Jensen's covering theorem
In set theory, Jensen's covering theorem states that if 0# does not exist then every uncountable set of ordinals is contained in a constructible set of the same cardinality. Informally this conclusion says that the constructible universe is close to the universe of all sets. The first proof appeared in (Devlin & Jensen 1975). Silver later gave a fine structure free proof using his machines and finally Magidor (1990) gave an even simpler proof.
The converse of Jensen's covering theorem is also true: if 0# exists then the countable set of all cardinals less than ℵω cannot be covered by a constructible set of cardinality less than ℵω.
In his book Proper Forcing, Shelah proved a strong form of Jensen's covering lemma.
- Devlin, Keith I.; Jensen, R. Björn (1975), "Marginalia to a theorem of Silver", ISILC Logic Conference (Proc. Internat. Summer Inst. and Logic Colloq., Kiel, 1974), Lecture Notes in Mathematics, 499, Berlin, New York: Springer-Verlag, pp. 115–142, doi:10.1007/BFb0079419, ISBN 978-3-540-07534-9, MR 0480036
- Magidor, Menachem (1990), "Representing sets of ordinals as countable unions of sets in the core model", Transactions of the American Mathematical Society, 317 (1): 91–126, doi:10.2307/2001455, ISSN 0002-9947, JSTOR 2001455, MR 0939805
- Mitchell, William (2010), "The covering lemma", Handbook of Set Theory, Springer, pp. 1497–1594, doi:10.1007/978-1-4020-5764-9_19, ISBN 978-1-4020-4843-2
- Shelah, Saharon (1982), Proper forcing, Lecture Notes in Mathematics, 940, Berlin, New York: Springer-Verlag, doi:10.1007/BFb0096536, hdl:10338.dmlcz/143570, ISBN 978-3-540-11593-9, MR 0675955