Cocountable
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In mathematics, a cocountable subset of a set X is a subset Y whose complement in X is a countable set. In other words, Y contains all but countably many elements of X.
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σ-algebras
The set of all subsets of X that are either countable or cocountable forms a σ-algebra, i.e., it is closed under the operations of countable unions, countable intersections, and complementation. This σ-algebra is the countable-cocountable algebra on X. It is the smallest σ-algebra containing every singleton set.
Topology
The cocountable topology (sometimes called the countable complement topology) on any set X consists of the empty set and all cocountable subsets of X. In the cocountable topology, the only closed subsets are countable sets, or the whole of X. Then X is automatically Lindelöf in this topology, since every open set only omits countably many points of X.
See also
References
- Lynn Arthur Steen and J. Arthur Seebach, Jr., Counterexamples in Topology. Springer-Verlag, New York, 1978. Reprinted by Dover Publications, New York, 1995. ISBN 0-486-68735-X (Dover edition). (See example 20)