# Gaifman: Epistemic and Ontological Problems Concerning Mathematics

HARVARD/MIT LOGIC SEMINAR
Epistemic and Ontological Problems Concerning Mathematics
Haim Gaifman (Columbia University)
Tuesday, April 30, 2013, at 4:15 PM
Fong Auditorium (Boylston Hall 110), Harvard University

Abstract. Philosophy of mathematics is confronted with two major questions: (i) How do we come to know mathematical propositions? (ii) What is the nature of mathematical truth? Attempts to give satisfactory answers to one of the questions have resulted in unsatisfactory accounts regarding the other. I shall outline an approach intended to do justice to both questions. This is an ongoing work.

# Hamkins: The theory of infinite games, with examples, including infinite chess

YESHIVA MATH/PHIL CLUB
The theory of infinite games, with examples, including infinite chess
Joel David Hamkins (CUNY)
Tuesday, April 30, 2013 5:45 pm
Furst Hall, Amsterdam Ave. & 185th Street, Yeshiva University

Abstract. I will give a general introduction to the theory of infinite games, suitable for mathematicians and philosophers. What does it mean to play an infinitely long game? What does it mean to have a winning strategy for such a game? Is there any reason to think that every game should have a winning strategy for one player or another? Could there be a game, such that neither player has a way to force a win? Must every computable game have a computable winning strategy? I will present several game paradoxes and example infinitary games, including an infinitary version of the game of Nim, and several examples from infinite chess.

# Baldwin: Axiomatic Set Theory and L_{omega_1,omega}

CUNY MODEL THEORY SEMINAR
Axiomatic Set Theory and $L_{\omega_1,\omega}$
John T. Baldwin (Mathematics, UIC)
Friday, April 20, 2012, 12:30 PM
Abstract. In the late 1960’s model theory and axiomatic set theory seemed to be inevitably intertwined. The fundamental notions of first order stability theory are absolute. We describe the role of this fact in the development of first order model theory independent from set theory since the 1970’s. The role of extensions of ZFC in infinitary logic is muddled. Important results are proved using weak extensions of ZFC; the use is not in general proved essential. We expound the following proof-scheme: (1) Prove an infinitary sentence is consistent with ZFC. (2) Prove there is a model of set theory for which this sentence is absolute. (3) Deduce the property it expresses is provable in ZFC. We will describe how this technique implies the following recent result: Theorem (Shelah) Let $\phi$ be a sentence of $L_{\omega_1, \omega}$. (a) If ‘algebraic closure’ fails exchange on models of phi then $\phi$ has many models in $\aleph_1$. (b) If $\phi$ is pseudo-minimal then it has model in the continuum. Here algebraic closure’ and pseudo-minimal’ are modifications of classical notions appropriate for the context.