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| In [[mathematics|mathematical]] [[order theory]], an '''ideal''' is a special subset of a [[partially ordered set]] (poset). Although this term historically was derived from the notion of a [[ring ideal]] of [[abstract algebra]], it has subsequently been generalized to a different notion. Ideals are of great importance for many constructions in order and [[lattice theory]].
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| == Basic definitions ==
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| A [[non-empty]] subset ''I'' of a partially ordered set (''P'',≤) is an '''ideal''', if the following conditions hold:
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| # For every ''x'' in ''I'', ''y'' ≤ ''x'' implies that ''y'' is in ''I''. (''I'' is a [[lower set]])
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| # For every ''x'', ''y'' in ''I'', there is some element ''z'' in ''I'', such that ''x'' ≤ ''z'' and ''y'' ≤ ''z''. (''I'' is a [[directed set]])
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| While this is the most general way to define an ideal for arbitrary posets, it was originally defined for [[lattice (order)|lattice]]s only. In this case, the following equivalent definition can be given:
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| a subset ''I'' of a lattice (''P'',≤) is an ideal [[if and only if]] it is a lower set that is closed under finite joins ([[suprema]]), i.e., it is nonempty and for all ''x'', ''y'' in ''I'', the element ''x''<math>\vee</math>''y'' of ''P'' is also in ''I''. | |
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| The [[duality (order theory)|dual]] notion of an ideal, i.e., the concept obtained by reversing all ≤ and exchanging <math>\vee</math> with <math>\wedge</math>, is a '''[[Filter (mathematics)|filter]]'''. The terms '''order ideal''', '''order filter''', '''semi-ideal''', '''down-set''' and '''decreasing subset''' are sometimes used for arbitrary lower or upper sets.<ref>Lawson (1998), {{Google books quote|id=2805q4tFiCkC|page=22|text=is said to be an order ideal if|p. 22}}</ref><ref>Stanley (2002), {{Google books quote|id=EvJg1VjIGyMC|page=100|text=order|p. 100}}</ref> Wikipedia uses only "ideal/filter (of order theory)" and "lower/upper set" to avoid confusion. | |
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| [[Frink ideal]]s, [[pseudoideal]]s and [[pseudoideal|Doyle pseudoideals]] are different generalizations of the notion of a lattice ideal.
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| An ideal or filter is said to be '''proper''' if it is not equal to the whole set ''P''.
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| The smallest ideal that contains a given element ''p'' is a '''principal ideal''' and ''p'' is said to be a '''principal element''' of the ideal in this situation. The principal ideal <math>\downarrow</math>''p'' for a principal ''p'' is thus given by <math>\downarrow</math>''p'' = {''x'' in ''P'' | ''x'' ≤ ''p''}.
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| == Prime ideals ==
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| An important special case of an ideal is constituted by those ideals whose set-theoretic complements are filters, i.e. ideals in the inverse order. Such ideals are called '''prime ideals'''. Also note that, since we require ideals and filters to be non-empty, every '''prime filter''' is necessarily proper. For lattices, prime ideals can be characterized as follows:
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| A subset ''I'' of a lattice (''P'',≤) is a prime ideal, if and only if
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| # ''I'' is a proper ideal of ''P'', and
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| # for every elements ''x'' and ''y'' of ''P'', ''x''<math>\wedge</math>''y'' in ''I'' implies that ''x'' is in ''I'' or ''y'' is in ''I''.
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| It is easily checked that this indeed is equivalent to stating that ''P''\''I'' is a filter (which is then also prime, in the dual sense).
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| For a [[complete lattice]] the further notion of a '''completely prime ideal'''
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| is meaningful. It is defined to be a proper ideal ''I'' with the additional
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| property that, whenever the meet ([[infimum]]) of some arbitrary set ''A''
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| is in ''I'', some element of ''A'' is also in ''I''. So this is just a
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| specific prime ideal that extends the above conditions to infinite meets.
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| The existence of prime ideals is in general not obvious, and often a satisfactory amount of prime ideals cannot be derived within [[Zermelo–Fraenkel set theory]]. This issue is discussed in various [[Boolean prime ideal theorem|prime ideal theorem]]s, which are necessary for many applications that require prime ideals.
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| == Maximal ideals ==
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| An ideal ''I'' is '''maximal''' if it is proper and there is no ''proper'' ideal ''J'' that is a strictly greater set than ''I''. Likewise, a filter ''F'' is maximal if it is proper and there is no proper filter that is strictly greater.
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| When a poset is a [[distributive lattice]], maximal ideals and filters are necessarily prime, while the converse of this statement is false in general.
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| Maximal filters are sometimes called [[ultrafilter]]s, but this terminology is often reserved for Boolean algebras, where a maximal filter (ideal) is a filter (ideal) that contains exactly one of the elements {''a'', ¬''a''}, for each element ''a'' of the Boolean algebra. In Boolean algebras, the terms ''prime ideal'' and ''maximal ideal'' coincide, as do the terms ''prime filter'' and ''maximal filter''.
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| There is another interesting notion of maximality of ideals: Consider an ideal ''I'' and a filter ''F'' such that ''I'' is [[disjoint sets|disjoint]] from ''F''. We are interested in an ideal ''M'' that is maximal among all ideals that contain ''I'' and are disjoint from ''F''. In the case of distributive lattices such an ''M'' is always a prime ideal. A proof of this statement follows.
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| ::'''Proof.''' Assume the ideal ''M'' is maximal with respect to disjointness from the filter ''F''. Suppose for a contradiction that ''M'' is not prime, i.e. there exists a pair of elements ''a'' and ''b'' such that ''a''<math>\wedge</math>''b'' in ''M'' but neither ''a'' nor ''b'' are in ''M''. Consider the case that for all ''m'' in ''M'', ''m''<math>\vee</math>''a'' is not in ''F''. One can construct an ideal ''N'' by taking the downward closure of the set of all binary joins of this form, i.e. ''N'' = { ''x'' | ''x''≤ ''m''<math>\vee</math>''a'' for some ''m'' in ''M''}. It is readily checked that ''N'' is indeed an ideal disjoint from ''F'' which is strictly greater than ''M''. But this contradicts the maximality of ''M'' and thus the assumption that ''M'' is not prime.
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| ::For the other case, assume that there is some ''m'' in ''M'' with ''m''<math>\vee</math>''a'' in ''F''. Now if any element ''n'' in ''M'' is such that ''n''<math>\vee</math>''b'' is in ''F'', one finds that (''m''<math>\vee</math>''n'')<math>\vee</math>''b'' and (''m''<math>\vee</math>''n'')<math>\vee</math>''a'' are both in ''F''. But then their meet is in ''F'' and, by distributivity, (''m''<math>\vee</math>''n'') <math>\vee</math>(''a''<math>\wedge</math>''b'') is in ''F'' too. On the other hand, this finite join of elements of ''M'' is clearly in ''M'', such that the assumed existence of ''n'' contradicts the disjointness of the two sets. Hence all elements ''n'' of ''M'' have a join with ''b'' that is not in ''F''. Consequently one can apply the above construction with ''b'' in place of ''a'' to obtain an ideal that is strictly greater than ''M'' while being disjoint from ''F''. This finishes the proof.
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| However, in general it is not clear whether there exists any ideal ''M'' that is maximal in this sense. Yet, if we assume the [[Axiom of Choice]] in our set theory, then the existence of ''M'' for every disjoint filter–ideal-pair can be shown. In the special case that the considered order is a [[Boolean algebra (structure)|Boolean algebra]], this theorem is called the [[Boolean prime ideal theorem]]. It is strictly weaker than the Axiom of Choice and it turns out that nothing more is needed for many order theoretic applications of ideals.
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| == Applications == | |
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| The construction of ideals and filters is an important tool in many applications of order theory.
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| * In [[Stone's representation theorem for Boolean algebras]], the maximal ideals (or, equivalently via the negation map, ultrafilters) are used to obtain the set of points of a [[topological space]], whose [[clopen set]]s are [[isomorphism|isomorphic]] to the original Boolean algebra.
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| * Order theory knows many [[completion (order theory)|completion procedures]], to turn posets into posets with additional [[completeness (order theory)|completeness]] properties. For example, the [[ideal completion]] of a given partial order ''P'' is the set of all ideals of ''P'' ordered by subset inclusion. This construction yields the [[free object|free]] [[directed complete partial order|dcpo]] generated by ''P''. An ideal is principal if and only if it is [[Compact element|compact]] in the ideal completion, so the original poset can be recovered as the sub-poset consisting of compact elements. Furthermore, every [[algebraic poset|algebraic dcpo]] can be reconstructed as the ideal completion of its set of compact elements.
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| == History ==
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| Ideals were introduced first by [[Marshall H. Stone]], who derived their name from the ring ideals of abstract algebra. He adopted this terminology because, using the [[isomorphism of categories|isomorphism of the categories]] of [[Boolean algebra (structure) | Boolean algebras]] and of [[Boolean ring]]s, the two notions do indeed coincide.
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| == Literature ==
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| Ideals and filters are among the most basic concepts of order theory. See the introductory books given for [[order theory]] and [[lattice theory]], and the literature on the [[Boolean prime ideal theorem]].
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| A monograph available free online:
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| * Burris, Stanley N.; and Sankappanavar, Hanamantagouda P.; 1981. ''[http://www.thoralf.uwaterloo.ca/htdocs/ualg.html A Course in Universal Algebra.]'' Springer-Verlag. ISBN 3-540-90578-2.
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| == See also ==
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| *[[Filter (mathematics)]]
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| *[[Ideal (ring theory)]]
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| *[[Ideal (set theory)]]
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| == Notes ==
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| <references/>
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| == References ==
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| * {{cite book |last1=Lawson |first1=M.V. |authorlink1= |last2= |first2= |authorlink2= |title=Inverse semigroups: the theory of partial symmetries |url= |edition= |series= |volume= |year=1998 |publisher=World Scientific |location= |isbn=978-981-02-3316-7 |id= }}
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| * {{cite book |last1=Stanley |first1=R.P. |authorlink1= |title=Enumerative combinatorics |url= |edition= |series=Cambridge studies in advanced mathematics |volume=1 |year=2002 |publisher=Cambridge University Press |location= |isbn=978-0-521-66351-9 |id= }}
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| [[Category:Order theory]]
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| [[Category:Ideals]]
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| [[Category:Articles containing proofs]]
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