In mathematics, a Lindelöf space is a topological space in which every open cover has a countable subcover. It is named for Ernst Leonard Lindelöf.
A Lindelöf space is a generalization of the more commonly used notion of compactness, which requires that the subcover be finite. In fact, we can define A-compact, where A is a cardinal, if every open cover has a subcover of cardinality strictly less than A; then compact is 
-compact and Lindelöf is 
-compact.
In general, no implications hold (in either direction) between the Lindelöf notion and other compactness notions, such as paracompact, (but regular Lindelöf implies paracompact, Morita Theorem) which are discussed in the compactness page. 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:
- The set of all points (x, y) with x < y
- The set of all points (x, y) with x+1 > y
- 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.
History
The Lindelöf space is named for Finnish mathematician Ernst Leonard Lindelöf.
References
- Michael Gemignani, Elementary Topology (ISBN 0-486-66522-4) (see especially section 7.2)
- Lynn Arthur Steen and J. Arthur Seebach, Jr., Counterexamples in Topology (ISBN 0-486-68735-X)