Extreme value: Difference between revisions
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I made a bad example in the previous comment. In ω+1 with order reversed, {ω} is the least element but there is no supremum of {ω} in ω+1. T.Miyasaka |
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#REDIRECT [[Maximum and minimum]] |
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{{redirect5|Maximum|the sense of the maximum of a [[function (mathematics)]]|Maxima and minima}} |
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The largest and the smallest element of a [[set]] are called '''extreme values''', '''absolute extrema''', or '''extreme records'''. |
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{{R hatnote}} |
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For a [[differentiable]] [[function (mathematics)|function]] <math>f</math>, if <math>f(x_0)</math> is an extreme value for the set of all values <math>f(x)</math>, and if <math>x_0</math> is in the [[interior]] of the [[domain (mathematics)|domain]] of <math>f</math>, then <math>(x_0, f(x_0))</math> is a [[stationary point]] or [[critical point]]. |
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== Extreme values in abstract spaces with order == |
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In the case of a general [[partial order]] one should not confuse a '''least element''' (smaller than all other) and a '''minimal element''' (nothing is smaller). Likewise, a '''[[greatest element]]''' of a [[poset]] is an [[upper bound]] of the set which is contained within the set, whereas a '''maximal element''' ''m'' of a poset ''A'' is an element of ''A'' such that if ''m'' ≤ ''b'' (for any ''b'' in ''A'') then ''m'' = ''b''. |
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Any least element or greatest element of a poset will be unique, but a poset can have several minimal or maximal elements. If a poset has more than one maximal element, then these elements will not be mutually comparable. |
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In a [[total order|totally ordered]] set, or chain, all elements are mutually comparable, so such a set can have at most one minimal element and at most one maximal element. Then, due to mutual comparability, the minimal element will also be the least element and the maximal element will also be the greatest element. |
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If a chain is finite then it will always have a maximum (maximal element, greatest element) and a minimum (minimal element, least element). If a chain is infinite then it need not have a maximum or a minimum. For example, the set of [[natural number]]s has no maximum, though it has a minimum. |
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If an infinite chain ''S'' is bounded, then the [[topological closure|closure]] ''Cl(S)'' of the set occasionally have a minimum and a maximum, in such case they are called the '''[[infimum|greatest lower bound]]''' and the '''[[supremum|least upper bound]]''' of the set ''S'', respectivelly. |
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In general, if an ordered set ''S'' has a greatest element m, m is a maximal element. Furthermore, if ''S'' is a subset of an ordered set ''T'' and m is the greatest element of ''S'' with respect to order induced by ''T'', m is a least upper bound of ''S'' in ''T''. The similar result holds for least element, minimal element and greatest lower bound. |
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'''See also:''' [[extreme value theorem]], [[extreme value theory]]. |
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'''Compare:''' [[extreme point]]. |
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[[Category:Calculus]] |
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[[Category:Order theory]] |
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[[ar:حدود عليا وحدود دنيا]] |
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[[de:Extremwert]] |
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[[fr:Plus grand élément]] |
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[[nl:Extreme waarden]] |
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[[pl:Ekstremum]] |
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[[pt:Mínimo]] |
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[[fi:Maksimi]] |
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Latest revision as of 00:34, 12 February 2023
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