Nordic Online Logic Seminar  page 2
An online seminar for logicians and logic aficionados worldwide.
The Nordic Online Logic Seminar (NOL Seminar) is a monthly seminar series initiated in 2021 presenting expository talks by logicians on topics of interest for the broader logic community. Initially the series focused on activities of the Nordic logic groups, but has since expanded to offer a variety of talks from logicians around the world. The seminar is open to professional or aspiring logicians and logic aficionados worldwide.
The tentative time slot is Monday, 16.0017.30 (Stockholm/Sweden time). If you wish to receive the Zoom ID and password for it, as well as regular announcements, please subscribe to the NOL Seminar mailing list.
NOL seminar organisers
Valentin Goranko and Graham Leigh

Sonja Smets (Institute for Logic, Language and Computation, University of Amsterdam)
Reasoning about Epistemic Superiority and Data Exchange
In this presentation I focus on a framework that generalizes dynamic epistemic logic in order to model a wider range of scenarios including those in which agents read or communicate (or somehow gain access to) all the information stored at specific sources, or possessed by some other agents (including information of a nonpropositional nature, such as data, passwords, secrets etc). The resulting framework allows one to reason about the state of affairs in which one agent (or group of agents) has ‘epistemic superiority’ over another agent (or group). I will present different examples of epistemic superiority and I will draw a connection to the logic of functional dependence by A. Baltag and J. van Benthem. At the level of group attitudes, I will further introduce the new concept of ‘common distributed knowledge’, which combines features of both common knowledge and distributed knowledge. This presentation is based on joint work with A. Baltag in [1].
[1] A. Baltag and S. Smets, Learning what others know, in L. Kovacs and E. Albert (eds.), LPAR23 proceedings of the International Conference on Logic for Programming, AI and Reasoning, EPiC Series in Computing, 73:90110, 2020. https://doi.org/10.29007/plm4

Dag Westerståhl (Stockholm University, Tsinghua University)
From consequence to meaning: the case of intuitionistic propositional logic (IPL)
One quarter of the talk presents background on how facts about entailments and nonentailments can single out the constants in a language, and in particular on an idea originating with Carnap that the standard relation of logical consequence in a formal language should fix the (modeltheoretic) meaning of its logical constants. Carnap’s focus was classical propositional logic (CPL), but his question can be asked for any logical language. The rest of the talk gives a very general positive answer to this question for IPL: the usual IPL consequence relation does indeed determine the standard intuitionistic meaning of the propositional connectives, according to most wellknown semantics for IPL, such as Kripke semantics, Beth semantics, Dragalin semantics, topological semantics, and algebraic semantics.

Thomas Bolander (Technical University of Denmark)
Epistemic Planning: Logical formalism, computational complexity, and robotic implementations
Dynamic Epistemic Logic (DEL) can be used as a formalism for agents to represent the mental states of other agents: their beliefs and knowledge, and potentially even their plans and goals. Hence, the logic can be used as a formalism to give agents, e.g. robots, a Theory of Mind, allowing them to take the perspective of other agents. In my research, I have combined DEL with techniques from automated planning in order to describe a theory of what I call Epistemic Planning: planning where agents explicitly reason about the mental states of others. The talk will introduce epistemic planning based on DEL, address issues of computational complexity, and demonstrate applications in cognitive robotics and humanrobot collaboration.

Thomas Ågotnes (University of Bergen and Shanxi University)
Somebody Knows and Weak Conjunctive Closure in Modal Logic
Normal modal logics are closed under conjunctive closure. There are, however, interesting nonnormal 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: somebodyknows. While something is general knowledge if it is known by everyone, this notion holds if it is known by someone. Somebodyknows is thus weaker than general knowledge but stronger than distributed knowledge. We introduce a modality for somebodyknows in the style of standard group knowledge modalities, and study its properties. Unlike most other group knowledge modalities, somebodyknows 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 somebodyknows modality, together with a completeness result: the somebodyknows 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 socalled 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.

Magdalena Ortiz (Umeå University)
A Short Introduction to SHACL for Logicians
The SHACL Shapes Constraint Language was recommended in 2017 by the W3C for describing constraints on web data (specifically, on RDF graphs) and validating them. At first glance, it may not seem to be a topic for logicians, but as it turns out, SHACL can be approached as a formal logic, and actually quite an interesting one. In this paper, we give a brief introduction to SHACL tailored towards logicians and frame key uses of SHACL as familiar logic reasoning tasks. We discuss how SHACL relates to description logics, which are the basis of the OWL Web Ontology Languages, a related yet orthogonal standard for web data. Finally, we summarize some of our recent work in the SHACL world, hoping that this may shed light on how ideas, results, and techniques from wellestablished areas of logic can advance the state of the art in this emerging field.

Neil Tennant (Ohio State University)
It's All or Nothing: Explosion v. Implosion
We set out five basic requirements for a logical system to be adequate for the regimentation of deductive reasoning in mathematics and science. We raise the question whether there are any further requirements, not entailed by these five, that ought to be imposed. One possible reply is dismissed: that the logical system should allow one to infer any proposition at all from an inconsistent set—i.e., it should have as primitive, or allow one to derive, the rule Ex Falso Quodlibet. We then propose that the logic should be implosive: it should not allow an inconsistent set to have any consequences other than absurdity. This proposal may appear to be very radical; but we hope to show that it is robust against objections.

Ali Enayat (University of Gothenburg)
Arithmetic and set theory through the lens of interpretability
The notion of (relative) interpretation for first order theories was introduced in a landmark 1953 monograph by Alfred Tarski, Andrzej Mostowski and Raphael Robinson, where it was developed as a powerful tool for establishing undecidability results. By now the domain of interest and applicability of interpretability theory far exceeds undecidability theory owing to its multifaceted interactions with both proof theory and model theory. Special attention will be paid to recent advances in the subject that indicate the distinctive character of Peano Arithmetic, ZermeloFraenkel set theory, and their higher order analogues in the realm of interpretability theory. This talk will present a personal overview of the interpretability analysis of arithmetical and set theoretical theories.

Sven Ove Hansson (KTH)
How to combine probabilities and full beliefs in a formal system
One of the major problems in formal epistemology is the difficulty of combining probabilistic and full (dichotomous, allornothing) beliefs in one and the same formal framework. Major properties of actual human belief systems, including how they are impacted by our cognitive limitations, are used to introduce a series of desiderata for realistic models of such belief systems. This leads to a criticism of previous attempts to combine representations of both probabilistic and full beliefs in one and the same formal model. Furthermore, a formal model is presented in which this is done differently. One of its major features is that contingent propositions can be transferred in both directions between full beliefs and lower degrees of belief, in a way that mirrors reallife acquisitions and losses of full beliefs. The subsystem consisting of full beliefs has a pattern of change that constitutes a credible system of dichotomous belief change.

Vann McGee (MIT)
Boolean Degrees of Truth and Classical Rules of Inference
Compositional semantics that acknowledge vagueness by positing degrees of truth intermediate between truth and falsity can retain classical sentential calculus, provided the degrees form a Boolean algebra. A valid deduction guarantees that the degree of truth of the conclusion be at least as great as every lower bound on the degrees of the premises. If we extend the language to allow infinite disjunctions and conjunctions, the Boolean algebra will need to be complete and atomic. If we extend further by adding quantifiers ranging over a fixed domain, we get the supervaluations proposed by Bas van Fraassen.

Alexandru Baltag (University of Amsterdam)
From Surprise Exams to Topological MuCalculus
I present a topological epistemic logic, motivated by a famous epistemic puzzle: the Surprise Exam Paradox. It is a fixedpoint modal logic, with modalities for knowledge (modelled as the universal modality), knowability of a proposition (represented by the topological interior operator), and (un)knowability of the actual world. The last notion has a nonselfreferential reading (modelled by Cantor derivative: the set of limit points of a given set) and a selfreferential one (modelled by the socalled perfect core of a given set: its largest subset which is a fixed point of relativized derivative). I completely axiomatize this logic, showing that it is decidable and PSPACEcomplete, as well as briefly explain how the same modeltheoretic method can be elaborated to prove the completeness and decidability of the full topological mucalculus. Finally, I apply it to the analysis of the Surprise Exam Paradox and other puzzles.
References:
 A. Baltag, N. Bezhanishvili, D. FernándezDuque. The Topology of Surprise. Proceedings of the International Conference on Principles of Knowledge Representation and Reasoning. Vol. 19 (1), 3342, 2022. Available online in ILLC Prepublication (PP) series PP202206.
 A. Baltag, N. Bezhanishvili, D. FernándezDuque. The Topological MuCalculus: Completeness and Decidability. LICS ‘21: Proceedings of the 36th Annual ACM/IEEE Symposium on Logic in Computer Science, vol 89: 113, 2021. doi:10.1109/lics52264.2021.9470560. Available online in ILLC Prepublication (PP) series PP202107.