Difference between revisions of "Partial ordering"
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==Definition== | ==Definition== | ||
| − | Given a [[relation]], {{M|\sqsubseteq}} in {{M|X}} (mathematically: {{M|\sqsubseteq\subseteq X\times X}}<ref group="Note">Here {{M|\sqsubseteq}} is the name of the relation, so {{M|(x,y)\in \sqsubseteq}} means {{M|x\sqsubseteq y}} - as usual for [[relation|relations]]</ref>) we say {{M|\sqsubseteq}} is a ''partial order''{{rAPIKM}} if: | + | Given a [[relation]], {{M|\sqsubseteq}} in {{M|X}} (mathematically: {{M|\sqsubseteq\subseteq X\times X}}<ref group="Note">Here {{M|\sqsubseteq}} is the name of the relation, so {{M|(x,y)\in \sqsubseteq}} means {{M|x\sqsubseteq y}} - as usual for [[relation|relations]]</ref>) we say {{M|\sqsubseteq}} is a ''partial order''{{rAPIKM}}{{rSTTJ}}{{rRAAAHS}} if: |
* The relation {{M|\sqsubseteq}} is all 3 of the following: | * The relation {{M|\sqsubseteq}} is all 3 of the following: | ||
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! 2 | ! 2 | ||
| − | | [[Relation#Types_of_relation|Identitive]] | + | | [[Relation#Types_of_relation|Identitive]] ({{AKA}}: [[Relation#Types_of_relation|antisymmetric]]) |
| {{M|1=\forall x,y\in X[((x,y)\in\sqsubseteq\wedge(y,x)\in\sqsubseteq)\implies (x=y)]}} or equivalently<br/> | | {{M|1=\forall x,y\in X[((x,y)\in\sqsubseteq\wedge(y,x)\in\sqsubseteq)\implies (x=y)]}} or equivalently<br/> | ||
{{M|1=\forall x,y\in X[(x\sqsubseteq y\wedge y\sqsubseteq x)\implies(x=y)]}} | {{M|1=\forall x,y\in X[(x\sqsubseteq y\wedge y\sqsubseteq x)\implies(x=y)]}} | ||
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* '''Note:''' {{M|\le}}, {{M|\preceq}} or {{M|\subseteq}}<ref group="Warning">I avoid using {{M|\subseteq}} for anything other than denoting [[subset|subsets]], the relation and the set it relates on will go together, so you'll already be using {{M|\subseteq}} to mean subset</ref> are all commonly used for partial relations too. | * '''Note:''' {{M|\le}}, {{M|\preceq}} or {{M|\subseteq}}<ref group="Warning">I avoid using {{M|\subseteq}} for anything other than denoting [[subset|subsets]], the relation and the set it relates on will go together, so you'll already be using {{M|\subseteq}} to mean subset</ref> are all commonly used for partial relations too. | ||
** The corresponding [[strict partial ordering|strict partial orderings]] are {{M|<}}, {{M|\prec}} and {{M|\subset}} | ** The corresponding [[strict partial ordering|strict partial orderings]] are {{M|<}}, {{M|\prec}} and {{M|\subset}} | ||
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==Induced [[strict partial ordering]]== | ==Induced [[strict partial ordering]]== | ||
Here, let {{M|\preceq}} be a ''partial ordering'' as defined above, then the relation, {{M|\prec}} defined by: | Here, let {{M|\preceq}} be a ''partial ordering'' as defined above, then the relation, {{M|\prec}} defined by: | ||
Revision as of 13:17, 1 January 2016
- Note to reader: this page defines [ilmath]\sqsubseteq[/ilmath] as the partial ordering under study, this is to try and make the concept distinct from [ilmath]\le[/ilmath], which the reader should have been familiar with from a young age (and thus can taint initial study)
Definition
Given a relation, [ilmath]\sqsubseteq[/ilmath] in [ilmath]X[/ilmath] (mathematically: [ilmath]\sqsubseteq\subseteq X\times X[/ilmath][Note 1]) we say [ilmath]\sqsubseteq[/ilmath] is a partial order[1][2][3] if:
- The relation [ilmath]\sqsubseteq[/ilmath] is all 3 of the following:
| Name | Definition | |
|---|---|---|
| 1 | Reflexive | [ilmath]\forall x\in X[(x,x)\in\sqsubseteq][/ilmath] or equivalently [ilmath]\forall x\in X[x\sqsubseteq x][/ilmath] |
| 2 | Identitive (AKA: antisymmetric) | [ilmath]\forall x,y\in X[((x,y)\in\sqsubseteq\wedge(y,x)\in\sqsubseteq)\implies (x=y)][/ilmath] or equivalently [ilmath]\forall x,y\in X[(x\sqsubseteq y\wedge y\sqsubseteq x)\implies(x=y)][/ilmath] |
| 3 | Transitive | [ilmath]\forall x,y,z\in X[((x,y)\in\sqsubseteq\wedge(y,z)\in\sqsubseteq)\implies(x,z)\in\sqsubseteq][/ilmath] or equivalently [ilmath]\forall x,y,z\in X[(x\sqsubseteq y\wedge y\sqsubseteq z)\implies(x\sqsubseteq z)][/ilmath] |
- Note: [ilmath]\le[/ilmath], [ilmath]\preceq[/ilmath] or [ilmath]\subseteq[/ilmath][Warning 1] are all commonly used for partial relations too.
- The corresponding strict partial orderings are [ilmath]<[/ilmath], [ilmath]\prec[/ilmath] and [ilmath]\subset[/ilmath]
Induced strict partial ordering
Here, let [ilmath]\preceq[/ilmath] be a partial ordering as defined above, then the relation, [ilmath]\prec[/ilmath] defined by:
- [ilmath](x,y)\in\prec\iff[x\ne y\wedge x\preceq y][/ilmath]
- Note: every strict partial ordering induces a partial ordering, given a strict partial ordering, [ilmath]<[/ilmath], we can define a relation [ilmath]\le[/ilmath] as:
- [ilmath]x\le y\iff[x=y\vee x<y][/ilmath] or equivalently (in relational form): [ilmath](x,y)\in\le\iff[x=y\vee (x,y)\in<][/ilmath]
In fact there is a 1:1 correspondence between partial and strict partial orderings, this is why the term "partial ordering" is used so casually, as given a strict you have a partial, given a partial you have a strict.
- See Overview of partial orders for more information
Notes
- ↑ Here [ilmath]\sqsubseteq[/ilmath] is the name of the relation, so [ilmath](x,y)\in \sqsubseteq[/ilmath] means [ilmath]x\sqsubseteq y[/ilmath] - as usual for relations
Warnings
- ↑ I avoid using [ilmath]\subseteq[/ilmath] for anything other than denoting subsets, the relation and the set it relates on will go together, so you'll already be using [ilmath]\subseteq[/ilmath] to mean subset
