A new dp-minimal expansion of the integers

We consider the structure $(\mathbb{Z},+,0,|_{p_{1}},\dots,|_{p_{n}})$, where $x|_{p}y$ means $v_{p}(x)\leq v_{p}(y)$ and $v_p$ is the $p$-adic valuation. We prove that its theory has quantifier elimination in the language $\{+,-,0,1,(D_{m})_{m\geq1},|_{p_{1}},\dots,|_{p_{n}}\}$ where $D_m(x)\leftrightarrow \exists y ~ my = x$, and that it has dp-rank $n$. In addition, we prove that a first order structure with universe $\mathbb{Z}$ which is an expansion of $(\mathbb{Z},+,0)$ and a reduct of $(\mathbb{Z},+,0,|_{p})$ must be interdefinable with one of them. We also give an alternative proof for Conant's analogous result about $(\mathbb{Z},+,0,<)$.Comment: 24 page

Axiomatic Theory of Independence Relations in Model Theory

This course introduces the fruitful links between model theory and a combinatoric of sets given by independence relations. An independence relation on a set is a ternary relation between subsets. Chapter 1 should be considered as an introductory chapter. It does not mention first-order theories or formulas. It introduces independence relations in a naive set theory framework. Its goal is to get the reader familiar with basic axioms of independence relations (which do not need an ambient theory to be stated) as well as introduce closure operators and pregeometries. Chapter 2 introduces the model-theoretic context. The two main examples (algebraically closed fields and the random graph) are described as well as independence relations in those examples. Chapter 3 gives the axioms of independence relations in a model-theoretic context. It introduces the general toolbox of the model-theorists (indiscernible sequences, Ramsey/Erdos-Rado and compactness) and the independence relations of heirs/coheirs with two main applications: Adler's theorem of symmetry (how symmetry emerges from a weaker set of axioms, which is rooted in the work of Kim and Pillay) and a criterion for NSOP4 using stationary independence relations in the style of Conant. Independence relations satisfying Adler's theorem of symmetry are here called 'Adler independence relations' or AIR. Chapter 4 treats forking and dividing. It is proved that dividing independence is always stronger than any AIR (even though it is not an AIR in general) a connection between the independence theorem and forking independence, which holds in all generality and is based on Kim-Pillay's approach. Then, simplicity is defined and the interesting direction of the Kim-Pillay theorem (namely that the existence of an Adler independence relation satisfying the independence theorem yields simplicity) is deduced from earlier results.Comment: 53 page

Generic multiplicative endomorphism of a field

We introduce the model-companion of the theory of fields expanded by a unary function for a multiplicative map, which we call ACFH. Among others, we prove that this theory is NSOP$_1$ and not simple, that the kernel of the map is a generic pseudo-finite abelian group. We also prove that if forking satisfies existence, then ACFH has elimination of imaginaries.Comment: 34 page

The Ax-Kochen-Ershov Theorem

These are the notes of a course for the summer school Model Theory in Bilbao hosted by the Basque Center for Applied Mathematics (BCAM) and the Universidad del Pa\'is Vasco/Euskal Herriko Unibertsitatea in September 2023. The goal of this course is to prove the Ax-Kochen-Ershov (AKE) theorem. This classical result in model theory was proven by Ax and Kochen and independently by Ershov in 1965-1966. The AKE theorem is considered as the starting point of the model theory of valued fields and witnessed numerous refinements and extensions. To a certain measure, motivic integration can be considered as such. The AKE theorem is not only an important result in model theory, it yields a striking application to $p$-adic arithmetics. Artin conjectured that all $p$-adic fields are $C_2$ (every homogeneous polynomial of degree $d$ and in $>d^2$ variable has a non trivial zero). A consequence of the AKE theorem is that the $p$-adics are asymptotically $C_2$. The conjecture of Artin has been disproved by Terjanian in 1966, yielding that the solution given by the AKE theorem is in a sense optimal. The proof presented here is due to Pas but the general strategy stays faithful to the original paper of Ax and Kochen, which consist in the study of the asymptotic first-order theory of the $p$-adics.Comment: 31 page

Note on a bomb dropped by Mr Conant and Mr Kruckman, and its consequences for the theory ACFG

This note is a reaction to Conant and Kruckman's recent preprint `Three surprising instance of dividing'. It mainly consists of an erratum to the author's paper `Forking, imaginaries and other features of ACFG', in light of the results of the aforementioned paper.Comment: 9 page

Existentially closed models of fields with a distinguished submodule

This paper deals with the class of existentially closed models of fields with a distinguished submodule (over a fixed subring). In the positive characteristic case, this class is elementary and was investigated by the first-named author. Here we study this class in Robinson's logic, meaning the category of existentially closed models with embeddings following Haykazyan and Kirby, and prove that in this context this class is NSOP$_1$ and TP$_2$