A set A of real numbers (shown as blue balls), a set of upper bounds of A (red balls), and the smallest such upper bound, that is, the supremum of A (shown as a red diamond).

In mathematics, given a subset S of a partially ordered set T, the supremum (sup) of S, if it exists, is the least element of T that is greater than or equal to each element of S. Mathematics is the body of Knowledge and Academic discipline that studies such concepts as Quantity, Structure, Space and In Mathematics, especially Order theory, a partially ordered set (or poset) formalizes the intuitive concept of an ordering sequencing or arrangement In Mathematics, especially in Order theory, the greatest element of a subset S of a Partially ordered set (poset is an element of S Consequently, the supremum is also referred to as the least upper bound, lub or LUB. If the supremum exists, it may or may not belong to S. If the supremum exists, it is unique.

Suprema are often considered for subsets of real numbers, rational numbers, or any other well-known mathematical structure for which it is immediately clear what it means for an element to be "greater-than-or-equal-to" another element. In Mathematics, the real numbers may be described informally in several different ways In Mathematics, a rational number is a number which can be expressed as a Ratio of two Integers Non-integer rational numbers (commonly called fractions The definition generalizes easily to the more abstract setting of order theory, where one considers arbitrary partially ordered sets. Order theory is a branch of Mathematics that studies various kinds of Binary relations that capture the intuitive notion of ordering providing a framework for saying In Mathematics, especially Order theory, a partially ordered set (or poset) formalizes the intuitive concept of an ordering sequencing or arrangement

The concept of supremum is not the same as the concepts of minimal upper bound, maximal element, or greatest element. In Mathematics, especially in Order theory, a maximal element of a subset S of some Partially ordered set is an element of S that In Mathematics, especially in Order theory, an upper bound of a Subset S of some Partially ordered set ( P, &le In Mathematics, especially in Order theory, a maximal element of a subset S of some Partially ordered set is an element of S that In Mathematics, especially in Order theory, the greatest element of a subset S of a Partially ordered set (poset is an element of S

Supremum of a set of real numbers

In analysis, the supremum or least upper bound of a set S of real numbers is denoted by sup(S) and is defined to be the smallest real number that is greater than or equal to every number in S. Analysis has its beginnings in the rigorous formulation of Calculus. In Mathematics, the real numbers may be described informally in several different ways An important property of the real numbers is its completeness: every nonempty subset of the set of real numbers that is bounded above has a supremum that is also a real number. In the mathematical area of Order theory, completeness properties assert the existence of certain infima or suprema of a given partially ordered In Mathematics, and more specifically Set theory, the empty set is the unique set having no ( Zero) members

Examples

In the last example, the supremum of a set of rationals is irrational, which means that the rationals are incomplete. In Mathematics, a rational number is a number which can be expressed as a Ratio of two Integers Non-integer rational numbers (commonly called fractions In Mathematics, an irrational number is any Real number that is not a Rational number — that is it is a number which cannot be expressed as a fraction In Mathematical analysis, a Metric space M is said to be complete (or Cauchy) if every Cauchy sequence of points in M has

One basic property of the supremum is

for any functionals f and g. In Mathematics, a functional is traditionally a map from a Vector space to the field underlying the vector space which is usually the Real

If, in addition, we define sup(S) = −∞ when S is empty and sup(S) = +∞ when S is not bounded above, then every set of real numbers has a supremum under the affinely extended real number system. In Mathematics, and more specifically Set theory, the empty set is the unique set having no ( Zero) members In Mathematics, the affinely extended real number system is obtained from the Real number system R by adding two elements +∞ and &minus∞ (pronounced

If the supremum belongs to the set, then it is the greatest element in the set. In Mathematics, especially in Order theory, the greatest element of a subset S of a Partially ordered set (poset is an element of S The term maximal element is synonymous as long as one deals with real numbers or any other totally ordered set. In Mathematics, especially in Order theory, a maximal element of a subset S of some Partially ordered set is an element of S that In Mathematics and Set theory, a total order, linear order, simple order, or (non-strict ordering is a Binary relation

To show that a = sup(S), one has to show that a is an upper bound for S and that any other upper bound for S is greater than a. Equivalently, one could alternatively show that a is an upper bound for S and that any number less than a is not an upper bound for S.

Suprema within partially ordered sets

Least upper bounds are important concepts in order theory, where they are also called joins (especially in lattice theory). Order theory is a branch of Mathematics that studies various kinds of Binary relations that capture the intuitive notion of ordering providing a framework for saying In mathematical Order theory, join is a Binary operation on a Partially ordered set that gives the Supremum (least upper bound of its arguments In Mathematics, a lattice is a Partially ordered set (also called a poset) in which every pair of elements has a unique Supremum (the elements' As in the special case treated above, a supremum of a given set is just the least element of the set of its upper bounds, provided that such an element exists. In Mathematics, especially in Order theory, an upper bound of a Subset S of some Partially ordered set ( P, &le

Formally, we have: For subsets S of arbitrary partially ordered sets (P, ≤), a supremum or least upper bound of S is an element u in P such that

1. x ≤ u for all x in S, and
2. for any v in P such that x ≤ v for all x in S it holds that u ≤ v. In Mathematics, especially Order theory, a partially ordered set (or poset) formalizes the intuitive concept of an ordering sequencing or arrangement

Thus the supremum does not exist if there is no upper bound, or if the set of upper bounds has two or more elements of which none is a least element of that set. It can easily be shown that, if S has a supremum, then the supremum is unique (as the least element of any partially ordered set, if it exists, is unique): if u₁ and u₂ are both suprema of S then it follows that u₁ ≤ u₂ and u₂ ≤ u₁, and since ≤ is antisymmetric, one finds that u₁ = u₂.

If the supremum exists it may or may not belong to S. If S contains a greatest element, then that element is the supremum; and if not, then the supremum does not belong to S. In Mathematics, especially in Order theory, the greatest element of a subset S of a Partially ordered set (poset is an element of S

The dual concept of supremum, the greatest lower bound, is called infimum and is also known as meet. In the mathematical area of Order theory, every Partially ordered set P gives rise to a dual (or opposite) partially ordered set which In Mathematics the infimum of a Subset of some set is the Greatest element, not necessarily in the subset that is less than or equal to all elements of In mathematics a meet on a set is defined either as the unique Infimum (greatest lower bound with respect to a Partial order on the set provided an infimum exists

If the supremum of a set S exists, it can be denoted as sup(S) or, which is more common in order theory, by S. Likewise, infima are denoted by inf(S) or S. In lattice theory it is common to use the infimum/meet and supremum/join as binary operators; in this case (and similarly for infima).

A complete lattice is a partially ordered set in which all subsets have both a supremum (join) and an infimum (meet). In Mathematics, a complete lattice is a Partially ordered set in which all subsets have both a Supremum (join and an Infimum (meet

In the sections below the difference between suprema, maximal elements, and minimal upper bounds is stressed. As a consequence of the possible absence of suprema, classes of partially ordered sets for which certain types of subsets are guaranteed to have least upper bound become especially interesting. This leads to the consideration of so-called completeness properties and to numerous definitions of special partially ordered sets. In the mathematical area of Order theory, completeness properties assert the existence of certain infima or suprema of a given partially ordered

Comparison with other order theoretical notions

Greatest elements

The distinction between the supremum of a set and the greatest element of a set may not be immediately obvious. In Mathematics, especially in Order theory, the greatest element of a subset S of a Partially ordered set (poset is an element of S The difference is that the greatest element must be a member of the set, whereas the supremum need not. For example, consider the set of negative real numbers. Since 0 is not a negative number, this set has no greatest element: for every element of the set, there is another, larger element. For instance, for any negative real number x, there is a negative real number x/2, which is greater. On the other hand, the upper bounds of the set of negative reals as a subset of the real numbers obviously constitute of all real numbers greater than or equal to 0. Hence, 0 is the least upper bound of the negative reals, and hence the supremum is 0.

In general, this situation occurs for all subsets that do not contain a greatest element. In contrast, if a set does contain a greatest element, then it also has a supremum given by the greatest element.

Maximal elements

For an example where there are no greatest but still some maximal elements, consider the set of all subsets of the set of natural numbers (the powerset). In Mathematics, especially in Order theory, a maximal element of a subset S of some Partially ordered set is an element of S that In Mathematics, given a set S, the power set (or powerset) of S, written \mathcal{P}(S P ( S) We take the usual subset inclusion as an ordering, i. e. a set is greater than another set if it contains all elements of the other set. Now consider the set S of all sets that contain at most ten natural numbers. The set S has many maximal elements, i. e. elements for which there is no greater element. In fact, all sets with ten elements are maximal. However, the supremum of S is the (only and therefore least) set which contains all natural numbers. One can compute least upper bounds of an element of a powerset (i. e. a set of sets) by just taking the union of its elements.

Minimal upper bounds

Finally, a set may have many minimal upper bounds without having a least upper bound (note that "minimal" and "least" are being used in their precise mathematical sense, not in their ordinary English usage sense). Minimal upper bounds are those upper bounds for which there is no strictly smaller element that also is an upper bound. This does not say that each minimal upper bound is smaller than all other upper bounds, it merely is not greater. The distinction between "minimal" and "least" is only possible when the given order is not a total one. In Mathematics and Set theory, a total order, linear order, simple order, or (non-strict ordering is a Binary relation In a totally ordered set, like the real numbers mentioned above, the concepts are the same.

As an example, let S be the set of all finite subsets of natural numbers and consider the partially ordered set obtained by taking all sets from S together with the set of integers Z and the set of positive real numbers R+, ordered by subset inclusion as above. The integers (from the Latin integer, literally "untouched" hence "whole" the word entire comes from the same origin but via French Then clearly both Z and R+ are greater than all finite sets of natural numbers. Yet, neither is R+ smaller than Z nor is the converse true: both sets are minimal upper bounds but none is a supremum.

Least-upper-bound property

The least-upper-bound property is an example of the aforementioned completeness properties which is typical for the set of real numbers. In the mathematical area of Order theory, completeness properties assert the existence of certain infima or suprema of a given partially ordered In fact, this is sometimes called Dedekind completeness.

If an ordered set S has the property that every nonempty subset of S having an upper bound also has a least upper bound, then S is said to have the least-upper-bound property. As noted above, the set R of all real numbers has the least-upper-bound property. Similarly, the set Z of integers has the least-upper-bound property; if S is a nonempty subset of Z and there is some number n such that every element s of S is less than or equal to n, then there is a least upper bound u for S, an integer that is an upper bound for S and is less than or equal to every other upper bound for S. A well-ordered set also has the least-upper-bound property, and the empty subset has also a least upper bound: the minimum of the whole set. In Mathematics, a well-order relation (or well-ordering) on a set S is a Total order on S with the property that every

An example of a set that lacks the least-upper-bound property is Q, the set of rational numbers. Let S be the set of all rational numbers q such that q² < 2. Then S has an upper bound (1000, for example, or 6) but no least upper bound in Q. For suppose p ∈ Q is an upper bound for S, so p² > 2. Then q = (2p+2)/(p + 2) is also an upper bound for S, and q < p. (To see this, note that q = p − (p² − 2)/(p + 2), and that p² − 2 is positive. )

There is a corresponding 'greatest-lower-bound property'; an ordered set possesses the greatest-lower-bound property if and only if it also possesses the least-upper-bound property; the least-upper-bound of the set of lower bounds of a set is the greatest-lower-bound, and the greatest-lower-bound of the set of upper bounds of a set is the least-upper-bound of the set.

If in a partially ordered set P every bounded subset has a supremum, this applies also, for any set X, in the function space containing all functions from X to P, where f ≤ g if and only if f(x) ≤ g(x) for all x in X. For example, it applies for real functions, and, since these can be considered special cases of functions, for real n-tuples and sequences of real numbers.

See also

• Infimum
• Essential suprema and infima
• Uniform norm (supremum norm)
• Limit superior and limit inferior (supremum limit)

References

• Walter Rudin, Principles of Mathematical Analysis, Third Edition, McGraw-Hill, 1976. In Mathematics the infimum of a Subset of some set is the Greatest element, not necessarily in the subset that is less than or equal to all elements of In Mathematics, the concepts of essential supremum and essential infimum are related to the notions of Supremum and Infimum, but the former are In Mathematical analysis, the uniform norm assigns to real- or complex -valued bounded functions f the nonnegative number In Mathematics, the limit inferior and limit superior (also called infimum limit and supremum limit, or liminf and limsup Walter Rudin (born 1921) is an American Mathematician, currently a Professor Emeritus of Mathematics at the University of Wisconsin-Madison

External links

• supremum (PlanetMath)