**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.