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The set of rational numbers is a proper subset of the set of real numbers. In this example, both sets are infinite, but the latter set has a larger cardinality (or power ) than the former set. Another example in an Euler diagram :
The real numbers can be generalized and extended in several different directions: The complex numbers contain solutions to all polynomial equations and hence are an algebraically closed field unlike the real numbers. However, the complex numbers are not an ordered field. The affinely extended real number system adds two elements +∞ and −∞.
An axiomatic definition of the real numbers consists of defining them as the elements of a complete ordered field. [2] [3] [4] This means the following: The real numbers form a set, commonly denoted , containing two distinguished elements denoted 0 and 1, and on which are defined two binary operations and one binary relation; the operations are called addition and multiplication of real ...
Fig. 1 The Hasse diagram of the set of all subsets of a three-element set ... On the real numbers ... (1, 2) ∪ (2, 3) as a subposet of the real numbers. The subset ...
Every non-empty subset of the real numbers which is bounded from above has a least upper bound.. In mathematics, the least-upper-bound property (sometimes called completeness, supremum property or l.u.b. property) [1] is a fundamental property of the real numbers.
The convex hull of a subset S of a real vector space is defined as the intersection of all convex sets that contain S. More concretely, the convex hull is the set of all convex combinations of points in S. In particular, this is a convex set. A (bounded) convex polytope is the convex hull of a finite subset of some Euclidean space R n.
Also, the real numbers form an ordered field, in which sums and products of positive numbers are also positive. Moreover, the ordering of the real numbers is total, and the real numbers have the least upper bound property: Every nonempty subset of that has an upper bound has a least upper bound that is also a real number.
The set M is a neighbourhood of the number a, because there is an ε-neighbourhood of a which is a subset of M. Given the set of real numbers with the usual Euclidean metric and a subset defined as := (; /), then is a neighbourhood for the set of natural numbers, but is not a uniform neighbourhood of this set.