Martin's maximum

In set theory, a branch of mathematical logic, Martin's maximum, introduced by (Foreman Magidor) and named after Donald Martin, is a generalization of the proper forcing axiom, itself a generalization of Martin's axiom. It represents the broadest class of forcings for which a forcing axiom is consistent. Martin's maximum [math]\displaystyle{ (\operatorname{MM}) }[/math] states that if D is a collection of [math]\displaystyle{ \aleph_1 }[/math] dense subsets of a notion of forcing that preserves stationary subsets of ω₁, then there is a D-generic filter. Forcing with a ccc notion of forcing preserves stationary subsets of ω₁, thus [math]\displaystyle{ \operatorname{MM} }[/math] extends [math]\displaystyle{ \operatorname{MA}(\aleph_1) }[/math]. If (P,≤) is not a stationary set preserving notion of forcing, i.e., there is a stationary subset of ω₁, which becomes nonstationary when forcing with (P,≤), then there is a collection D of [math]\displaystyle{ \aleph_1 }[/math] dense subsets of (P,≤), such that there is no D-generic filter. This is why [math]\displaystyle{ \operatorname{MM} }[/math] is called the maximal extension of Martin's axiom.

The existence of a supercompact cardinal implies the consistency of Martin's maximum. The proof uses Shelah's theories of semiproper forcing and iteration with revised countable supports.

[math]\displaystyle{ \operatorname{MM} }[/math] implies that the value of the continuum is [math]\displaystyle{ \aleph_2 }[/math] and that the ideal of nonstationary sets on ω₁ is [math]\displaystyle{ \aleph_2 }[/math]-saturated. It further implies stationary reflection, i.e., if S is a stationary subset of some regular cardinal κ ≥ ω₂ and every element of S has countable cofinality, then there is an ordinal α < κ such that S ∩ α is stationary in α. In fact, S contains a closed subset of order type ω₁.

References

• Foreman, M.; Magidor, M.; Shelah, Saharon (1988), "Martin's maximum, saturated ideals, and nonregular ultrafilters. I.", Annals of Mathematics, Second series 127 (1): 1–47, doi:10.2307/1971415  correction
• Jech, Thomas (2003), Set Theory, Springer Monographs in Mathematics (Third millennium ed.), Berlin, New York: Springer-Verlag, ISBN 978-3-540-44085-7 
• Moore, Justin Tatch (2011), "Logic and foundations: the proper forcing axiom", in Bhatia, Rajendra, Proceedings of the international congress of mathematicians (ICM 2010), Hyderabad, India, August 19–27, 2010. Vol. II: Invited lectures, Hackensack, NJ: World Scientific, pp. 3–29, ISBN 978-981-4324-30-4, http://www.math.cornell.edu/~justin/Ftp/ICM.pdf 

See also

• Transfinite number