# σ-compact space

(Redirected from Sigma-compact)

In mathematics, a topological space is said to be σ-compact if it is the union of countably many compact subspaces.[1]

A space is said to be σ-locally compact if it is both σ-compact and locally compact.[2]

## Properties and examples

• Every compact space is σ-compact, and every σ-compact space is Lindelöf (i.e. every open cover has a countable subcover).[3] The reverse implications do not hold, for example, standard Euclidean space (Rn) is σ-compact but not compact,[4] and the lower limit topology on the real line is Lindelöf but not σ-compact.[5] In fact, the countable complement topology is Lindelöf but neither σ-compact nor locally compact.[6]
• A Hausdorff, Baire space that is also σ-compact, must be locally compact at at least one point.
• If G is a topological group and G is locally compact at one point, then G is locally compact everywhere. Therefore, the previous property tells us that if G is a σ-compact, Hausdorff topological group that is also a Baire space, then G is locally compact. This shows that for Hausdorff topological groups that are also Baire spaces, σ-compactness implies local compactness.
• The previous property implies for instance that Rω is not σ-compact: if it were σ-compact, it would necessarily be locally compact since Rω is a topological group that is also a Baire space.
• Every hemicompact space is σ-compact.[7] The converse, however, is not true;[8] for example, the space of rationals, with the usual topology, is σ-compact but not hemicompact.
• The product of a finite number of σ-compact spaces is σ-compact. However the product of an infinite number of σ-compact spaces may fail to be σ-compact.[9]
• A σ-compact space X is second category (resp. Baire) if and only if the set of points at which is X is locally compact is nonempty (resp. dense) in X.[10]

## Notes

1. ^ Steen, p.19; Willard, p. 126.
2. ^ Steen, p. 21.
3. ^ Steen, p. 19.
4. ^ Steen, p. 56.
5. ^ Steen, p. 75–76.
6. ^ Steen, p. 50.
7. ^ Willard, p. 126.
8. ^ Willard, p. 126.
9. ^ Willard, p. 126.
10. ^ Willard, p. 188.