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{{expert-subject|logic|ex2=philosophy|ex3=mathematics|date=March 2011}} | |||
The '''Kripke–Platek set theory with urelements''' ('''KPU''') is an [[axiom system]] for [[set theory]] with [[urelement]]s, based on the traditional (urelement-free) [[Kripke-Platek set theory]]. It is considerably weaker than the (relatively) familiar system [[Zermelo–Fraenkel set theory|ZFU]]. The purpose of allowing urelements is to allow large or high-complexity objects (such as [[Baire space (set theory)|the set of all reals]]) to be included in the theory's transitive models without disrupting the usual well-ordering and recursion-theoretic properties of the [[constructible universe]]; KP is so weak that this is hard to do by [[constructible universe#relative constructibility|traditional means]]. | |||
==Preliminaries== | |||
The usual way of stating the axioms presumes a two sorted first order language <math>L^*</math> with a single binary relation symbol <math>\in</math>. | |||
Letters of the sort <math>p,q,r,...</math> designate urelements, of which there may be none, whereas letters of the sort <math>a,b,c,...</math> designate sets. The letters <math>x,y,z,...</math> may denote both sets and urelements. | |||
The letters for sets may appear on both sides of <math>\in</math>, while those for urelements may only appear on the left, i.e. the following are examples of valid expressions: <math>p\in a</math>, <math>b\in a</math>. | |||
The statement of the axioms also requires reference to a certain collection of formulae called <math>\Delta_0</math>-formulae. The collection <math>\Delta_0</math> consists of those formulae that can be built using the constants, <math>\in</math>, <math>\neg</math>, <math>\wedge</math>, <math>\vee</math>, and bounded quantification. That is quantification of the form <math>\forall x \in a</math> or <math> \exists x \in a</math> where <math>a</math> is given set. | |||
==Axioms== | |||
The axioms of KPU are the [[universal closure]]s of the following formulae: | |||
* [[Axiom of extensionality|Extensionality]]: <math>\forall x (x \in a \leftrightarrow x\in b)\rightarrow a=b</math> | |||
* [[Axiom of regularity|Foundation]]: This is an [[axiom schema]] where for every formula <math>\phi(x)</math> we have <math>\exists a \phi(a) \rightarrow \exists a\, (\phi(a) \wedge \forall x\in a\,(\neg \phi(x)))</math>. | |||
* [[Axiom of pairing|Pairing]]: <math>\exists a\, (x\in a \land y\in a )</math> | |||
* [[Axiom of union|Union]]: <math>\exists a \forall c \in b \forall y\in c\, (y \in a)</math> | |||
* [[Axiom schema of predicative separation|Δ<sub>0</sub>-Separation]]: This is again an [[axiom schema]], where for every <math>\Delta_0</math>-formula <math>\phi(x)</math> we have the following <math>\exists a \forall x \,(x\in a \leftrightarrow x\in b \wedge \phi(x) )</math>. | |||
* <math>\Delta_0</math>-[[Axiom schema of replacement|Collection]]: This is also an [[axiom schema]], for every <math>\Delta_0</math>-formula <math>\phi(x,y)</math> we have <math>\forall x \in a\exists y\, \phi(x,y)\rightarrow \exists b\forall x \in a\exists y\in b\, \phi(x,y) </math>. | |||
* Set Existence: <math>\exists a\, (a=a)</math> | |||
===Additional assumptions=== | |||
Technically these are axioms that describe the partition of objects into sets and urelements. | |||
* <math>\forall p \forall a \, (p \neq a)</math> | |||
* <math>\forall p \forall x \, (x \notin p)</math> | |||
==Applications== | |||
KPU can be applied to the model theory of [[infinitary language]]s. [[model theory|Models]] of KPU considered as sets inside a maximal universe that are [[transitive set|transitive]] as such are called [[admissible set]]s. | |||
== See also == | |||
* [[Axiomatic set theory]] | |||
* [[Admissible set]] | |||
* [[Admissible ordinal]] | |||
* [[Kripke–Platek set theory]] | |||
== References == | |||
* {{Citation | last = Barwise | first = Jon | authorlink = Jon Barwise | title = Admissible Sets and Structures | publisher = Springer-Verlag | year = 1975 | isbn = 3-540-07451-1}}. | |||
* {{Citation | last = Gostanian | first = Richard | title = Constructible Models of Subsystems of ZF | url = http://www.jstor.org/stable/2273185 | journal = [[Journal of Symbolic Logic]] | volume = 45 | pages = 237–250 | year = 1980}}. | |||
== External links == | |||
<!--* [http://www.cs.bilkent.edu.tr/~akman/jour-papers/air/node7.html Admissible Set Theory] (broken?)--> | |||
* [http://bureau.philo.at/phlo/199703/msg00185.html Logic of Abstract Existence] | |||
{{DEFAULTSORT:Kripke-Platek Set Theory With Urelements}} | |||
[[Category:Systems of set theory]] | |||
[[Category:Urelements]] |
Revision as of 11:48, 19 October 2013
Template:Expert-subject The Kripke–Platek set theory with urelements (KPU) is an axiom system for set theory with urelements, based on the traditional (urelement-free) Kripke-Platek set theory. It is considerably weaker than the (relatively) familiar system ZFU. The purpose of allowing urelements is to allow large or high-complexity objects (such as the set of all reals) to be included in the theory's transitive models without disrupting the usual well-ordering and recursion-theoretic properties of the constructible universe; KP is so weak that this is hard to do by traditional means.
Preliminaries
The usual way of stating the axioms presumes a two sorted first order language with a single binary relation symbol . Letters of the sort designate urelements, of which there may be none, whereas letters of the sort designate sets. The letters may denote both sets and urelements.
The letters for sets may appear on both sides of , while those for urelements may only appear on the left, i.e. the following are examples of valid expressions: , .
The statement of the axioms also requires reference to a certain collection of formulae called -formulae. The collection consists of those formulae that can be built using the constants, , , , , and bounded quantification. That is quantification of the form or where is given set.
Axioms
The axioms of KPU are the universal closures of the following formulae:
- Foundation: This is an axiom schema where for every formula we have .
- Δ0-Separation: This is again an axiom schema, where for every -formula we have the following .
- -Collection: This is also an axiom schema, for every -formula we have .
Additional assumptions
Technically these are axioms that describe the partition of objects into sets and urelements.
Applications
KPU can be applied to the model theory of infinitary languages. Models of KPU considered as sets inside a maximal universe that are transitive as such are called admissible sets.
See also
References
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Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
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15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
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