Least-upper-bound property

In mathematics, the least-upper-bound property (sometimes the completeness or supremum property or l.u.b) is a fundamental property of the real numbers. More generally, a partially ordered set X has the least-upper-bound property if every non-empty subset of X with an upper bound has a least upper bound (supremum) in X. In mathematics, the least-upper-bound property (sometimes the completeness or supremum property or l.u.b) is a fundamental property of the real numbers. More generally, a partially ordered set X has the least-upper-bound property if every non-empty subset of X with an upper bound has a least upper bound (supremum) in X. The least-upper-bound property is one form of the completeness axiom for the real numbers, and is sometimes referred to as Dedekind completeness. It can be used to prove many of the fundamental results of real analysis, such as the intermediate value theorem, the Bolzano–Weierstrass theorem, the extreme value theorem, and the Heine–Borel theorem. It is usually taken as an axiom in synthetic constructions of the real numbers (see least upper bound axiom), and it is also intimately related to the construction of the real numbers using Dedekind cuts. In order theory, this property can be generalized to a notion of completeness for any partially ordered set. A linearly ordered set that is dense and has the least upper bound property is called a linear continuum. Let S be a non-empty set of real numbers. The least-upper-bound property states that any non-empty set of real numbers that has an upper bound must have a least upper bound in real numbers. More generally, one may define upper bound and least upper bound for any subset of a partially ordered set X, with “real number” replaced by “element of X”. In this case, we say that X has the least-upper-bound property if every non-empty subset of X with an upper bound has a least upper bound. For example, the set Q of rational numbers does not have the least-upper-bound property under the usual order. For instance, the set has an upper bound in Q, but does not have a least upper bound in Q (since the square root of two is irrational). The construction of the real numbers using Dedekind cuts takes advantage of this failure by defining the irrational numbers as the least upper bounds of certain subsets of the rationals. The least-upper-bound property is equivalent to other forms of the completeness axiom, such as the convergence of Cauchy sequences or the nested intervals theorem. The logical status of the property depends on the construction of the real numbers used: in the synthetic approach, the property is usually taken as an axiom for the real numbers (see least upper bound axiom); in a constructive approach, the property must be proved as a theorem, either directly from the construction or as a consequence of some other form of completeness.