We are happy to announce that BCTCS is coming back to Swansea next year. More details to follow.
Today Adam Ó Conghaile will give a talk on “Games & Comonads in Finite Model Theory” as a part of our Theory Seminar Series.
Abstract. Model-comparison games (such as Ehrenfeucht-Fraïssé games, pebble games and modal bisimulation) capture various approximations to isomorphism and homomorphism between structures. These approximations are important and well-studied in computer science for their connections with logic and algorithms but their descriptions are often ad hoc and unified approach to these games is lacking in classical finite model theory. Game comonads, introduced by Abramsky, Dawar & Wang , provide a categorical semantics for these games which reveals connections between these games and relates them in a surprising and elegant way to structural parameters such as treewidth.
In my talk, I will survey the landscape of model-comparison games and developments in this new comonadic perspective to them, including contributions from my recent work with Anuj Dawar on game comonads for logics with generalised quantifiers. 
 – S. Abramsky, A. Dawar, P. Wang The pebbling comonad in finite model theory, LiCS 2017
 – A. Ó Conghaile, A. Dawar Game comonads & generalised quantifiers, CSL 2021
We are looking forward to welcome Paul Schafer, who will be visiting Swansea University on 18 to 23 January.
As a part of our Theory Seminars, today we welcomed Ryota Akiyoshi from Waseda University, who gave a talk on Takeuti’s finitism.
Abstract: In this talk, we address several mathematical and philosophical issues of Gaisi Takeuti’s proof theory, who is one of the most distinguished logicians in proof theory after Hilbert and Gentzen. He furthered the realization of Hilbert’s program by formulating Gentzen’s sequent calculus for higher-oder logics, conjecturing that the cut-elimination holds for it (Takeuti’s conjecture), and obtaining several stunning results in the 1950-60’s towards the solution of his conjecture.
This talk consists of two parts. (1) To summarize Takeuti’s background and the argument of the well-ordering proof of ordinals up to ε0 , (2) To evaluate it on philosophical grounds. Also, we will explain several mathematical and philosophical issues to be solved. This is joint work with Andrew Arana.
Tomorrow we are holding our end-of-summer logic minisymposium.
- Åsa Hirvonen
- Ken-etsu Fujita
- Masahiko Sato
2:00 Åsa Hirvonen (Helsinki University): On continuous logic
2:40 Coffee & Cake
3:00 Ken-etsu Fujita (Gunma University): A constructive proof of the Church—Rosser theorem
3:50 Masahiko Sato (Kyoto University): Unification of the Lambda-Calcululs and Combinatory Logic
On continuous logic, Åsa Hirvonen (Helsinki University)
Continuous first order logic in its current form was developed at the beginning of this millennium to offer a language for the model theoretic study of metric structures, such as Banach spaces and probability spaces. It is a many-valued logic but has a different motivation and semantic than, e.g., Lukasiewicz logic. Many model theoretical properties of first order logic generalise to it, such as compactness and Löwenheim-Skolem theorems. This talk is a general introduction to the syntax and semantics as well as some basic properties of continuous logic.
A constructive proof of the Church—Rosser theorem, Ken-etsu Fujita (Gunma University)
My motivation behind this talk comes from a quantitative analysis of reduction systems based on the two viewpoints, computational cost and computational orbit.
In the first part, we show that an upper bound function for the Church—Rosser theorem of type-free lambda-calculus with beta-reduction must be in the fourth level of the Grzegorczyk hierarchy. That is, the number of reduction steps to arrive at a common reduct is bounded by a function in the smallest Grzegorczyk class properly extending that of elementary functions. At this level we also find common reducts for the confluence property. The proof method developed here can be applied not only to type-free lambda-calculus with beta-eta-reduction
but also to typed lambda-calculi such as Pure Types Systems.
In the second part, we propose a formal system of reduction paths for parallel reduction, wherein reduction paths are generated from a quiver by means of three path-operators, concatenation, monotonicity, and cofinality. Next, we introduce an equational theory and reduction rules for the reduction paths, and show that the rules on paths are terminating and confluent so that normal paths are obtained. Following the notion of normal paths, a graphical representation of reduction paths is provided, based on which unique path and universal common-reduct properties are established. Finally, transformation rules from a conversion sequence to a reduction path leading to the universal common-reduct are given, and path matrices are also defined as block matrices of adjacency matrices in order to count reduction orbits.
Unification of the Lambda-Calcululs and Combinatory Logic, Masahiko Sato (Kyoto University):
The Lambda calculus and combinatory logic have been studied as two closely related but distinct systems of logic and computation. In this talk, however, we will argue that they are in fact one and the same calculus.
We substantiate our argument by introducing what we will call the external syntax and the internal syntax for the two calculi. The external syntax will be given as a natural common extension of syntax for the \lambda-calculus and combinatory logic.
The terms defined by the internal syntax will be characterized as the closed alpha eta normal terms of the external syntax. Thus, the external syntax provides us with human readable syntax containing all the traditional \lambda-terms and combinatory-terms, and the internal syntax is suitable for the infrastructure of a proof assistant.
This is an ongoing joint work with Takafumi Sakurai and Helmut Schwichtenberg.
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Ulrich, Monika and Olga attended Hausdorff Trimester Program Types, Sets and Constructions in Bonn, Germany. During this research trip they have participated in the Constructive Mathematics workshop and had a chance to collaborate with partners from the past and existing projects, including COMPUTAL, CONRCON and CID.
Irina and Markus attended SPIN symposium that took place in Malaga, Spain, where they gave a tutorial on Software Model Checking for Mobile Security.
Magne Haveraaen from the University of Bergen is visiting our Department in May-July 2018.
Ulrich and Olga visited Japan as part of the CID project in April 2018.