The Scandinavian Logic Society

NOL Seminar with Thomas Ågotnes

The Nordic Online Logic Seminar is organised monthly over Zoom, with expository talks on topics of interest for the broader logic community. The seminar is open for professional or aspiring logicians and logic aficionados worldwide. If you wish to receive the Zoom ID and password for it, as well as further announcements, please subscribe here:

Date: Monday, 28 August 2023 at 16:00 CEST (UTC+2) on Zoom
Speaker: Thomas Ågotnes, Professor of Information Science, University of Bergen (Norway) and Professor of Logic, Shanxi University (China)
Title: Somebody Knows and Weak Conjunctive Closure in Modal Logic

Normal modal logics are closed under conjunctive closure. There are, however, interesting non-normal logics that are not, but which nevertheless satisfy a weak form of conjunctive closure. One example is a notion of group knowledge in epistemic logic: somebody-knows. While something is general knowledge if it is known by everyone, this notion holds if it is known by someone. Somebody-knows is thus weaker than general knowledge but stronger than distributed knowledge. We introduce a modality for somebody-knows in the style of standard group knowledge modalities, and study its properties. Unlike most other group knowledge modalities, somebody-knows is not a normal modality; in particular it lacks the conjunctive closure property. We provide an equivalent neighbourhood semantics for the language with a single somebody-knows modality, together with a completeness result: the somebody-knows modalities are completely characterised by the modal logic EMN extended with a particular weak conjunctive closure axiom. The neighbourhood semantics and the completeness and complexity results also carry over other logics with weak conjunctive closure, including the logic of so-called local reasoning (Fagin et al., 1995) with bounded “frames of mind”, correcting an existing completeness result in the literature (Allen 2005). The talk is based on joint work with Yi N. Wang.