|
|
|
|
|
Lindelf SpaceIn mathematics, a Lindelf space is a topological space in which every open cover has a countable subcover. It is named for Ernst Leonard Lindelf. A Lindelf 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 Lindelf is -compact. In general, no implications hold (in either direction) between the Lindelf notion and other compactness notions, such as paracompact, (but regular Lindelf implies paracompact, Morita Theorem) which are discussed in the compactness page. Any second-countable space is a Lindelf space, but not conversely. However, the matter is simpler for metric spaces. A metric space is Lindelf if and only if it is separable if and only if it is second-countable. An open subspace of a Lindelf space is not necessarily Lindelf. However, a closed subspace must be Lindelf. Lindelf is preserved by continuous maps. However, it is not necessarily preserved by products, not even by finite products. Product of Lindelf spaces The product of Lindelf spaces is not necessarily Lindelf. 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 Lindelf space is named for Finnish mathematician Ernst Leonard Lindelf. 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)
|
 |
|
| Copyright 2005-2009 OnPedia.com. All Rights Reserved |
|
|