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The union of two intervals is an interval if and only if they have a non-empty intersection or an open end-point of one interval is a closed end-point of the other, for example (,) [,] = (,]. If R {\displaystyle \mathbb {R} } is viewed as a metric space , its open balls are the open bounded intervals ( c + r , c − r ) , and its closed balls ...
The lower limit topology is finer (has more open sets) than the standard topology on the real numbers (which is generated by the open intervals). The reason is that every open interval can be written as a (countably infinite) union of half-open intervals. For any real and , the interval [,) is clopen in (i.e., both open and closed).
The half-open intervals [a, b) and (a, b] are defined similarly. Whenever a ≤ b does not hold, all these intervals are empty. Every interval is a convex set, but the converse does not hold; for example, in the poset of divisors of 120, ordered by divisibility (see Fig. 7b), the set {1, 2, 4, 5, 8} is convex, but not an interval.
While there are many Borel measures μ, the choice of Borel measure that assigns ((,]) = for every half-open interval (,] is sometimes called "the" Borel measure on . This measure turns out to be the restriction to the Borel σ-algebra of the Lebesgue measure λ {\displaystyle \lambda } , which is a complete measure and is defined on the ...
Here, the basic open sets are the half open intervals [a, b). This topology on R is strictly finer than the Euclidean topology defined above; a sequence converges to a point in this topology if and only if it converges from above in the Euclidean topology. This example shows that a set may have many distinct topologies defined on it.
The open sets and closed sets of any topological space are closed under both unions and intersections. [1] On the real line R, the family of sets consisting of the empty set and all finite unions of half-open intervals of the form (a, b], with a, b ∈ R is a ring in the measure-theoretic sense.
Some sets are neither open nor closed, for instance the half-open interval [,) in the real numbers. The ray [ 1 , + ∞ ) {\displaystyle [1,+\infty )} is closed. The Cantor set is an unusual closed set in the sense that it consists entirely of boundary points and is nowhere dense.
the affinely extended real number system and order-isomorphic sets, for example the unit interval; the set of real numbers with only +∞ or only −∞ added, and order-isomorphic sets, for example a half-open interval; the long line; The set I × I (where × denotes the Cartesian product and I = [0, 1]) in the lexicographic order is a linear ...