# This Week in Logic at CUNY

Computational Logic Seminar
Tuesday, February 5, 2013 2:00 pm
Speaker: Konstantinos Georgatos John Jay College Of Criminal Justice
Title: Geodesic Semantics for Belief Change
Any logical system that models human reasoning needs to address the
possibility of error and the subsequent belief change once this error
is recognized. The need to deal with error-prone reasoning has only
been widely acknowledged in the last thirty years or so; witnesses the
popularity of the AGM postulates for Belief Revision. Despite the
variety of syntactical and semantical offerings, all seem to agree
that we need to model a concept of minimal change by choosing the most
similar epistemic state to the present one. The favorite choice
mechanisms are preferential orderings and, their generalization,
distance maps. Preferential orderings provide satisfactory
representation results but fail to model iteration. Distance maps
model iteration but fail to provide satisfactory completeness results.
In this talk, I will introduce a third semantical approach using
geodesic distance (length of shortest path on a graph) that lies
between the two and combines their best features: geodesic semantics
provide satisfactory completeness results like preferential orderings
do and deal with iteration, as distance maps do. Further, and perhaps
more important, geodesic semantics offer a novel, more natural
representation of similarity using distinguishability.

Set theory seminar
Friday, February 8, 2013 10:00 am
Speaker: Gunter Fuchs The City University of New York
Title: Magidor Forcing
In this talk, I am going to present a forcing designed by Magidor in
the late seventies to change the cofinality of a measurable cardinal
without collapsing cardinals. Previously, Prikry had introduced a
forcing that changes the cofinality of a measurable cardinal to
$omega$. Magidor’s forcing has more flexibility, but needs stronger
assumptions also, and it is quite complex. After giving some
background and showing the basic properties of Magidor forcing, I will
prove a combinatorial characterization of genericity of sequences
added by the forcing. There are some similarities to the situation of
Prikry forcing, where an $omega$-sequence of ordinals less than the
measurable cardinal is generic iff it is almost contained in any
measure one set, as was shown by Mathias. I will show or sketch a
proof of the corresponding characterization in the case of Magidor
forcing in the second part of the talk next week.

Model theory seminar
Friday, February 8, 2013 12:30 pm
Speaker: Alf Dolich The City University of New York
Title: Organizational Meeting
We’ll meet to discuss topics to be covered for the semester.

CUNY Logic Workshop
Friday, February 8, 2013 2:00 pm
Speaker: Roman Kossak The City University of New York
Title: Model theory of satisfaction classes
All countable recursively saturated models of Peano Arithmetic have
nonstandard satisfaction classes. In fact, each such model has a great
variety of nonstandard satisfaction classes. I will survey model
theoretic techniques that can be applied to construct many different
inductive satisfaction classes, and I will show how, in return,
inductive satisfaction classes are used to prove important result
about recursively saturated models of PA. I will also pose an open
problem concerning a possible converse to Tarski’s undefinability of
truth theorem.