Cumulative hierarchy

In mathematical set theory, a cumulative hierarchy is a family of sets W_α indexed by ordinals α such that

• W_α⊆W_α+1
• If α is a limit then W_α = ∪_β<α W_β

It is also sometimes assumed that W_α+1⊆P(W_α) or that W₀ is empty.

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

The phrase "the cumulative hierarchy" usually refers to the standard cumulative hierarchy V_α of the Von Neumann universe with V_α+1=P(V_α).

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.