Nordic Online Logic Seminar

The axioms of quantum mechanics present the state of a quantum system as a unit vector in complex Hilbert space. However, when Dirac [1] presented his bra- and ket-vectors, he had a more general space in mind. Schwartz [4] later gave a rigorous account for Dirac’s “vectors” as distributions, but in elementary physics books one still encounters presentations where ket-vectors are presented just as elements of a Hilbert space, and treated with methods from finite-dimensional linear algebra.

During the last years Tapani Hyttinen and I [2,3] have been looking at various models justifying the finite dimensional approaches from such textbooks. Our approaches are based on various embeddings of a Hilbert space into metric ultraproducts of finite-dimensional Hilbert spaces. In this talk I will present the basic ideas, their benefits and limitations. Time permitting, I will also contrast our approach to Boris Zilber’s work on the same questions, that was the original inspiration for us.

1. P.A.M. Dirac. The principles of Quantum Mechanics, 3rd ed, Clarendon Press, Oxford, 1947.
2. Å. Hirvonen, T. Hyttinen, On eigenvectors, approximations and the Feynman propagator, Ann. Pure Appl. Logic 170 (2019).
3. Å. Hirvonen, T. Hyttinen, On Ultraproducts, the Spectral Theorem and Rigged Hilbert Spaces, to appear in J. Symb. Log.
4. L. Schwartz, Théory de Distributions, Hermann, Paris, 1950.

Natural languages vary in their quantity expressions, but the variation seems to be constrained by general properties, so-called universals. Explanations thereof have been sought among constraints of human cognition, communication, complexity, and pragmatics. In this work, we examine whether perceptual constraints and coordination dynamics contribute to the development of two universals: monotonicity and convexity. Using a state-of-the-art multi-agent language coordination model (originally applied to colour terms) we evolve communicatively usable quantity terminologies. We compare the degrees of convexity and monotonicity of languages evolving in populations of agents with and without approximate number sense (ANS). The results suggest that ANS supports the development of monotonicity and, to a lesser extent, convexity. We relate our results to some classical observations about generalised quantifiers in mathematical logic and to the research on conceptual spaces.

This is joint work with Dariusz Kalociński, Franek Rakowski and Jakub Uszyński.

Voting on two alternatives appears unproblematic compared to voting on three (or more). When faced with only two alternatives, many arguments show that Majority Rule distinguishes itself from all other ways of making a group decision. For three or more alternatives, one faces the so-called “Paradox of Voting”: there may be elections with a majority cycle in which a majority of voters prefer A to B, a majority of voters prefer B to C, and a majority of voters prefer C to A. In this talk, I will explain a series of results that axiomatically characterize rules for resolving majority cycles in elections. These rules avoid the “Strong No Show Paradox” by responding properly to the addition of new voters to an election and mitigate spoiler effects by responding properly to the addition of new candidates to an election.

This talk is based on joint work with Wes Holliday.

Inquisitive modal logic is a generalization of standard modal logic where the language also contains questions, and modal operators that can be applied to them. In this talk, I will provide an introductory overview of inquisitive modal logic. I will review some motivations for the approach, present some prominent examples of inquisitive modal logics, mention some results about them, and outline directions for future work.

Sequential theories form a fundamental class of theories in logic. They have full coding possibilities and allow us to build partial satisfaction predicates for formulas of bounded depth-of-quantifier-alernations. In many respects, they are the proper domain of Gödelian metamathematics. We explain the notion of sequential theory.

A theory is restricted if it can be axiomatised by axioms of bounded depth-of-quantifier-alernations. All finitely axiomatised theories are restriced, but, for example, also Primitive Recursive Arithmetic. We explain the small-is-very-small principle for restricted sequential theories which says that, whenever the given theory shows that a definable property has a small witness, i.e., a witness in a sufficiently small definable cut, then it shows that the property has a very small witness, i.e., a witness below a given standard number.

We sketch two proofs that restricted theories are incomplete (however complex the axiom set). One uses the small-is-very-small principle and the other a direct Rosser argument. (The second argument was developed in collaboration with Ali Enayat.)

A game that characterizes definability of classes of structures by first-order sentences containing a given number of quantifiers was introduced by Immerman in 1981. In this talk I describe two other games that are equivalent with the Immerman game in the sense that they characterize definability by a given number of quantifiers.

In the Immerman game, Duplicator has a canonical optimal strategy, and hence Duplicator can be completely removed from the game by replacing her moves with default moves given by this optimal strategy. On the other hand, in the other two games there is no such optimal strategy for Duplicator. Thus, the Immerman game can be regarded as a one-player game, but the other two games are genuine two-player games.

The talk is based on joint work with Kerkko Luosto.

Switcher Semantics is a semantic framework that is basically characterised by allowing switching: when recursively applying a semantic function \(\mu\) to a complex term \(t\), the semantic function applying to an immediate subterm \(t'\) of \(t\) may be a function \(\mu'\), distinct from \(\mu\). An operator-argument-position pair is called a switcher if it induces such a switch. Switcher semantic systems do not satisfy the standard form of compositionality, but a generalized form, which allows greater flexibility. In earlier work (mostly published), some together with Kathrin Glüer, some with Dag Westerståhl, it has been applied to natural language constructions like proper names in modal contexts, general terms in modal contexts, indexicals in temporal contexts, quotation, and belief contexts. This talk will focus on quantifiers and quantification. First-order quantifiers can be regarded as switchers, switching from truth conditions to satisfaction conditions. The larger topic is quantification into switched contexts. I shall begin by giving an introduction to the framework.

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 non-propositional 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, 73:90-110, 2020. https://doi.org/10.29007/plm4