To see this we note that if x is a point outside the union of this locally finite collection of closed sets, we merely choose a neighbourhood V of x that intersects this collection at only finitely many of these sets. Define a bijective map from the collection of sets that V intersects to thus giving an index to each of these sets. Then for each set, choose an open set Ui containing x that doesn't intersect it. The intersection of all such Ui for 1 ≤ i ≤ k intersected with V, is a neighbourhood of x that does not intersect the union of this collection of closed sets.
Countably locally finite collections
A collection in a space is countably locally finite if it is the union of a countable family of locally finite collections of subsets of X. Countable local finiteness is a key hypothesis in the Nagata–Smirnov metrization theorem, which states that a topological space is metrizableif and only if it is regular and has a countably locally finite basis.
