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 , and prove that it holds for (the set of all ordinals below of cofinality ) provided that are regular uncountable cardinals and . An ad-hoc treatment of the case 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 where . This inevitably leads to the introduction of the idealized guessing principle , which in turn requires the study of amenable -sequences and nonconservative postprocessing functions. As an application, we waive an uncountability assumption from the first lecture, proving that holds for every pair of infinite regular cardinals such that .
In Part III, we combine idealized club-guessing with relative guessing, resulting in the principle . We explain how partitioned club-guessing can compensate for the inevitable shift to idealized . We show how anti-large-cardinal hypotheses on or on imply that a given witness to may be partitioned, and examine the impact of the sequence of ideals 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 to . Combining all the acquired machinery, we conclude that for every cardinal such that holds, holds for all stationary subsets of .
Lecture 1 ** Lecture 2 ** Lecture 3