Fodor's lemma
Concept in mathematical set theory / From Wikipedia, the free encyclopedia
In mathematics, particularly in set theory, Fodor's lemma states the following:
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If is a regular, uncountable cardinal, is a stationary subset of , and is regressive (that is, for any , ) then there is some and some stationary such that for any . In modern parlance, the nonstationary ideal is normal.
The lemma was first proved by the Hungarian set theorist, Géza Fodor in 1956. It is sometimes also called "The Pressing Down Lemma".