Closed set

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Real Numbers

A closed set of the real numbers, also known as a closed interval, is an interval of numbers which includes its endpoints, sets which consist only of isolated points, or the finite union of those two kinds of sets. For example, the set of numbers which are less than or equal to 1 but greater than or equal to 0 is a closed interval, because it includes its endpoints 1 and 0.

Topological Spaces

In mathematical topology, a closed set is the complement of an open set.