Editing Upper and lower bounds

{{short description|Majorant and minorant in mathematics}}
{{About|precise bounds|asymptotic bounds|Big O notation}}
[[File:Illustration of supremum.svg|thumb|300px|A set with upper bounds and its least upper bound]]

In [[mathematics]], particularly in [[order theory]], an '''upper bound''' or '''majorant'''<ref name=schaefer/> of a [[subset]] {{mvar|S}} of some [[Preorder|preordered set]] {{math|(''K'', ≤)}} is an [[Element (mathematics)|element]] of {{mvar|K}} that is {{nobr|[[greater than or equal to]]}} every element of {{mvar|S}}.<ref name="MacLane-Birkhoff" /><ref>{{Cite web|url=https://www.mathsisfun.com/definitions/upper-bound.html|title=Upper Bound Definition (Illustrated Mathematics Dictionary)|website=Math is Fun|access-date=2019-12-03}}</ref> 
[[Duality (order theory)|Dually]], a '''lower bound''' or '''minorant''' of {{mvar|S}} is defined to be an element of {{mvar|K}} that is less than or equal to every element of {{mvar|S}}. 
A set with an upper (respectively, lower) bound is said to be '''bounded from above''' or '''majorized'''<ref name=schaefer/> (respectively '''bounded from below''' or '''minorized''') by that bound. 
The terms '''bounded above''' ('''bounded below''') are also used in the mathematical literature for sets that have upper (respectively lower) bounds.<ref>{{Cite web|url=http://mathworld.wolfram.com/UpperBound.html|title=Upper Bound|last=Weisstein|first=Eric W.|website=mathworld.wolfram.com|language=en|access-date=2019-12-03}}</ref>

== Examples ==

For example, {{math|5}} is a lower bound for the set {{math|1=''S'' = {{mset|5, 8, 42, 34, 13934}}}} (as a subset of the [[integers]] or of the [[real numbers]], etc.), and so is {{math|4}}. On the other hand, {{math|6}} is not a lower bound for {{mvar|S}} since it is not smaller than every element in {{mvar|S}}. {{math|13934}} and other numbers ''x'' such that {{math|x ≥ 13934}} would be an upper bound for ''S''.

The set {{math|1=''S'' = {{mset|42}}}} has {{math|42}} as both an upper bound and a lower bound; all other numbers are either an upper bound or a lower bound for that {{mvar|S}}.

Every subset of the [[natural number]]s has a lower bound since the natural numbers have a least element (0 or 1, depending on convention). An infinite subset of the natural numbers cannot be bounded from above. An infinite subset of the integers may be bounded from below or bounded from above, but not both. An infinite subset of the [[rational number]]s may or may not be bounded from below, and may or may not be bounded from above.

Every finite subset of a non-empty [[totally ordered set]] has both upper and lower bounds.

==Bounds of functions==

The definitions can be generalized to [[Function (mathematics)|functions]] and even to sets of functions.

Given a function {{italics correction|{{mvar|f}}}} with [[Domain of a function|domain]] {{mvar|D}} and a preordered set {{math|(''K'', ≤)}} as [[codomain]], an element {{mvar|''y''}} of {{mvar|K}} is an upper bound of {{italics correction|{{mvar|f}}}} if {{math|''y'' ≥ {{italics correction|''f''}}(''x'')}} for each {{mvar|x}} in {{mvar|D}}. The upper bound is called ''[[Mathematical jargon#sharp|sharp]]'' if equality holds for at least one value of {{mvar|x}}. It indicates that the constraint is optimal, and thus cannot be further reduced without invalidating the inequality.

Similarly, a function {{mvar|g}} defined on domain {{mvar|D}} and having the same codomain {{math|(''K'', ≤)}} is an upper bound of {{italics correction|{{mvar|f}}}}, if {{math|''g''(''x'') ≥ {{italics correction|''f''}}(''x'')}} for each {{mvar|x}} in {{mvar|D}}. The function {{mvar|g}} is further said to be an upper bound of a set of functions, if it is an upper bound of ''each'' function in that set.

The notion of lower bound for (sets of) functions is defined analogously, by replacing ≥ with ≤.

==Tight bounds==

An upper bound is said to be a ''tight upper bound'', a ''least upper bound'', or a ''[[supremum]]'', if no smaller value is an upper bound. Similarly, a lower bound is said to be a ''tight lower bound'', a ''greatest lower bound'', or an ''[[infimum]]'', if no greater value is a lower bound.

==Exact upper bounds==
An upper bound {{mvar|u}} of a subset {{mvar|S}} of a preordered set {{math|(''K'', ≤)}} is said to be an ''exact upper bound'' for {{mvar|S}} if every element of {{mvar|K}} that is strictly majorized by {{mvar|u}} is also majorized by some element of {{mvar|S}}. Exact upper bounds of [[reduced product|reduced products]] of [[linear order|linear orders]] play an important role in [[PCF theory]].<ref>{{Cite journal|url=https://www.sciencedirect.com/science/article/pii/S0168007298000116|title=Exact upper bounds and their uses in set theory|last=Kojman|first=Menachem|journal=Annals of Pure and Applied Logic |date=21 August 1998 |volume=92 |issue=3 |pages=267–282 |doi=10.1016/S0168-0072(98)00011-6 }}</ref>

== See also ==

* [[Greatest element and least element]]
* [[Infimum and supremum]]
* [[Maximal and minimal elements]]

== References ==
{{reflist | refs=
<ref name="MacLane-Birkhoff">{{cite book | last1 = Mac Lane| first1 = Saunders | author1-link = Saunders Mac Lane | last2 = Birkhoff| first2 = Garrett | author2-link = Garrett Birkhoff | title = Algebra | url = https://archive.org/details/algebra00lane| url-access = limited| place = Providence, RI | publisher = [[American Mathematical Society]] | page = [https://archive.org/details/algebra00lane/page/n164 145] | year = 1991 | isbn = 0-8218-1646-2 }}</ref>
<!-- <ref name="Davey-Priestley"> {{cite book| author = B. A. Davey and H. A. Priestley| year =2002| title = Introduction to Lattices and Order| edition = 2nd | publisher = Cambridge University Press|  isbn= 0-521-78451-4}} </ref> -->
<ref name=schaefer>{{cite book | last1=Schaefer | first1=Helmut H. | author-link=Helmut H. Schaefer | last2=Wolff | first2=Manfred P. | title=Topological Vector Spaces | publisher=Springer New York Imprint Springer | series=[[Graduate Texts in Mathematics|GTM]] | volume=8 |page=3 | publication-place=New York, NY | year=1999 | isbn=978-1-4612-7155-0 | oclc=840278135 }} <!-- {{sfn | Schaefer | 1999 | p=}} --> </ref>

}}

[[Category:Mathematical terminology]]
[[Category:Order theory]]
[[Category:Real analysis]]

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