Revision as of 16:30, 27 November 2008 by imported>Richard Pinch
In mathematics, a set , where is some topological space, is said to be closed if , the complement of in , is an open set.
Examples
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Let X be the open interval (0, 1) with the usual topology induced by the Euclidean distance. Open sets are then of the form
where and is an arbitrary index set (if then the open interval (a, b) is defined to be the empty set). The definition now implies that closed sets are of the form
- .
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As a more interesting example, consider the function space (with a < b). This space consists of all real-valued continuous functions on the closed interval [a, b] and is endowed with the topology induced by the norm
In this topology, the sets
and
are open sets while the sets
and
are closed (the sets and are the closure of the sets and respectively).