Daniel T. Soukup: Monochromatic sumsets for colouring of the real numbers

Thursday, Dec 21, 2017,  10:30–12.00

Main Lecture Hall , Alfréd Rényi Institute of Mathematics

Abstract: The aim of this talk is to review certain additive Ramsey-theoretic results on colouring the set of natural, rational and real numbers. In particular, we will prove that, under certain set theoretic assumptions, for any finite colouring f of the real number R there is an infinite X so that f is constant on X+X (where repetitions are allowed in the sumset). This work is joint with P. Komjáth, I. Leader, P. A. Russell, S. Shelah, Z. Vidnyánszky.

Dorottya Sziráki: Open colorings on generalized Baire spaces

Thursday, July 20, 2017,  10:30–12.00

Main Lecture Hall , Alfréd Rényi Institute of Mathematics

Abstract: We study the uncountable version of a natural variant of the Open Coloring Axiom. More concretely, suppose that $\kappa$ is an uncountable cardinal such that $\kappa^{<\kappa}=\kappa$ and X is a subset of the generalized Baire space $\kappa^\kappa$ (the space of functions from $\kappa$ to $\kappa$ equipped with the bounded topology). Let OCA*(X) denote the following statement: for every partition of $[X]^2$ as the union of an open set R and a closed set S, either X is a union of $\kappa$ many S-homogeneous sets, or there exists a $\kappa$-perfect R-homogeneous set. We show that after Lévy-collapsing an inaccessible $\lambda>\kappa$ to $\kappa^+$, OCA*(X) holds for all $\kappa$-analytic subsets X of $\kappa^\kappa$. Furthermore, the Silver dichotomy for ${\Sigma}^0_2(\kappa)$ equivalence relations on $\kappa$-analytic subsets also holds in this model. Thus, both of the above statements are equiconsistent with the existence of an inaccessible $\lambda>\kappa$. We also examine games related to the above partition properties.

Dániel T. Soukup: Uncountable strongly surjective linear orders

Thursday, July 13, 2017,  10:30

Seminar Room, Alfréd Rényi Institute of Mathematics

Abstract: A linear order $L$ is strongly surjective if $L$ can be mapped onto any of its suborders in an order preserving way. We review various results on the existence and non-existence of uncountable strongly surjective linear orders answering questions of Camerlo, Carroy and Marcone. In particular, $\diamondsuit^+$ implies the existence of a lexicographically ordered Suslin-tree which is strongly surjective and minimal; every strongly surjective linear order must be an Aronszajn type under $2^{\aleph_0} <2^{\aleph_1}$ or in the Cohen and other canonical models (where $2^{\aleph_0}=2^{\aleph_1}$); finally, we show that it is consistent with CH that there are no uncountable strongly surjective linear orders at all. Further details and open problems can be found in https://arxiv.org/abs/1706.10171