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.
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