Winter School in Abstract Analysis, January 2023

I gave a 3-lecture tutorial at the Winter School in Abstract Analysis in Steken, January 2023.

Title: Club guessing

Abstract. Club guessing principles were introduced by Shelah as a weakening of Jensen’s diamond. Most spectacularly, they were used to prove Shelah’s ZFC bound on the power of the first singular cardinal. These principles have found many other applications: in cardinal arithmetic and PCF theory; in the construction of combinatorial objects on uncountable cardinals such as Jonsson algebras, strong colorings, Souslin trees, and pathological graphs; to the non-existence of universals in model theory; to the non-existence of forcing axioms at higher uncountable cardinals; and many more.
In this tutorial, we will give a gentle introduction to the subject.

In Part I, we shall present the basic club-guessing principle CG(S), and prove that it holds for S:=Eθκ (the set of all ordinals below κ of cofinality θ) provided that θ<κ are regular uncountable cardinals and θ+<κ. An ad-hoc treatment of the case θ=0 will be given, and raising the concept of relative club-guessing. We shall also present postprocessing functions, and compare Ulam matrices with a coloring obtained by Sirepinski under a GCH-type assumption, motivating an application of club-guessing that will appear in the second lecture.

 

In Part II, we shall construct an optimal Sirepinski-type coloring in ZFC, using pcf scales and club-guessing. We then move on to address the extreme case of CG(Eθκ) where κ=θ+. This inevitably leads to the introduction of the idealized guessing principle CG(S,J), which in turn requires the study of amenable C-sequences and nonconservative postprocessing functions. As an application, we waive an uncountability assumption from the first lecture, proving that CG(Eθκ) holds for every pair θ<κ of infinite regular cardinals such that θ+<κ.

 

In Part III, we combine idealized club-guessing with relative guessing, resulting in the principle CG(S,T,J). We explain how partitioned club-guessing can compensate for the inevitable shift to idealized CG. We show how anti-large-cardinal hypotheses on θ or on κ imply that a given witness to CG(Eθκ,T,J) may be partitioned, and examine the impact of the sequence of ideals J on one’s ability to partition. This leads to a Solovay-type decomposition theorem for club-guessing and to a general discussion on how to move from CG(S,T,J) to CG(S,T,J). Combining all the acquired machinery, we conclude that for every cardinal λω such that ◻(λ+) holds, CG(S,T,Jbd[δ]δS) holds for all stationary subsets S,T of λ+.

 

Lecture 1 ** Lecture 2 ** Lecture 3

This entry was posted in Invited Talks, Open Problems and tagged , , , , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *