Das Donau-Rhein-Modelltheorie-Seminar wurde auf Initiative von Salma Kuhlmann (Universität Konstanz) gegründet und wird organisiert von Philipp Habegger (Universität Basel), Amador Martin-Pizarro (Universität Freiburg), Salma Kuhlmann und Margaret Thomas (Universität Konstanz) sowie Tobias Kaiser (Universität Passau). Das Seminar stellt eine gemeinsame Plattform für die in der Modelltheorie und deren Anwendungen forschenden Mathematiker des oberdeutschen Sprachraums dar und findet einmal im Semester an wechselnden Orten statt. Vorträge werden sowohl von internationalen Gästen als auch von Nachwuchswissenschaftlern gehalten.
Im Sommersemester 2017 war die Auftaktveranstaltung an der Universität Konstanz.
Im Wintersemester 2017/2018 ist die Universität Passau der Ausrichter.
Termin: Freitag, 1. Dezember 2017, und Samstag, 2. Dezember 2017
Freitag, 1. Dezember:
19:00: Abendessen (Heilig Geist Stiftschenke)
Samstag, 2. Dezember
Ort: Universität Passau, Informatik und Mathematik, Hörsaal 12
09:00 - 09:50: Christian Urech (Basel)
10:00 - 10:50: Zaniar Ghadernezhad (Freiburg)
11:00 - 11:50: Merlin Carl (Konstanz)
12:00 - 13:00: Mittagspause (Restaurant Akropolis Athen)
13:00 - 13:50: Piotr Kowalski (Wroclaw)
14:00 - 14:50: Gabriel Dill (Basel)
Titel und Abstracts:
Merlin Carl (Konstanz)
Titel: Real closed fields, models of arithmetic and exponentiation
Abstract: Roughly, an integer part of a real closed field K is a subring that relates to K in a similar way that Z relates to R. By a classical theorem of Shepherdson, integer parts of real closed fields coincide with models of open induction, a weak fragment of arithmetic. This motivates the question how models of stronger fragments of arithmetic relate to real closed fields of which they are integer parts, which was started by D'Aquino, Knight and Starchenko and then carried on by Marker, Schmerl, Steinhorn, Kuhlmann and others. In this talk, we will show that real closed fields with an integer part that is a model of Peano arithmetic allow a weak form of exponentiation, called "left exponentiation" (which was proved in joint work with S. Kuhlmann and P. D'Aquino, building on work of D. Marker) and that, conversely, models of true arithmetic are integer parts of real closed fields that are elementary equivalent to the reals with exponentiation.
Gabriel Dill (Basel)
Titel: Unlikely intersections between isogeny orbits and curves
Abstract: In the spirit of the Mordell-Lang conjecture, we consider the intersection of a curve in a family of abelian varieties with the images of a finite-rank subgroup of a fixed abelian variety A_0 under all isogenies between A_0 and some member of the family. After excluding certain degenerate cases, we can prove that this intersection is finite by applying the Pila-Zannier method with a variant of the Pila-Wilkie theorem due to Habegger and Pila. This proves a conjecture of André-Pink-Zannier in the case of curves. We can even allow translates of the finite-rank subgroup by abelian subvarieties of controlled dimension if we strengthen the degeneracy hypotheses suitably.
Zaniar Ghadernezhad (Freiburg)
Titel: Non-amenablility of automorphism groups of generic structures
The study of amenable groups is originated in the works of von Neumann in his analysis of Banach-Tarski paradox. Since then amenability, non-amenability and paradoxicality has been studied for various groups appearing in different parts of mathematics. A topological group G is amenable if every G-flow has an invariant Borel probability measure. Well-known examples of amenable groups are finite groups, solvable groups and locally compact abelian groups. The study of amenability of topological groups benefit from various viewpoints that ranges from analytic approach to combinatorial. Kechris, Pestov, and Todorcevic established a very general correspondence which equates a stronger form of amenability, called extreme amenability, of the automorphism group of an ordered Fraisse structure with the Ramsey property of its finite substructures. In the same spirit Moore showed a correspondence between the automorphism groups of countable structure and a a structural Ramsey property, which englobes Fölner's existing treatment. In this talk we will consider automorphism groups of certain Hrushovski's generic structures. We will show that they are not amenable by exhibiting a combinatorial/geometric criterion which forbids amenability.
Piotr Kowalski (Wroclaw)
Titel: Virtually free groups and Galois actions
Abstract: I will talk about joint work with Özlem Beyarslan. We show that for a finitely generated virtually free group G, the theory of actions of G on fields has a model companion, which we call G-TCF. We also give an algebraic condition on G, which is equivalent to simplicity of the theory G-TCF.
Christian Urech (Basel)
Titel: Subgroups of elliptic elements of the Cremona group
Abstract: The Cremona group is the group of birational transformations of the complex projective plane. This classical topic has recently again turned into a popular subject of research and in the last decade many classicalproblems were solved with modern tools from birational geometry and dynamics. In this talk I will explain how a basic application of the compactness theorem allows the classification of groups of elliptic elements of the Cremona group - a class of groups that hasn't been understood well so far. In particular, one can deduce from this classification the Tits alternative for the Cremona group. We will also see that every simple subgroup of the Cremona group is isomorphic to a simple subgroup of PGL(3,C).