Archives of: Carnegie Mellon Logic Seminar

Jing Zhang: A Ramsey theorem for (repeated) sums

Mathematical logic seminar – Nov 13 2018
Time: 3:30pm – 4:30 pm

Room: Wean Hall 8220

Speaker: Jing Zhang
Department of Mathematical Sciences
CMU

Title: A Ramsey theorem for (repeated) sums

Abstract:

The motivation is the question: for any finite coloring f: R -> r
does there exist an infinite X such that X + X is monochromatic under f?
Hindman, Leader and Strauss showed the answer is negative if CH holds.
Komjáth, Leader, Russell, Shelah, Soukup and Vidnyánszky showed the
positive answer is consistent relative to the existence of a certain large
cardinal. I will demonstrate how to eliminate the use of large cardinals.
Other variations of the statement will also be discussed, including some
ZFC results.

Jing Zhang: A Ramsey theorem for (repeated) sums

Mathematical logic seminar – Oct 30 2018
Time:     3:30pm – 4:30 pm

Room:     Wean Hall 8220

Speaker:         Jing Zhang
Department of Mathematical Sciences
CMU

Title:     A Ramsey theorem for (repeated) sums

Abstract:

The motivation is the question: for any finite coloring f: R -> r , does there exist an infinite X such that X + X is monochromatic under f? Hindman, Leader and Strauss showed the answer is negative if CH holds. Komjáth, Leader, Russell, Shelah, Soukup and Vidnyánszky showed the positive answer is consistent relative to the existence of a certain large cardinal. I will demonstrate how to eliminate the use of large cardinals. Other variations of the statement will also be discussed, including some ZFC results.

Anush Tserunyan: Hyperfinite subequivalence relations of treed equivalence relations

Mathematical logic seminar – Oct 16 2018
Time:     3:30pm – 4:30 pm

Room:     Wean Hall 8220

Speaker:         Anush Tserunyan
Department of Mathematics
University of Illinois at Urbana-Champaign

Title:     Hyperfinite subequivalence relations of treed equivalence relations

Abstract:

A large part of measured group theory studies structural properties of countable groups that hold “on average”. This is made precise by studying the orbit equivalence relations induced by free Borel actions of these groups on probability spaces. In this vein, the cyclic (more generally, amenable) groups correspond to hyperfinite equivalence relations, and the free groups to the treeable ones. In joint work with R. Tucker-Drob, we give a detailed analysis of the structure of hyperfinite subequivalence relations of a treed equivalence relation, deriving some of analogues of structural properties of cyclic subgroups of a free group. In particular, just like any cyclic subgroup is contained in a unique maximal one, we show that any hyperfinite subequivalence relation is contained in a unique maximal one.

Clinton Conley: Realizing abstract systems of congruence II

Mathematical logic seminar – Oct 9 2018
Time: 3:30pm – 4:30 pm

Room: Wean Hall 8220

Speaker: Clinton Conley
Department of Mathematical Sciences
CMU

Title: Realizing abstract systems of congruence II

Abstract:

An abstract system of congruence (ASC) is simply an equivalence relation
on the power set of a finite set F satisfying some nondegeneracy
conditions. Given such an ASC and an action of a group G on a set X, a
realization of the ASC is a partition of X into pieces indexed by F such
that whenever two subsets A, B are asc-equivalent, the corresponding
subsets XA and XB of X can be translated to one another in the action.
Familiar notions like paradoxical decompositions can be easily formalized
and refined by the ASC language. Wagon, upon isolating this notion,
characterized those ASCs which can be realized by rotations of the sphere.
He asks whether there is an analogous characterization for realizing ASCs
using partitions with the property of Baire. We provide such a
characterization. This is joint work with Andrew Marks and Spencer Unger.

Nigel Pynn-Coates: Asymptotic valued differential fields and differential-henselianity

Seminar will meet at 4:30pm Monday next week.

Mathematical logic seminar – Oct 1 2018 – NOTE UNUSUAL DAY AND TIME!
Time: 4:30pm – 5:30 pm

Room: Wean Hall 8220

Speaker: Nigel Pynn-Coates
Department of Mathematics
UIUC

Title: Asymptotic valued differential fields and differential-henselianity

Abstract:

It is well known that henselianity plays a fundamental role in the algebra and model theory of valued fields. The notion of differential-henselianity, introduced by Scanlon and developed in further generality by Aschenbrenner, van den Dries, and van der Hoeven, is a natural generalization to the setting of valued differential fields. I will present three related theorems justifying the position that differential-henselianity plays a similarly fundamental role in the algebra of asymptotic valued differential fields, a class arising naturally from the study of Hardy fields and transseries, and that have potential applications to the model theory of such fields.

Clinton Conley: Realizing abstract systems of congruence

Mathematical logic seminar – Sep 25 2018
Time:     3:30pm – 4:30 pm

Room:     Wean Hall 8220

Speaker:         Clinton Conley
Department of Mathematical Sciences
CMU

Title:     Realizing abstract systems of congruence

Abstract:

An abstract system of congruence (ASC) is simply an equivalence relation on the power set of a finite set F satisfying some nondegeneracy conditions. Given such an ASC and an action of a group G on a set X, a realization of the ASC is a partition of X into pieces indexed by F such that whenever two subsets A, B are asc-equivalent, the corresponding subsets XA and XB of X can be translated to one another in the action. Familiar notions like paradoxical decompositions can be easily formalized and refined by the ASC language. Wagon, upon isolating this notion, characterized those ASCs which can be realized by rotations of the sphere. He asks whether there is an analogous characterization for realizing ASCs using partitions with the property of Baire. We provide such a characterization. This is joint work with Andrew Marks and Spencer Unger.

Jing Zhang: How to get the brightest rainbows from the darkest colorings

Mathematical logic seminar – Sep 18 2018
Time:     3:30pm – 4:30 pm

Room:     Wean Hall 8220

Speaker:         Jing Zhang
Department of Mathematical Sciences
CMU

Title:     How to get the brightest rainbows from the darkest colorings

Abstract:

We will discuss the rainbow (sometimes called polychromatic) variation of the Ramsey theorem on uncountable cardinals. It roughly says if a given coloring on n-tuples satisfies that each color is not used too many times, we can always find a rainbow subset, that is a set in which no two n-tuples from the set get the same color. We will use problems in different settings (inaccessible cardinals, successors of singular cardinals, small uncountable cardinals etc) to demonstrate that the rainbow variation is a “strict combinatorial weakening” of Ramsey theory.

Anton Bernshteyn: From finite combinatorics to descriptive set theory and back

Mathematical logic seminar – Sep 4 2018
Time:     3:30pm – 4:30 pm

Room:     Wean Hall 8220

Speaker:         Anton Bernshteyn
Department of Mathematical Sciences
CMU

Title:     From finite combinatorics to descriptive set theory and back

Abstract:

Many results in finite combinatorics can be extended to infinite structures via compactness – but this transfer is powered by the Axiom of Choice and leads, in general, to highly “pathological” objects. It is natural to ask, which combinatorial constructions can be performed in a “well-behaved” fashion, say, in a Borel or measurable way? This question is addressed in a young branch of descriptive set theory called descriptive combinatorics. We will discuss a class of coloring problems with the requirement that the desired coloring be Baire measurable (i.e., “topologically well-behaved”). The central result of this talk is that the existence of a Baire measurable coloring is equivalent to a purely combinatorial statement, analogs of which have for a long time been studied in finite graph theory with no relation to descriptive set theory.

David Fernández-Bretón: Variations and analogs of Hindman’s theorem

Mathematical logic seminar – May 22 2018
Time:     3:30pm – 4:30 pm

Room:     Wean Hall 8220

Speaker:         David Fernández-Bretón
Department of Mathematics
University of Michigan

Title:     Variations and analogs of Hindman’s theorem

Abstract:

Hindman’s theorem is a Ramsey-theoretic result asserting that, whenever one colours the set of natural numbers with finitely many colours, there will be an infinite set such that all numbers that can be obtained by adding finitely many elements from the set (no repetitions allowed) have the same colour. I will explore generalizations and extensions of this theorem: replacing “natural numbers” with “abelian group” and varying the number of colours, as well as the size of the desired monochromatic set, yields a plethora of very interesting results.

François Le Maître: Ample generics and full groups

Mathematical logic seminar – May 15 2018
Time:     3:30pm – 4:30 pm

Room:     Wean Hall 8220

Speaker:         François Le Maître
Institut de Mathématiques de Jussieu-PRG
Université Paris Diderot

Title:     Ample generics and full groups

Abstract:

In this talk, I will explain why the full group of the action of the rationals on the real line is an example of a connected topologically simple Polish group with ample generics, providing a natural answer to a question of A. Kechris and C. Rosendal. If time permits, I will discuss the following open question: is there a Polish group with ample generics which is not quasi non-archimedean ? The talk is based on a joint work with Adriane Kaïchouh.