Axiom:Axiom of Replacement/Class Theory/Formulation 2

Axiom

For every mapping $f$ and set $x$, the restriction $f \restriction x$ is a set.

Also known as

The axiom of replacement is also known as the axiom of substitution.

Also see

• Equivalence of Formulations of Axiom of Replacement

• Results about the Axiom of Replacement can be found here.

Historical Note

The axiom of replacement was added to the axioms of Zermelo set theory by Abraham Halevi Fraenkel, and also independently by Thoralf Albert Skolem.

The resulting system of axiomatic set theory is now referred to as Zermelo-Fraenkel Set Theory.

Sources

• 2010: Raymond M. Smullyan and Melvin Fitting: Set Theory and the Continuum Problem (revised ed.) ... (previous) ... (next): Chapter $6$: Order Isomorphism and Transfinite Recursion: $\S 3$ The axiom of substitution: Exercise $3.1 \ \text {(a)}$