## You are here

Homeaxiom of foundation

## Primary tabs

# axiom of foundation

The *axiom of foundation* (also called the *axiom of regularity*) is an axiom of ZF set theory prohibiting circular sets and sets with infinite levels of containment. Intuitively, it states that every set can be built up from the empty set. There are several equivalent formulations, for instance:

For any nonempty set $X$ there is some $y\in X$ such that $y\cap X=\emptyset$.

For any set $X$, there is no function $f$ from $\omega$ to the transitive closure of $X$ such that for every $n$, $f(n+1)\in f(n)$.

For any formula $\phi$, if there is any set $x$ such that $\phi(x)$ then there is some $X$ such that $\phi(X)$ but there is no $y\in X$ such that $\phi(y)$.

Sets which satisfy this axiom are called *artinian*. It is known that, if ZF without this axiom is consistent, then this axiom does not add any inconsistencies.

One important consequence of this property is that no set can contain itself. For instance, if there were a set $X$ such that $X\in X$ then we could define a function $f(n)=X$ for all $n$, which would then have the property that $f(n+1)\in f(n)$ for all $n$.

## Mathematics Subject Classification

03C99*no label found*

- Forums
- Planetary Bugs
- HS/Secondary
- University/Tertiary
- Graduate/Advanced
- Industry/Practice
- Research Topics
- LaTeX help
- Math Comptetitions
- Math History
- Math Humor
- PlanetMath Comments
- PlanetMath System Updates and News
- PlanetMath help
- PlanetMath.ORG
- Strategic Communications Development
- The Math Pub
- Testing messages (ignore)

- Other useful stuff

## Recent Activity

new question: Prove that for any sets A, B, and C, An(BUC)=(AnB)U(AnC) by St_Louis

Apr 20

new image: information-theoretic-distributed-measurement-dds.png by rspuzio

new image: information-theoretic-distributed-measurement-4.2 by rspuzio

new image: information-theoretic-distributed-measurement-4.1 by rspuzio

new image: information-theoretic-distributed-measurement-3.2 by rspuzio

new image: information-theoretic-distributed-measurement-3.1 by rspuzio

new image: information-theoretic-distributed-measurement-2.1 by rspuzio

Apr 19

new collection: On the Information-Theoretic Structure of Distributed Measurements by rspuzio

Apr 15

new question: Prove a formula is part of the Gentzen System by LadyAnne

Mar 30

new question: A problem about Euler's totient function by mbhatia