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- Genearlizations of Martin’s Axiom and the well-met condition January 11, 2015
- Many diamonds from just one January 6, 2015
- Happy new jewish year! September 24, 2014
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- Walk on countable ordinals: the characteristics December 1, 2013
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### Keywords

Knaster Generalized Clubs Almost-disjoint famiy free Boolean algebra incompactness Singular Cofinality Absoluteness Whitehead Problem Successor of Singular Cardinal Erdos Cardinal PFA Sakurai's Bell inequality Cardinal function Singular cardinals combinatorics Antichain Cardinal Invariants Rainbow sets Non-saturation reflection principles b-scale very good scale Kurepa Hypothesis Poset stationary hitting Club Guessing Almost countably chromatic sap Large Cardinals Forcing Axioms Rock n' Roll Mandelbrot set Square-Brackets Partition Relations Prikry-type forcing Cohen real Hedetniemi's conjecture polarized partition relation Souslin Tree tensor product graph Constructible Universe projective Boolean algebra Prevalent singular cardinals Uniformization Hereditarily Lindelöf space Weakly compact cardinal L-space Diamond S-Space approachability ideal weak square Ostaszewski square Martin's Axiom Forcing Aronszajn tree diamond star Chromatic number Shelah's Strong Hypothesis Axiom R Rado's conjecture ccc Dushnik-Miller PFA(S)[S] Singular Density Successor of Regular Cardinal Foundations Small forcing Minimal Walks weak diamond Universal Sequences OCA middle diamond P-Ideal Dichotomy Partition Relations square stationary reflection Erdos-Hajnal graphs

# Tag Archives: approachability ideal

## Same Graph, Different Universe

Abstract. May the same graph admit two different chromatic numbers in two different universes? how about infinitely many different values? and can this be achieved without changing the cardinals structure? We answer these questions in the affirmative. In this paper, … Continue reading

Posted in Preprints
Tagged approachability ideal, Chromatic number, Constructible Universe, Forcing, Ostaszewski square
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## Shelah’s approachability ideal (part 2)

In a previous post, we defined Shelah’s approachability ideal $I[\lambda]$. We remind the reader that a subset $S\subseteq\lambda$ is in $I[\lambda]$ iff there exists a collection $\{ \mathcal D_\alpha\mid\alpha<\lambda\}\subseteq\mathcal [\mathcal P(\lambda)]^{<\lambda}$ such that for club many $\delta\in S$, the union … Continue reading

Posted in Blog, Expository, Open Problems
Tagged approachability ideal, Club Guessing
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## Shelah’s approachability ideal (part 1)

Given an infinite cardinal $\lambda$, Shelah defines an ideal $I[\lambda]$ as follows. Definition (Shelah, implicit in here). A set $S$ is in $I[\lambda]$ iff $S\subseteq\lambda$ and there exists a collection $\{ \mathcal D_\alpha\mid\alpha<\lambda\}\subseteq\mathcal [\mathcal P(\lambda)]^{<\lambda}$, and some club $E\subseteq\lambda$, so … Continue reading

## Jensen’s diamond principle and its relatives

This is chapter 6 in the book Set Theory and Its Applications (ISBN: 0821848127). Abstract: We survey some recent results on the validity of Jensen’s diamond principle at successor cardinals. We also discuss weakening of this principle such as club … Continue reading

Posted in Open Problems, Publications
Tagged 03E05, 03E35, 03E50, approachability ideal, Club Guessing, Diamond, diamond star, Non-saturation, sap, Souslin Tree, square, stationary hitting, Uniformization, Whitehead Problem
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## The failure of diamond on a reflecting stationary set

Joint work with Moti Gitik. Abstract: It is shown that the failure of $\diamondsuit_S$, for a subset $S\subseteq\aleph_{\omega+1}$ that reflects stationarily often, is consistent with GCH and $\text{AP}_{\aleph_\omega}$, relatively to the existence of a supercompact cardinal. This should be comapred with … Continue reading

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

Abstract: Let $\lambda$ denote a singular cardinal. Zeman, improving a previous result of Shelah, proved that $\square^*_\lambda$ together with $2^\lambda=\lambda^+$ implies $\diamondsuit_S$ for every $S\subseteq\lambda^+$ that reflects stationarily often. In this paper, for a subset $S\subset\lambda^+$, a normal subideal of … Continue reading