# Set Theory

### Previous Lectures

We shall present a new method for obtaining the tree property at w2 from the consistency of a weakly compact cardinal.

The method is due to Stejskalova, and uses Grigorieff forcing.

We address the question whether, assuming the generalized continuum hypothesis (GCH), the existence of a k-Aronszajn tree entails the existence of a distributive one.

Biblography: see here.

We are going to continue the analysis of generalised scattered orders, proving the result described towards the end of Chris Lambie-Hanson’s talk. This states that consistently, for every sigma-scattered linear ordering there is a colouring of its pairs in black & white such that every triple contains a white pair and every copy of one of the following order-types contains a black pair:

- omega_1^omega
- (omega_1^omega)^*
- omega_1 * (omega * omega^*)^omega
- omega_1^* * (omega * omega^*)^omega
- (omega * omega^*)^omega * omega_1
- (omega * omega^*)^omega * omega_1^*

This generalises a 46-year-old Theorem of Erdős & Rado about ordinals. A sufficient hypothesis implying this theorem is the existence of a colouring of the pairs of omega_1 * omega in black & white such that every triple contains a black pair and every subset of full order-type contains a white one. Time permitting we may present a proof that stick = b = Aleph_1 implies the existence of such a colouring. Here b is the unbounding number and stick = Aleph_1 is a weakening of the club principle which was considered by Baumgartner 41 years ago, named by Broverman, Ginsburg, Kunen & Tall two years thereafter and twenty years ago reconsidered as a cardinal characteristic by Fuchino, Shelah & Soukup.

The class of scattered linear orders, isolated by Hausdorff, plays a prominent role in the study of general linear orders. In 2006, Dzamonja and Thompson introduced classes of orders generalizing the class of scattered orders. For a regular cardinal kappa, they defined the classes of kappa-scattered and weakly kappa-scattered linear orders. For kappa = omega, these two classes coincide and are equal to the classical class of scattered orders. For larger values of kappa, though, the two classes are provably different. In this talk, we will investigate properties of these generalized scattered orders with respect to partition relations, in particular the extent to which the classes of kappa-scattered or weakly kappa-scattered linear orders of size kappa are closed under partition relations of the form tau -> (phi, n) for n < omega. We will show that, assuming kappa^{<kappa} = kappa, the class of weakly kappa-scattered orders is closed under all such partition relations while, for uncountable values of kappa, the class of kappa-scattered orders consistently fails to be closed. Along the way, we will prove a generalization of the Milner-Rado paradox and look at some results regarding ordinal partition relations. This is joint work with Thilo Weinert.

This is an expository presentation following the paper "Minimality of non $\sigma$-scattered orders" by Ishiu and Moore. In the first part of the talk we will introduce the invariant $\Omega(L)$ of a linear order $L$, and characterize $\sigma$-scattered linear orders in terms of this invariant. In the second part, we will prove under the forcing axiom $\mathsf{PFA}^+$ that any linear order which is minimal with respect to embedding among the non $\sigma$-scattered orders must be either a real or Aronszajn type.

This is an expository presentation following the paper "Minimality of non $\sigma$-scattered orders" by Ishiu and Moore. In the first part of the talk we will introduce the invariant $\Omega(L)$ of a linear order $L$, and characterize $\sigma$-scattered linear orders in terms of this invariant. In the second part, we will prove under the forcing axiom $\mathsf{PFA}^+$ that any linear order which is minimal with respect to embedding among the non $\sigma$-scattered orders must be either a real or Aronszajn type.

This is a continuation of last week's talk. This time, I shall prove that square(kappa) give rise to a partition of kappa into kappa many fat sets

A subset F of a regular uncountable cardinal kappa is said to be *fat *iff for every club C in kappa, and every ordinal alpha<kappa, F\cap C contains a closed copy of alpha+1.

By a theorem of H. Friedman from 1974, every stationary subset of w1 is fat. In particular, w1 may be partitioned into w1 many pairwise disjoint fat sets.

In this talk, I shall prove that square(kappa) give rise to a partition of kappa into kappa many pairwise disjoint fat sets. In particular, the following are equiconsistent:

- w2 cannot be partitioned into w2 many pairwise disjoint fat sets;
- w2 cannot be partitioned into two disjoint fat sets;
- there exists a weakly compact cardinal.

We are going to prove a classical result of Baumgartner - the existence of a linear order type every uncountable subtype of which contains a copy of omega_1 yet fails to be the union of countably many well-ordered types.

The paper may be found in here.

Abstract: We are going to prove a classical result of Baumgartner - the existence of a linear order type every uncountable subtype of which contains a copy of omega_1 yet fails to be the union of countably many well-ordered types.

The paper may be found in here.

The paper may be found in here.

Last week, we presented a construction scheme which is based on well-behaving delta-systems of finite subsets of w1. In this lecture, we shall present an application to the theory of uncountable trees.

The results are taken from the following paper.

We present a construction scheme which is based on well-behaving delta-systems of finite subsets of w1, and use it to construct uncountable trees.

The results are taken from the following paper.

We present Komjath's theorem that there exists a coloring of the real line in 2 colors such that for any uncountable subset A of reals, there exist 4 distinct elements a,b,c,d in A such that a+b gets color 0, and c+d gets color 1.

The results are taken from the following paper.

We shall complete the verification of existence of obligatory graphs for infinite colouring numbers.

In our previous talk, we covered the case of graphs of regular cardinality. This time, we shall address graphs of singular cardinality.

We show that every graph with inﬁnite colouring number has a well-ordering of its vertices that simultaneously witnesses its colouring number and its cardinality.

The lecture will be based on the following paper.

A tree is said to be rigid if it has a trivial automorphism group. It is said to be homogeneous if any two nodes of the same level can be sent one to the other via an automorphism of the tree. In this talk, we shall present Larson's proof that the existence of a strongly homogeneous Souslin tree entails the existence of a strongly rigid Souslin tree.

We shall resume the presention of Ostaszewski's construction of a perfectly normal, hereditarily separable, first countable, locally countable, locally compact, Hausdorff topological space in which every open set is either countable or co-countable.

We shall present Ostaszewski's construction of a perfectly normal, hereditarily separable, first countable, locally countable, locally compact, Hausdorff topological space in which every open set is either countable or co-countable.

We shall present a recent theorem of Raghavan and Todorcevic that uses a Souslin tree to refute a particular generalization of the Erdos-Dushnik-Miller theorem.

We shall present a construction (due to Milner and Shelah) of a very large graph which has no unfriendly 2-partition, and in which every vertex has infinite degree.

We shall show that any ladder system on w1 induces a certain uncountable topological space, and then present sufficient conditions on the ladder system that makes the corresponding space into a Dowker space.

We shall describe a construction of a Souslin tree, following our recent paper.

We shall describe a construction of a Souslin tree, following our recent paper.

Infinite trees and partition calculus (aka, Ramsey theory) are well-known to be intertwined. For instance, Ramsey theorem implies Konig's lemma that asserts that every infinite tree which is finitely branching has an infinite path.

In this talk, we shall deal with uncountable trees such as Souslin trees and Aronszajn trees, and show how to derive negative partition relations from them.

The purpose of this talk is to present the main developments in Cardinal Arithmetic from 1960 to 1975. After a brief review of the basic independence results, we will review the basic definitions and results about ultrapowers and measurable cardinals and proceed to Scott's and Vopenka's results in Cardinal Arithmetic regarding measurable cardinals and singular cardinals of measurable cofinality. These results are generalizable to all singular cardinals of uncountable cofinality and this is what we will look at next. For that will start with the basic definitions and examples regarding the Galvin-Hajnal norm and finish with the application of the Galvin-Hajnal bound for families of almost disjoint functions to Cardinal Arithmetic.

We introduce Menger and Hurewicz covering properties, which are generalizations of sigma-compactness. Menger and Hurewicz conjectured that, for subsets of the real line, the above properties were equivalent to sigma-compactness. Using topological and an elegant combinatorial method (coherent omission of intervals), we show (in ZFC) that they are false. We consider also stronger covering properties, relations between them and we give examples of such sets of reals. After that we obtain the solution to the Hurewicz problem: Is there in ZFC an example of set of reals which is Menger but not Hurewicz? Finally we show some results concerning behavior of Menger and Hurewicz properties in finite products.

Gray's combinatorial principle SD_k is a strong combination of Jensen's Square_k and Diamond(k^+) principles. This principle proved itself very useful in constructing uncountable graphs of counter-intuitive nature.

By a 35 year old theorem of Shelah, Square_k+Diamond(k^+) does not imply SD_k for regular uncountable cardinals k. In this talk, I will prove that they are equivalent whenever k is singular.

Which Isbell-Mrowka spaces spaces satisfy the star version of Menger’s covering property?

Following Bonanzinga and Matveev, this question is considered here from a combinatorical point of view. We give an answer to a problem thay have stated, and present some related open problems.

All is taken from this paper by Boaz Tsaban.

The slides are available here.

Some combinatorial principles were invented by Jensen, in his analysis of Godel's constructible universe. One of them is Diamond^*.

We will introduce variants and generalizations of Diamond^* and discuss when these principles hold and when they do not hold.

We shall present the P-hierarchy of ultrafilters, that was posed by Andrzej Starosolski.

The P-hierarchy of ultrafilters is one of many ways to classify ultrafilters on natural numbers and it is composed of ℵ1 disjoint classes P(α) where α is ordinal number <ω1. The class P(1) is just a class of principal ultrafilters. The class P(2) is composed of P-points, which were isolated by Rudin in order to prove non-homogeneity of the remainder of Cech-Stone compactification of natural numbers. Next, in higher classes of P-hierarchy, one can find ultrafilters with more and more complicated structures.

In this talk, we will disscuss relations between classes P(α) of P-hierarchy and other special types of ultrafilters, including: Baumgartner’s I-ultrafilters, thin ultrafilters, summable ultrafilters, and van der Waerden ultrafilters.

A topological space is called k-resolvable if it is the union of k many disjoint dense subsets. In this second lecture, we shall survey some of the results obtained throughout the years and record some open questions.

A topological space is called *resolvable* if it is the union of two disjoint dense subsets. Since the concept was first defined and explored by Edwin Hewitt in 1943, much effort has been invested in obtaining general results concerning the resolvability or irresolvability of certain types of spaces, and in generating examples and counterexamples.

In the present lecture we will take a leisurely tour through the subject. We will discuss generalizations of the original concept, display some of the results obtained throughout the years and mention questions which are still open.

We shall discuss generalizations of Ramsey's theorem to the context of trees of high chromatic number. A detailed abstract is available here.

Komjath has asked the following question: Let X be a subset of Euclidean space. Must there exist a subset Y of X such that X and Y have same outer measure and the distance between any two points in Y is irrational?

The amalgamation property is a topic of fundamental interest in model theory and is still imperfectly understood. In the 1980s, Grossberg asked a question, which remains open to this day, about the existence of a Hanf number for amalgamation in abstract elementary classes. We introduce a new class of structures, called well-colorings, and use them to give a partial answer to Grossberg’s question, significantly improving upon previous work of Baldwin, Kolesnikov, and Shelah. We shall start the talk by briefly discussing the relevant model-theoretic definitions (no prior model-theoretic knowledge will be assumed) and will then give proofs of the main results, which are entirely set-theoretic and combinatorial in nature and of interest in their own right. This is joint work with Alexei Kolesnikov.

We shall survey the history of the study of the productivity of the k-chain-condition in partial orders, topological spaces, and Boolean algebras. We shall address a conjecture that tries to characterize such a productivity in Ramsey-type language. For this, a new oscillation function for successor cardinals, and a new characteristic function for walks on ordinals will be proposed and investigated.

We shall present the notion of a Luzin Set, various generalizations, as well as applications to strong and not-so-strong colorings.

Dani shall present costructions (due to Hajnal) of Anti-Ramsey colorings which are not universal. That is, these colorings fail to embed particular finite patterns. Unlike Shelah's construction (that Michal presented), these construction will be carried in ZFC.

We shall present various concepts of being a "large" subset of w_{1}.

Can you tell the present by knowing the future? That is, can there be a function f:[X]^{w}-->X so that given a sequence <x_{0},x_{1},x_{2},...>, we would have x_{n}=f(x_{n+1},x_{n+2},x_{n+3},....) for (almost) all n?

This type of problems was considered by Galvin, Erdos-Hajnal, Prikry, and Solovay in the 1960's and 1970's, and regained interest more recently in the study of generalized hat problems.

Tomer's talk will present this line of research.

Michal shall prove Shelah's thereom that the Continuum Hypothesis entails a coloring c:[w_{1}]^{2}-->w such that c``[A]^{2}=w for every uncountable subset A of w_{1}, and yet c admits no 3-sized set X on which c|[X]^{2} is one-to-one.

We shall prove that for every infinite cardinal k, there exists a coloring c:[_{k}]^{w}→_{X} satisfying the following:

- c is 2-to-1;
- c restricted to [A]
^{w}is not injective for every infinite A.

We shall provide sufficient conditions for the existence of a function f:[w_{1}]^{2}→w_{1} satisfying the following:

- f is 2-to-1;
- f restricted to any uncountable square [X]
^{2}is not injective.

An L-space is a regular topological space which is hereditary Lindelof, but not separable. Yuval will present a sufficient condition for the existence of an L-space: a combination of an uncountable b-universal sequence, and an L-syndetic coloring give rise to such a space.

Yuval will show how to read a b-universal binary sequence of length continuum from Kronecker's theorem on simultaneous diophantine approximation.

Lidor will present a proof of Todorcevic's theorem stating that there is a *continuous *coloring of all triples of rational numbers in coutnably many colors, in such a way that for any topological copy C of the rationals and any possible color k, there exists a triple in C that is colored with the desired color k.

Lidor will present a proof of Baumgartner's theorem stating that there is a coloring of all pairs of rational numbers in coutnably many colors, in such a way that for any topological copy C of the rationals and any possible color k, there exists a pair in C that is colored with the desired color k.

במפגש הראשון דנו במשפט רמזי הסופי והאינסופי, ובדוגמא של שרפינסקי המראה כי ההכללה המתבקשת למקרה שאיננו בן מניה - איננה נכונה. דיברנו על סוגי צביעות המעידות על כשלון תופעות מסוג רמזי, ועל הגרסא האולטימטיבית של "צביעה חזקה", כמו גם, גרסאות אסימטריות.

דיברנו על שמורות מונים של מרחבים טופולוגיים, והשוונו בין המושגים: "בן מניה שתיים", "ספרבילי" ו"לינדלוף". הזכרנו שהמושגים שקולים בהקשר של מרחבים מטריים, ובחרנו להתמקד במקרה של מרחבים רגולריים. רמזנו שצביעות חזקות מאפשרות להגדיר מרחבים רגולריים המקיימים תכונה אחת, ולא את השניה: למשל מרחב רגולרי ספרבילי תורשתית, שאיננו לינדלוף. מנגד, הזכרנו כי הטענה כי "כל מרחב רגולרי ספרבילי תורשתית הוא לינדלוף" מתיישבת עם האקסיומות הרגילות של תורת הקבוצות.

- תאריך עדכון אחרון: 19/10/2017