Definition:Inductive Set
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Definition
Let $S$ be a set of sets.
Then $S$ is inductive if and only if:
\((1)\) | $:$ | $S$ contains the empty set: | \(\ds \quad \O \in S \) | ||||||
\((2)\) | $:$ | $S$ is closed under the successor mapping: | \(\ds \forall x:\) | \(\ds \paren {x \in S \implies x^+ \in S} \) | where $x^+$ is the successor of $x$ | ||||
That is, where $x^+ = x \cup \set x$ |
Axiomatic Set Theory
The concept of an inductive set is axiomatised in the Axiom of Infinity in Zermelo-Fraenkel set theory:
- $\exists x: \paren {\paren {\exists y: y \in x \land \forall z: \neg \paren {z \in y} } \land \forall u: u \in x \implies \paren {u \cup \set u \in x} }$
Also known as
Some sources refer to this concept as a successor set.
However, note that this term has already been used on this site.
Examples
Inductive Set as Subset of Real Numbers
A specific instance of such an inductive set is defined by some authors as follows:
Let $I$ be a subset of the real numbers $\R$.
Then $I$ is an inductive set if and only if:
- $1 \in I$
and
- $x \in I \implies \paren {x + 1} \in I$
Also see
- Definition:Inductive Class, of which this is an instance
- Results about inductive sets can be found here.
Sources
- 1960: Paul R. Halmos: Naive Set Theory ... (previous) ... (next): $\S 11$: Numbers
- 2002: Thomas Jech: Set Theory (3rd ed.) ... (previous) ... (next): Chapter $1$: Infinity