Journal of Symbolic Logic

A relative of the approachability ideal, diamond and non-saturation

Assaf Rinot

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Let λ denote a singular cardinal. Zeman, improving a previous result of Shelah, proved that □*λ together with 2λ=λ⁺ implies ♢S for every S⊆λ⁺ that reflects stationarily often. In this paper, for a set S⊆λ⁺, a normal subideal of the weak approachability ideal is introduced, and denoted by I[S;λ]. We say that the ideal is fat if it contains a stationary set. It is proved:

1. if I[S;λ] is fat, then NSλ⁺↾ S is non-saturated;

2. if I[S;λ] is fat and 2λ=λ⁺, then ♢S holds;

3. □*λ implies that I[S;λ] is fat for every S⊆λ⁺ that reflects stationarily often;

4. it is relatively consistent with the existence of a supercompact cardinal that □*λ fails, while I[S;λ] is fat for every stationary S⊆λ⁺ that reflects stationarily often.

The stronger principle ♢*λ⁺ is studied as well.

Article information

J. Symbolic Logic, Volume 75, Issue 3 (2010), 1035-1065.

First available in Project Euclid: 9 July 2010

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Digital Object Identifier

Mathematical Reviews number (MathSciNet)

Zentralblatt MATH identifier

Primary: 03E35: Consistency and independence results
Secondary: 03E05: Other combinatorial set theory

diamond diamond star saturation approachability ideal weak square reflection principles stationary hitting sap


Rinot, Assaf. A relative of the approachability ideal, diamond and non-saturation. J. Symbolic Logic 75 (2010), no. 3, 1035--1065. doi:10.2178/jsl/1278682214.

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