# Category:Definitions/Zermelo Set Theory

This category contains definitions related to Zermelo Set Theory.

Related results can be found in Category:Zermelo Set Theory.

**Zermelo set theory** is a system of axiomatic set theory.

Its basis consists of a system of Aristotelian logic, appropriately axiomatised, together with the following axioms:

### The Axiom of Extension

Let $A$ and $B$ be sets.

The **axiom of extension** states that:

- $A$ and $B$ are equal

- they contain the same elements.

That is, if and only if:

and:

This can be formulated as follows:

- $\forall x: \paren {x \in A \iff x \in B} \iff A = B$

### The Axiom of the Empty Set

- $\exists x: \forall y \in x: y \ne y$

### The Axiom of Pairing

For any two sets, there exists a set to which only those two sets are elements:

- $\forall a: \forall b: \exists c: \forall z: \paren {z = a \lor z = b \iff z \in c}$

### The Axiom of Specification

For any well-formed formula $\map P y$, we introduce the axiom:

- $\forall z: \exists x: \forall y: \paren {y \in x \iff \paren {y \in z \land \map P y} }$

where each of $x$, $y$ and $z$ range over arbitrary sets.

### The Axiom of Unions

For every set of sets $A$, there exists a set $x$ (the **union set**) that contains all and only those elements that belong to at least one of the sets in the $A$:

- $\forall A: \exists x: \forall y: \paren {y \in x \iff \exists z: \paren {z \in A \land y \in z} }$

### The Axiom of Powers

For every set, there exists a set of sets whose elements are all the subsets of the given set.

- $\forall x: \exists y: \paren {\forall z: \paren {z \in y \iff \paren {w \in z \implies w \in x} } }$

### The Axiom of Infinity

There exists a set containing:

That is:

- $\exists x: \paren {\paren {\exists y: y \in x \land \forall z: \neg \paren {z \in y} } \land \forall u: u \in x \implies u^+ \in x}$

## Pages in category "Definitions/Zermelo Set Theory"

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