Logic

Set theory has proven useful in the study of derived limits. These functors are widely studied for their applications in algebraic topology, and their behavior is to some extent independent from ZFC. As already shown by Bergfalk and Lambie-Hanson in the case of ordinals, the derived limits associated with some set-theoretic objects tend not to vanish in $𝕃$. This corresponds to some form of incompactness. Here I present a similar nonvanishing result for ${}^κ ω$ that uses diamonds and special Aronszajn trees. This is work in progress with Jeffrey Bergfalk.

One of the foundational results of voting theory is the Gibbard–Satterthwaite theorem: for finite societies, every function selecting a winner from a finite set of candidates that is immune to manipulation by the misrepresentation of preferences is either constant or dictatorial. In infinite societies, the Gibbard–Satterthwaite theorem can fail: Pazner and Wesley (1977) showed that when the set of voters is infinite, there exist social choice functions that are both non-manipulable ('strategyproof') and non-dictatorial. Their proof rests on the existence of non-principal ultrafilters, a consequence of the axiom of choice, and hence clearly has a nonconstructive aspect. In this talk I will examine the Pazner–Wesley possibility theorem in the context of reverse mathematics, and show that for countable societies it is equivalent over RCA0 to arithmetical comprehension. I will then formulate a seemingly weaker version of the Pazner–Wesley possibility theorem, using individual strategyproofness rather than coalitional strategyproofness, and raise some open questions regarding the relationship between these two statements.

NOTES: unusual room.
In the last 20 years there has been a resurgence of interest in fixed points in both mathematical and computational logic. At the interface of both these traditions is the extension of type theories with (co)induction. Similar theories now underlie popular proof assistants such as Coq and Lean. However, until recently, little was known about the expressivity of theories with fixed points: what can they prove? what can they compute? In this talk I will talk about a recent line of work that classifies the expressivity of type theories with fixed points. This talk is based on joint work with Gianluca Curzi.

Partial combinatory algebras (pcas) are abstract models of computation. They are one of the earliest type of model that emerged in the 1930's when mathematicians were trying to define what it means to be computable. Notions such as recursive functions (used by Gödel in the proof of his incompleteness theorems), Turing machines, combinatory algebra and lambda calculus each turned out to be useful in the development of the theory of computation in their own way. Pcas are also used in the study of constructive mathematics. They play a key role in the connections between constructive mathematics, proof theory, and computability, and also serve as a basis for models of constructive mathematics.

In this talk we will give a quick review of the basics of combinatory algebra, and discuss some of the key examples, such as Kleene's models $\mathcal{K}_1$ (which describes the classical setting for computation on the natural numbers), $\mathcal{K}_2$ (which does the same for the real numbers), and Scott's graph model. We then discuss recent results about embeddings and completions of pcas, and in particular the complexity of various embeddings. Depending on time, we will also discus the complexity of the isomorphism problem, extensionality, and ordinal analysis of pcas.

Everyone loves a good decomposition. How can we break down a mathematical object — a graph, a group, or a function — efficiently into well-behaved (or regular) parts? And what conditions can we place on these objects to guarantee a higher degree of regularity, such as homogeneity? It turns out an excellent source of such conditions is model-theoretic dividing lines, that is, tameness properties of (first-order) structures. This is not a coincidence. In this talk, I will dive into the deep theory of these dividing lines in search of the source of homogeneity.

A graph has a countable chromatic number if the vertices can be labeled by natural numbers so that no edge has the same label on both ends. Verifying this property for a given graph can be somewhat difficult; consider e.g. the graph of points in the Euclidean space $ℝ^n$ connected with an edge iff their distance is a rational number. The countable chromatic number of this graph is an easy observation for $n=1$, but a nontrivial result for $n>1$ (Komjath, Schmerl). I will introduce an infinite determined game which provides a simple criterion for a definable (analytic on a Polish space) graph to be countably chromatic; it is sufficient to verify that the second player has a winning strategy. Using this criterion we can e.g. easily resolve the example of $ℝ^n$, as well as deduce interesting consequences on the structure of uncountably chromatic definable graphs.

The talk is based on the paper Chodounský, Zapletal: Two graph games (2024).

Given a topology on the natural numbers, how complicated is it to describe? To answer this question with tools from computability theory, we will restrict to the context of countable second-countable (CSC) topological spaces. One approach is to assign an index to each computable CSC space and determine the arithmetic complexity of the set of CSC spaces with some property. Another approach comes from computable structure theory; for example, given two computable copies of a CSC space, does there exist a computable homeomorphism between them? In this talk, we will explore these approaches and apply them in three running examples: the indiscrete, discrete, and initial segment topologies.

NOTES: the speaker will be online.
I will give a talk on several recent results regarding ultrahomogeneous graphs. A relational structure R is ultrahomogeneous if every isomorphism of finite induced substructures of R extends to an automorphism of R. We classify the finite vertex-colored oriented ultrahomogeneous graphs and the finite vertex-colored undirected graphs. The classifications comprise new general methods which govern how graphs can be combined or extended to create new ultrahomogeneous graphs. Further, we extend a classic theorem of Cameron (every 5-tuple regular graph is already ultrahomogeneous) to the setting of colored graphs.

This is joint work with Sofia Brenner, Eda Kaja, Thomas Schneider, and Pascal Schweitzer.

Fix a sequence of countable structures $\mathcal{M}_n$, for $n$ in $ℕ$, in a given first-order language $\mathcal{L}$. The reduced product of $(\mathcal{M}_n)$, denoted $\prod_n\mathcal{M}_n/\mathrm{Fin}$ is the $\mathcal{L}$-structure obtained by quotienting the product of the $\mathcal{M}_n$ by the equivalence relation of 'eventual equality'. This is similar to the ultraproduct construction, yet its theory is fairly less understood.

We consider the following question: if two reduced products $\prod_n\mathcal{M}_n/\mathrm{Fin}$ and $\prod_n\mathcal{N}_n/\mathrm{Fin}$ are isomorphic, what can be said about relations between structures we started with? Of course, one can obtain isomorphisms by taking fiberwise isomorphisms, and/or shuffling the indexes, but is that all? We discuss how, in specific interesting cases (such as fields, or certain graphs), answers to this question depend on the set theoretic ambient.

The concept of a monster model in model theory is a helpful tool for avoiding cumbersome bookkeeping and notation. Instead of needing to repeatedly take elementary extensions to realise types, or find automorphisms, one can simply say that there is a model M of a theory such that any 'small' model of that theory embeds into M, and any 'small' partial isomorphism in M extends to an automorphism. While usually 'small' means 'cardinality less than M' (and M is then taken to be 'big enough'), sometimes there are special cases in which 'small' means 'set-sized'. In particular, the surreal numbers acts as a monster model for the theory of dense linear orders, real-closed fields, and more.

I will introduce the concept of a monster model and its uses, expand somewhat on the hidden set-theoretic baggage associated with these objects, and show off the monstrosity of the surreal numbers. I will then show how, without the axiom of choice, the surreal numbers may no longer be a monster, using a simple symmetric extension argument.