## Asaf Karagila: Spectra of Uniformity

The FG1 Seminar of TU Wien

On Friday Oct 6 at 15:00,  we will meet in the “Dissertantenraum” of the Institute for Discrete Mathematics and Geometry, TU Wien, Wiedner Hauptstr 8-10, 8th floor, green area.

Speaker:  Asaf Karagila

Title: Spectra of Uniformity

Abstract: The existence of uniform ultrafilters on infinite sets is predicated
on the axiom of choice. We discuss some related results, and prove an
Easton-like theorem showing the class of cardinals which carry uniform
ultrafilters has very few provable properties without the axiom of choice.

This is a joint work with Yair Hayut, and can be found on
https://arxiv.org/abs/1709.04824

## Mohammad Golshani: Some properties of Cohen and random reals

Kurt Gödel Research Center – Research Seminar
Thursday, October 5 – 4:00pm in the KGRC lecture room

“Some properties of Cohen and random reals”

(Institute for Research in Fundamental Sciences (IPM), Tehran, Iran)

We discuss some properties of Cohen and random reals. We prove a general combinatorial fact about them which in particular implies that they are large, they contain arbitrary large arithmetical or geometrical progressions, satisfy the Hindman’s conclusion and so on. We also show that they are wild in the sense of o-minimal theory. We also investigate how badly approximable are they by algebraic numbers.

This is a work in progress with Will Brian.

## Witold Marciszewski: On factorization properties of function spaces

KGRC Research seminar on 2017-06-29 at 4pm

Speaker: Witold Marciszewski (University of Warsaw, Poland)

Abstract: For a Tychonoff space $X$, by $C_p(X)$ we denote the space of all continuous real-valued functions on $X$, equipped with the topology of pointwise convergence. One of the important questions (due to A.V. Arhangel’skii), stimulating the theory of $C_p$-spaces for almost 30 years and leading to interesting results in this theory, is the problem whether the space $C_p(X)$ is (linearly, uniformly) homeomorphic to its own square $C_p(X)\times C_p(X)$, provided $X$ is an infinite compact or metrizable space.

In my talk I will present some recent developments concerning these type of questions. In particular, I will show a metrizable counterexample to this problem for homeomorphisms. I will also show that, for every infinite zero-dimensional Polish space $X$, spaces $C_p(X)$ and $C_p(X)\times C_p(X)$ are uniformly homeomorphic.

This is a joint research with Rafal Gorak and Mikolaj Krupski.

## Peter Nyikos: Cardinality restrictions on some kinds of locally compact spaces

KGRC Research seminar on 2017-06-29 at 2pm

Speaker: Peter Nyikos (University of South Carolina, Columbia, USA)

Abstract: In what follows, “space” means “Hausdorff ($T_2$) topological space.” Some of the theorems and problems to be discussed include:

Theorem 1. It is ZFC-independent whether every locally compact, $\omega_1$-compact space of
cardinality $\aleph_1$ is the union of countably many countably compact spaces.

[$\omega_1$-compact means that every closed discrete subspace is countable. This is obviously implied by being the union of countably many countably compact spaces, but the converse is not true.]

Problem 1. Is it consistent that every locally compact, $\omega_1$-compact space of cardinality $\aleph_2$ is the union of countably many countably compact spaces?

Problem 2. Is ZFC enough to imply that there is a normal, locally countable, countably compact space of cardinality greater than $\aleph_1$?

Problem 3. Is it consistent that there exists a normal, locally countable, countably compact space of cardinality greater than $\aleph_2$?

The spaces involved in Problem 2 and Problem 3 are automatically locally compact, because regularity is already enough to give every point a countable countably compact (hence compact) neighborhood.

Problem 4 [Problem 5]. Is there an upper bound on the cardinalities of regular [resp. normal],
locally countable, countably compact spaces?

Theorem 2. The axiom $\square_{\aleph_1}$ implies that there is a normal, locally countable, countably compact space of cardinality $\aleph_2$.

The statement in Theorem 1 was shown consistent by Lyubomyr Zdomskyy, assuming $\mathfrak p > \aleph_1$ plus P-Ideal Dichotomy (PID). Counterexamples have long been known to exist under
$\mathfrak b = \aleph_1$, under $\clubsuit$, and under the existence of a Souslin tree.

Theorem 2 may be the first application of $\square_{\aleph_1}$ to construct a topological space whose existence in ZFC is unknown.

## David Schrittesser: News on mad families

KGRC Research Seminar  – 2017-06-22 at 4pm.

Speaker: David Schrittesser (University of Copenhagen, Denmark)

Abstract: This talk is about two results on mad families (dating from this year): Firstly, in joint work with Karen Bakke Haga and Asger Törnquist and, we link madness of certain definable sets to forcing and use this to show that under the Axiom of Projective Determinacy there are no projective mad families. Moreover, the results generalize: we may replace “being almost disjoint” by “being $J$-disjoint”, for certain ideals $J$ on the natural numbers including, e.g., Fin $\times$ Fin. The other result is an improvement of Horowitz and Shelah’s construction of a Borel maximal eventually different family of functions. We obtain a closed such family, and the result even generalizes to certain compact spaces.

## Diana Carolina Montoya Amaya: Some cardinal invariants of the generalized Baire spaces

The successful PhD defense of Diana Carolina Montoya Amaya took place Wednesday, June 14 at the KGRC. Congratulations!

Abstract: The central topic of this talk is the well-known Cardinal invariants of the continuum and it is divided in two parts: In the first one we focus on the generalization of some of these cardinals to the generalized Baire spaces $\kappa^\kappa$, when $\kappa$ is a regular uncountable cardinal. First, we present a generalization of some of the cardinals in Cichon’s diagram to this context and some of the provable ZFC relationships between them. Further, we study their values in some generic extensions corresponding to $<\!\!\kappa$-support and $\kappa$-support iterations of generalized classical forcing notions. We point out the similarities and differences with the classical case and explain the limitations of the classical methods when aiming for such generalizations. Second, we study a specific model where the ultrafilter number at $\kappa$ is small, $2^\kappa$ is large and in which a larger family of cardinal invariants can be decided and proven to be $<\!2^\kappa$.

The second part deals exclusively with the countable case: We present a generalization of the method of matrix iterations to find models where various constellations in Cichon’s diagram can be obtained and the value of the almost disjointness number can be decided. The method allows us also to find a generic extension where seven cardinals in Cichon’s diagram can be separated.

Board of examiners:

Professor Mirna Džamonja (University of East Anglia)
o.Univ.-Prof. Sy-David Friedman (Universität Wien)
ao.Univ.Prof. Martin Goldstern (TU Wien)

## Zoltán Vidnyánszky: Borel chromatic numbers: finite vs infinite

KGRC Research Seminar – 2017-06-08 at 4pm.

Speaker: Zoltán Vidnyánszky (York University, Toronto, Canada)

Abstract: One of the most interesting results of Borel graph combinatorics is the $G_0$ dichotomy, i. e., the fact that a Borel graph has uncountable Borel chromatic number if and only if it contains a Borel homomorphic image of a graph called $G_0$. It was conjectured that an analogous statement could be true for graphs with infinite Borel chromatic number. Using descriptive set theoretic methods we answer this question and a couple of similar questions negatively, showing that one cannot hope for the existence of a Borel graph whose embeddability would characterize Borel (or even closed) graphs with infinite Borel chromatic number. We will also discuss a positive result and its relation to Hedetniemi’s conjecture.

## Chris Scambler: On Ineffable Liars

KGRC Friday seminar on 2017‑06‑02 – 12 pm

Speaker: Chris Scambler (New York University, USA)

Abstract: The most promising non-classical approaches to the theory of truth build on that of Saul Kripke (1975) by adding a conditional satisfying reasonable laws. Among the attractive features of such approaches are their capacity to offer object-language means for classifying the defectiveness of paradoxical sentences and formulas; in (2007), Hartry Field shows his approach yields a transfinite hierarchy of determinacy operators of increasing strength that seem to play exactly this role. There are, however, difficult technical questions about the extent of the hierarchy of such operators that turn on the availability of reasonable ordinal notation systems, and these may yield philosophical issues for Field’s approach to the paradoxes. According to Field, the extent of the hierarchy is inherently ‘fuzzy’, because of indeterminacy concerning the unrestricted notion of definability. As a result, Field argues, one can’t diagonalize out of the hierarchy of determinacy operators in any meaningful sense, since the hierarchy in question is not bivalently definable. In (2014), Philip Welch has argued that on the contrary the hierarchy of determinacy operators breaks down precisely at the least $\Sigma_2$-extendible ordinal (relative to a given model M); moreover, Welch has shown how to use this result to produce “ineffable liars”, that diagonalize out of the hierarchy: these are sentences that are indeterminate on Field’s theory, but whose defectiveness is not measured by any determinacy operator in the object language.

The task of this paper is to assess the significance of Welch’s result, and to adjudicate the dispute between Field and Welch. In the opening sections, I will review the Kripke and Field constructions, focussing especially on the hierarchy of determinacy operators and their behaviour. After that, I will give an overview of Welch’s construction, culminating in the construction of an ineffable liar sentence. Finally, I will scrutinize Welch’s argument from a philosophical perspective, and suggest that Field’s project is not adversely affected by Welch’s results. Nevertheless, I will show some ways in which that the latter are still of considerable philosophical interest.

References

Saul A. Kripke: Outline of a theory of truth. Journal of Philosophy 72 (19):690-716 (1975)

Hartry Field: Solving the paradoxes, escaping revenge. In J. C. Beall (ed.), Revenge of the Liar: New Essays on the Paradox. Oxford University Press (2007)

P. D. Welch: Some observations on truth hierarchies. Review of Symbolic Logic 7 (1):1-30 (2014)

## Vincenzo Dimonte: Rank-into-rank axioms and forcing

KGRC Research Seminar – 2017‑06‑01 at 4pm

Speaker: Vincenzo Dimonte (University of Udine, Italy)

Abstract: Rank-into-rank axioms sit on the top of the large cardinal hierarchy, and their fringe status makes them quite mysterious and evasive. In particular, research on I0 started to gain momentum just in the last few years.

In this talk we will give an overview of what is known at the moment about the interaction between such axioms and forcing, in four steps, in increasing order of complexity. The main result most of the time would be that the rank-into-rank axiom is not destroyed by the forcing, therefore providing many independence results (for example involving the behaviour of the power function, tree structures, pcf theory…). We will also note how such results pose an actual problem for the main branch of the research on I0, i.e, the quest for finding similarities between I0 and the Axiom of Determinacy.

Part of this work is joint with Sy Friedman and Liuzhen Wu.

## Victoria Gitman: A model of second-order arithmetic with the choice scheme in which $\Pi^1_2$-dependent choice fails

KGRC research seminar – 2017‑05‑18 at 4pm

Speaker: Victoria Gitman (CUNY Graduate Center, New York, USA)

Abstract:

In second-order arithmetic, the choice scheme is the scheme of assertions, for every second-order formula $\varphi(n,X,A)$, that if for every $n$ there is a set $X$ such that $\varphi(n,X,A)$ holds, then there is a single set $Y$ whose $n$-th slice $Y_n$ witnesses $\varphi(n,Y_n,A)$. While full second-order arithmetic ${\textrm Z}_2$ implies the choice scheme for $\Sigma^1_2$-assertions, the reals of the Feferman-Lévy model form a model of ${\textrm Z}_2$ in which $\Pi^1_2$-choice fails. The dependent choice scheme is the analogue ${\textrm DC}$ for second-order arithmetic and it asserts, for every second-order formula $\varphi(X,Y,A)$, that if for every set $X$ there is another set $Y$ such that $\varphi(X,Y,A)$ holds, then there is a single set $Z$, viewed as an $\omega$-sequence of sets, such that for every $n$, $\varphi(Z\upharpoonright n,Z_n,A)$ holds. The theory ${\textrm Z}_2$ implies $\Sigma^1_2$-dependent choice, and Simpson has conjectured that there is a model of ${\textrm Z}_2$ with the choice scheme in which $\Pi^1_2$-dependent choice fails. We prove Simpson’s conjecture by constructing a symmetric submodel of a forcing extension in which ${\textrm AC}_\omega$ holds, but ${\textrm DC}$ fails for a $\Pi^1_2$-definable relation on the reals.

We force over $L$ with a tree iteration of Jensen’s forcing (a ccc subposet
of Sacks forcing adding a unique generic real) along the tree ${}^{\lt\omega}\omega_1$, adding a tree, isomorphic to ${}^{\lt\omega}\omega_1$, of finite sequences of reals ordered by extension, such that that the sequences on level $n$ are $L$-generic for the $n$-length iteration of Jensen’s forcing. We extend the uniqueness of generic reals properties of Jensen’s forcing (obtained earlier by Jensen and later by Lyubetsky and Kanovei) by showing that in the tree iteration extension, the only sequences of reals $L$-generic for the $n$-length iteration of Jensen’s forcing are those explicitly added on level $n$ of the generic tree. The uniqueness property implies that the generic tree is $\Pi^1_2$-definable.

The theorem arose out of our attempts to separate the analogues of the
choice scheme and the dependent choice scheme over Kelley-Morse set theory,
and we conjecture that an appropriate generalization of our arguments will
now achieve this result.

This is joint work with Sy-David Friedman.