Kripke–Platek set theory

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The Kripke–Platek axioms of set theory (KP) are a system of axioms of axiomatic set theory, developed by Saul Kripke and Richard Platek. The axiom system is written in first-order logic; it has an infinite number of axioms because an axiom schema is used.

1 The axioms of KP
2 Proof that Cartesian products exist
3 Admissible sets
4 See also
5 References

[edit] The axioms of KP

Axiom of extensionality: Two sets are the same if and only if they have the same elements.
Axiom of induction: If φ(a) is a proposition, and if for all sets x it follows from the fact that φ(y) is true for all elements y of x that φ(x) holds, then φ(x) holds for all sets x.
Axiom of empty set: There exists a set with no members, called the empty set and denoted {}.
Axiom of pairing: If x, y are sets, then so is {x, y}, a set containing x and y as its only elements.
Axiom of union: For any set x, there is a set y such that the elements of y are precisely the elements of the elements of x.
Axiom of Σ₀-separation: Given any set and any Σ₀-proposition φ(x), there is a subset of the original set containing precisely those elements x for which φ(x) holds. (This is an axiom schema.)

Here, a Σ₀, or Π₀, or Δ₀ proposition is one whose quantifiers are all bound, that is, are of the form $\forall u \in v$ or $\exist u \in v.$ (More generally, we would say that a formula is Σ_n+1 when it is obtained by adding existential quantifiers in front of a Π_n formula, and that it is Π_n+1 when it is obtained by adding universal quantifiers in front of a Σ_n formula: this is related to the arithmetical hierarchy but in the context of set theory.)

Axiom of Σ₀-collection: Given any Σ₀-proposition φ(x, y), if for every set x there exists a set y such that φ(x, y) holds, then for all sets u there exists a set v such that for every x in u there is a y in v such that φ(x, y) holds. (See Axiom of collection.)

If one knew that some set existed, then the axiom of separation would give us the empty set.

These axioms differ from ZFC in as much as they exclude the axioms of: infinity, powerset, and choice. Also the axioms of separation and collection here are weaker than the corresponding axioms in ZFC because the predicates φ used in these are limited to bound quantifiers only.

The axiom of induction here is stronger than the usual axiom of regularity (which amounts to applying induction to the complement of a set (the class of all sets not in the given set)).

[edit] Proof that Cartesian products exist

Theorem: If A and B are sets, then there is a set A×B which consists of all ordered pairs (a, b) of elements a of A and b of B.

{a} = {a, a} exists by the axiom of pairing. {a, b} exists by the axiom of pairing. Thus (a, b) = { {a}, {a, b} } exists by the axiom of pairing.

If p is intended to stand for (a, b), then a Δ₀ formula expressing that is: $\exist r \in p (a \in r \and \forall x \in r (x = a)) \and \exist s \in p (a \in s \and b \in s \and \forall x \in s (x = a \or x = b))$ and $\forall t \in p ((a \in t \and \forall x \in t (x = a)) \or (a \in t \and b \in t \and \forall x \in t (x = a \or x = b))).$

Thus a superset of A×{b} = {(a, b) | a in A} exists by the axiom of collection.

Abbreviate the formula above by $\psi (a, b, p)\!.$ Then $\exist a \in A \psi (a, b, p)$ is Δ₀. Thus A×{b} itself exists by the axiom of separation.

If v is intended to stand for A×{b}, then a Δ₀ formula expressing that is: $\forall a \in A \exist p \in v \psi (a, b, p) \and \forall p \in v \exist a \in A \psi (a, b, p).$

Thus a superset of {A×{b} | b in B} exists by the axiom of collection.

Putting $\exist b \in B$ in front of that last formula and we get that the set {A×{b} | b in B} itself exists by the axiom of separation.

Finally, A×B = $\cup$ {A×{b} | b in B} exists by the axiom of union. This is what was to be proved.

[edit] Admissible sets

If L_α is a standard model of KP set theory, then it is said to be an "admissible set" and α is called an "admissible ordinal". α is an admissible ordinal if and only if α is a limit ordinal and there does not exist a γ<α for which there is a Σ₁(L_α) mapping from γ onto α. If M is a standard model of KP, then the set of ordinals in M is an admissible ordinal.

If L_α is a standard model of KP set theory without the axiom of Σ₀-collection, then it is said to be an "amenable set".