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In mathematics, and particularly in axiomatic set theory, the diamond principle ◊ is a combinatorial principle introduced by Jensen (1972) that holds in the constructible universe and that implies the continuum hypothesis. Jensen extracted the diamond principle from his proof that V=L implies the existence of a Suslin tree.

[edit] Definition

The diamond principle ◊ says that there exists a ◊-sequence, in other words sets A_α⊆α for α<ω₁ such that for any subset A of ω₁ the set of α with A∩α = A_α is stationary in ω₁.

More generally, or a given cardinal number $κ$ and a stationary set $S\subseteq\kappa$ , the statement ◊_S (sometimes written ◊(S) or ◊_κ(S)) is the statement that there is a sequence $\langle A_\alpha: \alpha \in S \rangle$ such that

each $A_\alpha \subseteq \alpha$
for every $A \subseteq \kappa, \{\alpha \in S: A \cap \alpha = A_\alpha\}$ is stationary in $κ$

The principle ◊_ω₁ is the same as ◊.

[edit] Properties and use

Jensen (1972) showed that the diamond principle ◊ implies the existence of Suslin trees. He also showed that ◊ implies the CH. Also ♣ + CH implies ◊, but Shelah gave models of ♣ + ¬ CH, so ◊ and ♣ are not equivalent (rather, ♣ is weaker than ◊).

Akemann & Weaver (2004) used ◊ to construct a C^*-algebra serving as a counterexample to Naimark's problem.

For all cardinals $κ$ and stationary subsets $S \subseteq \kappa^+$ , ◊_S holds in the constructible universe. Recently Shelah proved that for $\kappa>\aleph_0$ , ◊_κ⁺ follows from $2 κ = κ +$ .

[edit] See also

[edit] References

Akemann, Charles; Weaver, Nik (2004), "Consistency of a counterexample to Naimark's problem", Proceedings of the National Academy of Sciences of the United States of America 101 (20): 7522–7525, doi:10.1073/pnas.0401489101, MR 2057719, ISSN 0027-8424, http://arxiv.org/abs/math.OA/0312135
Jensen, R. Björn (1972), "The fine structure of the constructible hierarchy", Annals of Mathematical lLogic 4: 229–308, doi:10.1016/0003-4843(72)90001-0, MR 0309729
Assaf Rinot, Jensen's diamond principle and its relatives, online
S. Shelah: Whitehead groups may not be free, even assuming CH, II, Israel J. Math., 35(1980), 257–285.

Contents

[edit] Definition

[edit] Properties and use

[edit] See also

[edit] References