Cocountability

In mathematics, a cocountable subset of a set X is a subset Y whose complement in X is a countable set.Since the rational numbers are a countable subset of the reals, for example, the irrational numbers are a cocountable subset of the reals.It is the smallest σ-algebra containing every singleton set.[2] The cocountable topology (also called the "countable complement topology") on any set X consists of the empty set and all cocountable subsets of X.This set theory-related article is a stub.
mathematicssubsetcomplementcountable setcofiniteσ-algebrasingleton setcocountable topologyempty setset theory