Clubsuit

In mathematics, and particularly in axiomatic set theory, ♣_S (clubsuit) is a family of combinatorial principles that are a weaker version of the corresponding ◊_S; it was introduced in 1975 by Adam Ostaszewski.[1]

Definition

For a given cardinal number ${\displaystyle \kappa }$ and a stationary set ${\displaystyle S\subseteq \kappa }$, ${\displaystyle \clubsuit _{S}}$ is the statement that there is a sequence ${\displaystyle \left\langle A_{\delta }:\delta \in S\right\rangle }$ such that

• every A_δ is a cofinal subset of δ
• for every unbounded subset ${\displaystyle A\subseteq \kappa }$, there is a ${\displaystyle \delta }$ so that ${\displaystyle A_{\delta }\subseteq A}$

${\displaystyle \clubsuit _{\omega _{1}}}$ is usually written as just ${\displaystyle \clubsuit }$.

♣ and ◊

It is clear that ◊ ⇒ ♣, and it was shown in 1975 that ♣ + CH ⇒ ◊; however, Saharon Shelah gave a proof in 1980 that there exists a model of ♣ in which CH does not hold, so ♣ and ◊ are not equivalent (since ◊ ⇒ CH).[2]

See also

• Club set

References

1. Ostaszewski, Adam J. (1975). "On countably compact perfectly normal spaces". Journal of the London Mathematical Society. 14 (3): 505–516. doi:10.1112/jlms/s2-14.3.505.
2. Shelah, S. (1980). "Whitehead groups may not be free even assuming CH, II". Israel Journal of Mathematics. 35 (4): 257–285. doi:10.1007/BF02760652.