Woodin cardinal
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In set theory, a Woodin cardinal is a cardinal number κ such that for all
- f : κ → κ
there exists
- α < κ with f[α] ⊆ α
and an elementary embedding
- j : V → M
from V into a transitive inner model M with critical point α and
- Vj(f)(α) ⊆ M.
Woodin cardinals are important in descriptive set theory. Existence of infinitely many Woodin cardinals implies projective determinacy, which in turn implies that every projective set is measurable, has the Baire property (differs from an open set by a meager set, that is, a set which is a countable union of nowhere dense sets), and the perfect subset property (is either countable or contains a perfect subset).