Cumulative hierarchy

In mathematical set theory, a cumulative hierarchy is a family of sets W_α indexed by ordinals α. It is often required to satisfy the conditions

• W_α⊆W_α+1⊆P(W_α)
• If α is a limit or 0 then W_α = ∪_β<α W_β

The union of the sets of a cumulative hierarchy is often used as a model of set theory.

Reflection principle

A cumulative hierarchy satisfies a form of the reflection principle: any formula of the language of set theory that holds in the union W of the hierarchy also holds in some stages W_α.

Examples

• The Von Neumann universe is built from a cumulative hierarchy V_α.
• The sets L_α of the constructible universe form a cumulative hierarchy.
• The Boolean valued models constructed by forcing are built using a cumulative hierarchy.
• The well founded sets in a model of set theory (possibly not satisfying the axiom of foundation) form a cumulative hierarchy whose union satisfies the axiom of foundation.

References

• Jech, Thomas (2003). Set Theory. Springer Monographs in Mathematics (Third Millennium ed.). Berlin, New York: Springer-Verlag. ISBN 978-3-540-44085-7. Zbl 1007.03002.