LIRa Session: Rasmus Rendsvig

Speaker: Rasmus Rendsvig (Center for Information and Bubble Studies, University of Copenhagen)

Date and Time: Thursday, November 28th 2019, 13:00-14:30 (NOTE THE UNUSUAL TIME)

Venue: ILLC Seminar Room F1.15, Science Park 107.

Title: A Topological Perspective on Common Knowledge and on Dynamic Epistemic Logic. 

Abstract. This talk discusses how analyzing logical dynamics as unfolding in a topological space provides an interesting perspective. The talk falls in two parts.

The first part exemplifies a topological analysis by taking a look at the attainment of common knowledge. Common knowledge, as is well known, is not attainable in finite time by unreliable communication, thus hindering perfect coordination. Focusing on the coordinated attack problem modeled using dynamic epistemic logic, this part asks

“If the generals may communicate indefinitely, will they then converge to a state of common knowledge?”

We answer by making precise and showing the following:
Common knowledge is attainable if, and only if, we do not care about common knowledge.

The analysis relies on a particular interpretation of qualitative distance. The first part will motivate one specific such interpretation, which in turn motivates working with a particular topology, known as the Stone topology.

The second part of the talk takes an abstract view of functions and relations on Stone topological spaces—one realm inhabited by many model transformations known from dynamic epistemic logic—and discusses connections between recursive reduction law existence, computability and continuity.