Fodor's lemma

In mathematics, particularly in set theory, Fodor's lemma (or the pressing-down lemma) states:

In modern parlance, the nonstationary ideal is normal. The lemma was first proved by the Hungarian set theorist, Géza Fodor in 1956.

There is a Fodor's lemma for trees:

Fodor's lemma also holds for Thomas Jech's notion of stationary sets as well as for the general notion of stationary set.

References

• Preliminary version of an unpublished book: