Clubsuit

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

Definition

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

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

$\clubsuit_{\omega_1}$ is usually written as just $\clubsuit$.

♣ 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).

References

• A. J. Ostaszewski, On countably compact perfectly normal spaces, Journal of London Mathematical Society, 1975 (2) 14, pp. 505-516.
• S. Shelah, Whitehead groups may not be free, even assuming CH, II, Israel Journal of Mathematics, 1980 (35) pp. 257-285.

See also

• Club set