In mathematics, a cocountablesubset 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. Since the rational numbers are a countable subset of the reals, for example, the irrational numbers are a cocountable subset of the reals. If the complement is finite, then one says Y is cofinite.[1]
σ-algebrasedit
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.[2]
Topologyedit
The cocountable topology (also called the "countable complement topology") on any set X consists of the empty set and all cocountable subsets of X.[3]
Referencesedit
^Halmos, Paul; Givant, Steven (2009), "Chapter 5: Fields of sets", Introduction to Boolean Algebras, Undergraduate Texts in Mathematics, New York: Springer, pp. 24–30, doi:10.1007/978-0-387-68436-9_5, ISBN9780387684369
cocountability, mathematics, cocountable, subset, subset, whose, complement, countable, other, words, contains, countably, many, elements, since, rational, numbers, countable, subset, reals, example, irrational, numbers, cocountable, subset, reals, complement,. 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 Since the rational numbers are a countable subset of the reals for example the irrational numbers are a cocountable subset of the reals If the complement is finite then one says Y is cofinite 1 s algebras editThe set of all subsets of X that are either countable or cocountable forms a s algebra i e it is closed under the operations of countable unions countable intersections and complementation This s algebra is the countable cocountable algebra on X It is the smallest s algebra containing every singleton set 2 Topology editThe cocountable topology also called the countable complement topology on any set X consists of the empty set and all cocountable subsets of X 3 References edit Halmos Paul Givant Steven 2009 Chapter 5 Fields of sets Introduction to Boolean Algebras Undergraduate Texts in Mathematics New York Springer pp 24 30 doi 10 1007 978 0 387 68436 9 5 ISBN 9780387684369 Halmos amp Givant 2009 Chapter 29 Boolean s algebras pp 268 281 doi 10 1007 978 0 387 68436 9 29 James Ioan Mackenzie 1999 Topologies and Uniformities Springer Undergraduate Mathematics Series London Springer 33 doi 10 1007 978 1 4471 3994 2 ISBN 9781447139942 nbsp This set theory related article is a stub You can help Wikipedia by expanding it vte Retrieved from https en wikipedia org w index php title Cocountability amp oldid 1217811202, wikipedia, wiki, book, books, library,