Open set

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An open set in Euclidean space is a set that contains a ball (mathematics) around each of its points.^{[Citation Needed]} In Euclidean space, a "ball" of radius r centered at a point p is the set of all points which are distance at most r from p. Intuitively, if a point is contained in an open set, then all points sufficiently close to it are also contained.

Open sets are the basic objects in topology, in terms of which all other objects are defined.