In mathematics, a Lindelöf space is a topological space in which every open cover has a countable subcover. A Lindelöf space is a generalization of the more commonly used notion of compactness, which requires that the subcover be finite.

Lindelöf spaces are named for the Finnish mathematician Ernst Leonard Lindelöf.

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## Properties of Lindelöf spaces

In general, no implications hold (in either direction) between the Lindelöf property and other compactness properties, such as paracompactness. But by the Morita theorem, every regular Lindelöf space is paracompact. Also, any second-countable space is a Lindelöf space, but not conversely.

However, the matter is simpler for metric spaces. A metric space is Lindelöf if and only if it is separable if and only if it is second-countable.

An open subspace of a Lindelöf space is not necessarily Lindelöf. However, a closed subspace must be Lindelöf.

Lindelöf is preserved by continuous maps. However, it is not necessarily preserved by products, not even by finite products.

## Product of Lindelöf spaces

The product of Lindelöf spaces is not necessarily Lindelöf. The usual example of this is the Sorgenfrey plane S, which is the product of R under the half-open interval topology with itself. Open sets in the Sorgenfrey plane are unions of half-open rectangles that include the south and west edges and omit the north and east edges, including the northwest, northeast, and southeast corners.

Consider the open covering of S which consists of:

1. The set of all points (x, y) with x < y
2. The set of all points (x, y) with x + 1 > y
3. For each real x, the half-open rectangle [x, x + 2) × [−x, −x + 2)

The thing to notice here is that each rectangle [x, x + 2) × [−x, −x + 2) covers exactly one of the points on the line x = −y. None of the points on this line is included in any of the other sets in the cover, so there is no proper subcover of this cover, which therefore contains no countable subcover.

## Generalisation

The following definition generalises the definitions of compact and Lindelöf: a topological space is κ-compact, where κ is any cardinal, if every open cover has a subcover of cardinality strictly less than κ. Compact is then [itex]\aleph_0[itex]-compact and Lindelöf is then [itex]\aleph_1[itex]-compact.

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