## Fudan Logic Seminar 2024

### November 7

Location: HGX204

Speaker: Theodore A. Slaman

Time: 18:30 - 20:10

#### On the Turing Jump

There is a clear method to show that a set of integers $A$ is computable: exhibit an algorithm which given input $k$ halts in finitely many steps and returns whether $k$ is an element of $A$. Normally, this algorithm uncovers a reason that $k \in A$ or $k \notin A$. Surprisingly, in most cases, proofs that sets are not computable also follow a clear method and uncover a common reason for non-computability: any algorithm to compute $A$ could be converted to a program to compute the Halting Problem,
\[H = \{e : \text{the $e$th program halts on input $e$}\},\]
a canonical non-computable set. We will discuss a few of these images of $H$, show how the Halting Problem can be extended to a function on sets of integers called the Turing jump, and make the mathematical case that the jump is intrinsic to the concept of computability.

This talk is intended for a general audience and will not assume prior knowledge of computability or mathematical logic.

### October 25

Location: HGW2409

Speaker: Jan Dobrowolski

Time: 11:00 - 12:35

#### Some connections between model theory and algebraic combinatorics

I will first talk about a joint work with M. Bays and T. Zou in which we studied a spatial version of the orchard problem, proving that, roughly speaking, all configurations of finitely many points on a smooth cubic surface S which admit quadratically many triple lines must concentrate on an intersection of S with a plane.

I will then outline a (closely related, though not obviously at a first glance) work in progress in which I attempt to classify correspondences between complex algebraic groups for which the generalised sum-product phenomenon holds. In dimension one, by the results of Bays and Breuillard, this is known to hold if and only if the groups in question are not isogenous. In the general case, the notion of weak general position, used in our work on the orchard problem described above, seems to provide a promising strategy for tackling the problem.

### September 20

Location: HGW2403

Speaker: FANG, Nan (方楠)

Time: 13:30 - 15:10

#### Array Noncomputability: A Modular Approach

In the study of c.e. Turing degrees, array noncomputable degrees are those in which the multiple permitting argument can be effectively applied. Typically, in such arguments, a finite fragment of the desired set is constructed for a single requirement, during which multiple permissions are requested. By treating the construction for a single requirement as a module, a set that satisfies all requirements can be built by appropriately arranging and combining these modules. In this talk, we introduce a new perspective and tool for handling array noncomputable degrees, allowing for a more natural modularization of the multiple permitting construction. Additionally, we extend the concept of array noncomputability from c.e. sets to left-c.e. reals. We will present examples to illustrate how this new approach can simplify proofs involving array noncomputable degrees. If time permits, we will further explore extensions to uniformly multiple permitting and non-totally \omega-c.e. degrees. Slides.

### July 5

Location: HGW2403

Speaker: Krzysztof Krupiński

Time: 13:30 - 14:30

#### Revised Newelski's conjecture and the convolution semigroup of finitely satisfiable Keisler measures on a definable group

Let $G$ be a 0-definable group in a model $M$ of a NIP theory, and $N \succ M$ be $|M|^+$-saturated. On the space $S_{G,M}(N)$ of all complete types over $N$ concentrated on $G$ and finitely satisfiable in $M$, there is a natural left continuous semigroup operation $*$. By Ellis theorem, there is always a minimal left ideal $I$ in this semigroup and an idempotent $u \in I$, and then $uI$ is a group whose isomorphism type does not depend on the choice of $I$ and $u$. We call it the ideal (or Ellis) group of $S_{G,M}(N)$. Newelski's conjecture predicted that a certain natural epimorphism $f$ from $uI$ to $G(U)/G(U)^{00}_M$ is an isomorphism, where $U \succ N$ is a monster model. This conjecture is known to hold for definably amenable groups in NIP theories, but fails in general for NIP theories.

The ideal group $uI$ is equipped with the so-called $\tau$-topology which makes it a separately continuous, quasi-compact, $T_1$ group. The map $f$ factors through the maximal Hausdorff quotient of $uI$, which lead Anand Pillay and myself to the weakening of Newelski's conjecture saying that under NIP, $uI$ is Hausdorff in the $\tau$-topology.

I will discuss a proof of this revised Newelski's conjecture under the assumption that $M$ is countable. This is based on the theorem of Glasner describing the structure of tame, metrizable, minimal flows.

There is a natural extension of the semigroup operation $*$ from $S_{G,M}(N)$ to the space $M_{G,M}(N)$ of Keisler measures over $N$ concentrated on $G$ and finitely satisfiable in $M$, called the convolution product, which was intensively studied in earlier papers by Chernikov and Gannon. Assuming that $M$ is countable (and NIP), we provide an explicit construction of a minimal left ideal in the convolution semigroup $M_{G,M}(N)$ from a minimal left ideal in $S_{G,M}(N)$ and the unique Haar measure on the ideal group of $S_{G,M}(N)$ which exists by the above revised Newelski's conjecture for countable $M$.

This is joint work with Artem Chernikov and Kyle Gannon.

### June 7

Location: HGW2403

Speaker: Daisuke IKEGAMI (池上 大祐)

Time: 13:30 - 15:00

#### Preservation of AD via forcings

The Axiom of Determinacy (AD) has been investigated extensively especially on its connections with descriptive set theory, large cardinals, and inner model theory. Forcing over models of AD produces various models of ZFC together with some fragments of forcing axioms as well as failures of squares. In this talk, we focus on the question what kind of forcings preserve the truth of AD, and we present several results regarding this question. Using our main result, we produce a model of ZF + AD where Mouse Capturing fails, where Mouse Capturing states that for all reals $x$ and $y$, $x$ is ordinal definable from $y$ if and only if $x$ is in some mouse over $y$. This is joint work with Nam Trang.

Speaker: Liping TANG (唐丽萍)

Time: 15:15 - 16:45

#### A Communication Game for Linguistic Politeness in Social Networks

From the viewpoint of information transaction models in linguistic pragmatics, expressions of linguistic politeness (LP) induce costs upon speakers. That speakers regularly “pay” such cost is what formal models of LP typically explain either by individual-level strategic considerations (e.g., the speaker's aim of avoiding a face-threat to the hearer) or community-level conventional considerations (e.g., the use of LP as a relation-acknowledging device). Because these explanations are compatible, as each relates to the speaker and hearer's social relation, we combine them into a single game-theoretical model enriched by three types of social network structures (ring-shaped, star-shaped, and complete). Using simulation studies of (single and repeated) speech acts of requesting, we let the degree of LP be determined by (i) the degree of social imposition associated with a request, (ii) the number of interlocutors' past interactions, and (iii) the relative importance of strategic and conventional considerations. The greatest average optimal degree of LP is obtained in the star-shaped network, which intuitively corresponds to a power-centered, hierarchical society.

### May 14

Speaker: Thomas Scanlon

Time: 13:30 - 15:30. Location: HGW2301.

#### (Un)decidability in function fields

Starting with J. Robinson's 1949 proof of the undecidability of the first-order theory of the field of rational numbers through a reduction to Gödel's first incompleteness theorem, various other theories of fields have been shown to be undecidable. On the other hand, A. Tarski's 1929 theorem on the decidability of Euclidean geometry is really a theorem on the decidability of the first-order theory of the field of real numbers. Theories of various other fields, including $p$-adic fields, fields of formal power series over decidable fields of characteristic zero, and pseudofinite fields, amongst others, have been shown to be decidable. It is a long standing open problem, appearing in print in 1963, whether the theory of the field $\mathbb{C}(t)$ of rational functions in one variable over the complex numbers is decidable.

In this lecture, I will describe some attempts to resolve this problem. In particular, I will discuss some methods using the theory of elliptic curves to define complicated structures in this field, but then also why those structures are likely decidable in their own right. I will also discuss other approaches to the decidability problem based on the theories of algebraic curves and more precisely an attempt to encode finite sets using families of hyperelliptic curves. Finally, I will speculate about the connections between this problem and the theory of rational curves on algebraic varieties.

### April 30

Location: HGW2403.

Speaker: Bokai YAO (姚博凯)

Time: 13:30 - 15:30

#### 什么是无素集合论？

在传统的集合论框架中，一切皆为集合。所谓“无素”（urelements）是指那些非集合的元素。若在集合论宇宙中引入无素，其对集合论公理系统会产生何种影响？这些公理与无素的数量是否存在某种联系？选择公理在其中扮演怎样的角色？最后，一个涵盖所有存在的集合论宇宙背后的哲学图景又是什么？本报告将通过介绍无素集合论的基本概念及方法，对这些问题进行探讨。handout

Speaker: Pierre-Louis Curien

Time: 15:30 - 17:30

#### Combinatorics and geometry of opetopes

Opetopes were introduced by Baez and Dolan at the end of the last millenium as a foundational tool for higher-dimensional algebra. They are shapes featuring associativity, coherence of asscoiativity, coherence of coherence of associativity, etc., in an unbiased setting. Here, unbiased refers to a product of varying arity, as opposed to a biased, binary one. While opetopes can be defined in a compact way by means of categorical abstractions (e.g. via polynomial functors as introduced by Joyal and Gambino), they are difficult to grasp and characterise combinatorially or syntactically. I shall report on work of Louise Leclerc (some of which joint with me) on proving equivalences of different definitions of opetopes. I shall also sketch how opetopes may label a suitable triangular subdivision of the family of associahedra, that refines the historically important Boardman-Vogt cubical subdivision (work in progress).

### April 26

Location: HGW2403. Tencent Meeting: 711-524-519, code: 666666

Speaker: Jason CHEN (陈泽晟)

Time: 13:30 - 15:30

#### Metamathematical methods in descriptive set theory: a case study in the taxonomy of proofs

This project has a two-fold goal: on the practical side, to provide a rough collection and classification of the various types of proofs in descriptive set theory that can be characterized by their use of metamathematical methods. In doing so, I also aim to track the degree of involvement of metamathematical tools in these proofs, and hope to demonstrate that there is a sui generis methodology, in full analogy with, say, "algebraic methods". And on the philosophical side, the goal is to probe perhaps more general questions about the nature proofs and methods, particularly regarding what it means to characterize a proof by the method used, and what a method imports in a proof.

To do this, I will first motivate this project a little by relating it to a broader philosophical project born out of Dawson's monograph "Why Prove it Again? Alternative Proofs in Mathematical Practice" about the values of having multiple proofs. Next, I will consider an obvious non-example of metamathematical proof (that makes use of metamathematical methods), which will allow me to sketch a list of putative objections to calling such proofs metamathematical. These putative objections will shed light on the practice of organizing mathematical proofs by their methodology. Following that, I will present a series theorems and proofs that involve increasingly substantial use of metamathematical methods. At each turn, I will analyze how the proof in question addresses the earlier objections, honing in on the issue of whether the metamathematical methods can be translated away without loss of insight. This sort of dialectics will hopefully shed light on our practical taxonomy of proofs by their methodology, as well as on the specific question of whether a proof can be said to make substantial use of metamathematical methods. Slides

### April 24

Location: HGW2501. Tencent Meeting: 459-205-619, code: 666666

Speaker: Jason CHEN (陈泽晟)

Time: 15:30 - 17:30

#### The emergence of invariant descriptive set theory

The common story of the history of descriptive set theory tells of Cantor's ingenious venture into the transfinite, how his work found palpable expression with the French analysts, the ill-fated program of the Moscow school to understand the projective hierarchy, and its marvellous revival in the west with large cardinals and determinacy. Nevertheless, a cursory glance at some of the recent publications in generalist mathematics journal such as the Annals of Mathematics and Inventiones Mathematicae reveals that the field has been making remarkable contributions to a wide range of mathematical areas, including ergodic theory, functional analysis, measure theory, and graph combinatorics, with techniques and concerns that have little motivation in ways of classical concerns such as the continuum hypothesis or large cardinals.

The present project is a partial attempt to fill this obvious gap in the literature, aiming to trace the pre-history of the rich theory, developed in the past 30 years, of invariant descriptive set theory (better known as the theory of Borel or definable equivalence relations). My goal is to shed light on a host of questions that naturally arise, for example: is there a clear historical narrative of the development of the field? What was in the air mathematically from the Moschow school's study of the projective sets to the rise of definable equivalence relations? When did descriptive set theorists turn to abstract structures among classification problems, rather than working on the concrete classification problems themselves? What technical tools were available when this turn took place, and where did they originate?

One pleasant finding is that the field of Borel equivalence relations is undergirded by two separate threads of development: on the one hand, the development of abstract techniques establishing non-reductions, motivated by Luzin and his complexity-regularity program, as well as Sierpinski's effort to ratify the axiom of choice into mathematical canon; on the other hand, the development of the actual reductions, motivated by non-set-theoretic concerns such as ergodic theory and the theory of Polish groups. As an interlude, we will also briefly touch upon the role that the infamous Luzin Affair played in this history. Slides

### April 23

Location: HGW2403

Speaker: Baptiste Mélès

Time: 13:30 - 15:30

#### The rationale of axioms: Peano's arithmetic in Coq's standard library

In logic and mathematics, axioms provide the rationale for theorems, but they obviously do not need to have a rationale themselves. However, some logicians and mathematicians attempted to justify each axiom on the basis of the global properties of the logic or the theory they intended to construct. Starting with Gentzen and the requirement for cut-elimination, I will show that this requirement is now the basis of the Coq proof assistant. Due to global properties of Coq's logic, the number of Peano's axioms can be reduced from 5 to 2. As a result, some axiomatics are governed by a more demanding structure than a simple "aggregate" of axioms.

### April 2

Location: HGW2403

Speaker: Yuan LI (李元)

Time: 13:30 - 15:30

#### On the Minimum Depth of Circuits with Linear Number of Wires Encoding Good Codes

Let $S_d(n)$ denote the minimum number of wires of a depth-$d$ (unbounded fan-in) circuit encoding an error-correcting code $C:\{0, 1\}^n \to \{0, 1\}^{32n}$ with distance at least $4n$. Gál, Hansen, Koucký, Pudlák, and Viola [IEEE Trans. Inform. Theory 59(10), 2013] proved that $S_d(n) = \Theta_d(\lambda_d(n)\cdot n)$ for any fixed $d \ge 3$. By improving their construction and analysis, we prove $S_d(n)= O(\lambda_d(n)\cdot n)$. Letting $d = \alpha(n)$, a version of the inverse Ackermann function, we obtain circuits of linear size. This depth $\alpha(n)$ is the minimum possible to within an additive constant 2; we credit the nearly-matching depth lower bound to Gál et al., since it directly follows their method (although not explicitly claimed or fully verified in that work), and is obtained by making some constants explicit in a graph-theoretic lemma of Pudlák [Combinatorica, 14(2), 1994], extending it to super-constant depths.

We also study a subclass of MDS codes $C: \mathbb{F}^n \to \mathbb{F}^m$ characterized by the Hamming-distance relation $\textrm{dist}(C(x), C(y)) \ge m - \textrm{dist}(x, y) + 1$ for any distinct $x, y \in \mathbb{F}^n$. (For linear codes this is equivalent to the generator matrix being totally invertible.) We call these superconcentrator-induced codes, and we show their tight connection with superconcentrators. Specifically, we observe that any linear or nonlinear circuit encoding a superconcentrator-induced code must be a superconcentrator graph, and any superconcentrator graph can be converted to a linear circuit, over a sufficiently large field (exponential in the size of the graph), encoding a superconcentrator-induced code.

This is a joint work with Andrew Drucker.