Hereditary set

Inset theory,ahereditary set(orpure set) is asetwhose elements are all hereditary sets. That is, all elements of the set are themselves sets, as are all elements of the elements, and so on.

Examples

For example, it isvacuously truethat the empty set is a hereditary set, and thus the set$\{\varnothing \}$containing only theempty set$\varnothing$is a hereditary set. Similarly, a set$\{\varnothing,\{\varnothing \}\}$that contains two elements: the empty set and the set that contains only the empty set, is a hereditary set.

In formulations of set theory

In formulations of set theory that are intended to be interpreted in thevon Neumann universeor to express the content ofZermelo–Fraenkel set theory,allsets are hereditary, because the only sort of object that is even a candidate to be an element of a set is another set. Thus the notion of hereditary set is interesting only in a context in which there may beurelements.

Assumptions

The inductive definition of hereditary sets presupposes that set membership iswell-founded(i.e., theaxiom of regularity), otherwise the recurrence may not have a unique solution. However, it can be restated non-inductively as follows: a set is hereditary if and only if itstransitive closurecontains only sets. In this way the concept of hereditary sets can also be extended tonon-well-founded set theoriesin which sets can be members of themselves. For example, a set that contains only itself is a hereditary set.

See also

• Hereditarily countable set
• Hereditarily finite set
• Well-founded set

References

• Kunen, Kenneth(1980).Set Theory: An Introduction to Independence Proofs.North-Holland.ISBN0-444-85401-0.