# Kripke–Platek set theory with urelements

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Kripke–Platek set theory with urelements

The Kripke–Platek set theory with urelements (KPU) is an axiom system for set theory with urelements that is considerably weaker than the familiar system ZF.

Preliminaries

The usual way of stating the axioms presumes a two sorted first order language $L^*$ with a single binary relation symbol $in$. Letters of the sort $p,q,r,...$ designate urelements, of which there may be none, whereas letters of the sort $a,b,c,...$ designate sets. The letters $x,y,z,...$ may denote both sets and urelements.

The letters for sets may appear on both sides of $in$, while those for urelements may only appear on the left, i.e. the following are examples of valid expressions: $pin a$, $bin a$.

The statement of the axioms also requires reference to a certain collection of formulae called $Delta_0$-formulae. The collection $Delta_0$ consists of those formulae that can be built using the constants, $in$, $eg$, $wedge$, $vee$, and bounded quantification. That is quantification of the form $forall x in a$ or $exists x in a$ where $a$ is given set.

Axioms

The axioms of KPU are the universal closures of the following formulae:

* Extensionality: $forall x \left(x in a leftrightarrow xin b\right) ightarrow a=b$

* Foundation: This is an axiom schema where for every formula $phi\left(x\right)$ we have $exists x phi\left(x\right) ightarrow exists x, \left(phi\left(x\right) wedge forall yin x,\left( eg phi\left(y\right)\right)\right)$.

* Pairing: $exists a, \left(xin a land yin a \right)$

* Union: $exists a forall x in b forall yin x, \left(y in a\right)$

* &Delta;0-Separation: This is again an axiom schema, where for every $Delta_0$-formula $phi\left(x\right)$ we have the following $exists a forall x ,\left(xin a leftrightarrow xin b wedge phi\left(x\right) \right)$.

* $Delta_0$-Collection: This is also an axiom schema, for every $Delta_0$-formula $phi\left(x,y\right)$ we have $forall x in aexists y, phi\left(x,y\right) ightarrow exists bforall x in aexists yin b, phi\left(x,y\right)$.

* Set Existence: $exists a, \left(a=a\right)$

Technically these are axioms that describe the partition of objects into sets and urelements.

* $forall p forall a , \left(p eq a\right)$

* $forall p forall x , \left(x otin p\right)$

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.

* Axiomatic set theory
* Kripke–Platek set theory

References

* Gostanian, Richard, 1980, "Constructible Models of Subsystems of ZF," "Journal of Symbolic Logic 45" (2): .
* Jon Barwise, "Admissible Sets and Structures". Springer Verlag. ISBN 3540074511

* [http://bureau.philo.at/phlo/199703/msg00185.html Logic of Abstract Existence]

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