# Upcoming Events

Logic Colloquium

February 22, 2019, 4:10 PM (60 Evans Hall)

Rizos Sklinos

Lecturer, Stevens Institute of Technology

Elementary theories of hyperbolic groups

The discovery of non euclidean geometry in the early nineteenth century had shaken the beliefs and conjectures of more than two thousand years and changed the picture we had for mathematics, physics and even philosophy. Lobachevsky and Bolyai independently around 1830 discovered hyperbolic geometry. A notable distinguish feature of hyperbolic geometry is its negative curvature in a way that the sum of angles of a triangle is less than π. Gromov much later in 1987 introduced hyperbolic groups which are groups acting “nicely” on hyperbolic spaces, or equivalently finitely generated groups whose Cayley graphs are “negatively curved”. Main examples are free groups and almost all surface groups. The fascinating subject of hyperbolic groups touches on many mathematical disciplines such as geometric group theory, low dimensional topology and combinatorial group theory. It is connected to model theory through a question of Tarski.

Tarski asked around 1946 whether non abelian free groups have the same common first order theory. This question proved extremely hard to answer and only after more than fifty years in 2001 Sela and Kharlampovich-Myasnikov answered it positively. Both works are voluminous and have not been absorbed yet. The techniques almost exclusively come from the disciplines mentioned above, hence it is no wonder that the question had to wait for their development. The great novelty of the methods and the depth of the needed results have made it hard to streamline any of the proofs. Despite the difficulties there is some considerable progress in the understanding of the first order theory of “the free group” and consequently first order theories of hyperbolic groups from the scopes of basic model theory, Shelah’s classification theory and geometric stability. In this talk I will survey what is known about these theories and what are the main open questions.

Logic Colloquium

March 08, 2019, 4:10 PM (60 Evans Hall)

Dana S. Scott, FBA, FNAS

Distinguished Research Associate, UC Berkeley Department of Philosophy

Free Logic: its formalization and some applications

Prof. J. Karel Lambert (Emeritus, UC Irvine) gave the subject its name and its profile as a well defined field of research over 60 years ago. “Free logic” is logic free of existential presuppositions in general and with respect to singular terms in particular. This talk will discuss its formalization and models for both classical and intuitionistic logic. Applications to understanding properties of descriptions, virtual classes, and structures with partial functions will be presented.