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Erik Walsberg

Publications and source records attributed to Erik Walsberg.

At least 37 records · Page 2Linked to original sources
arXiv2021

A tetrachotomy for expansions of the real ordered additive group

Let $\mathcal{R}$ be an expansion of the ordered real additive group. When $\mathcal{R}$ is o-minimal, it is known that either $\mathcal{R}$ defines an ordered field isomorphic to $(\mathbb{R},<,+,\cdot)$ on some open subinterval $I\subseteq \mathbb{R}$, or $\mathcal{R}$ is a reduct of an ordered vector space. We say $\mathcal{R}$ is field-type if it satisfies the former condition. In this paper, we prove a more general result for arbitrary expansions of $(\mathbb{R},<,+)$. In particular, we show that for expansions that do not define dense $ω$-orders (we call these type A expansions), an appropriate version of Zilber's principle holds. Among other things we conclude that in a type A expansion that is not field-type, every continuous definable function $[0,1]^m \to \mathbb{R}^n$ is locally affine outside a nowhere dense set.

math.LO↗
arXiv2021

Directed sets and topological spaces definable in o-minimal structures

We study directed sets definable in o-minimal structures, showing that in expansions of ordered fields these admit cofinal definable curves, as well as a suitable analogue in expansions of ordered groups, and furthermore that no analogue holds in full generality. We use the theory of tame pairs to extend the results in the field case to definable families of sets with the finite intersection property. We then apply our results to the study of definable topologies. We prove that all definable topological spaces display properties akin to first countability, and give several characterizations of a notion of definable compactness due to Peterzil and Steinhorn generalized to this setting.

math.LO↗
arXiv2020

Dp-minimal expansions of discrete ordered abelian groups

If $\mathcal{Z}$ is a dp-minimal expansion of a discrete ordered abelian group $(Z,<,+)$ and $\mathcal{Z}$ does not admit a nontrivial definable convex subgroup then $\mathcal{Z}$ is interdefinable with $(Z,<,+)$ and $(Z,<,+)$ is elementarily equivalent to $(\mathbb{Z},<,+)$.

math.LO↗
arXiv2020

Nippy proofs of p-adic results of Delon and Yao

Let $K$ be an elementary extension of $\mathbb{Q}_p$, $V$ be the set of finite $a \in K$, $\mathrm{st}$ be the standard part map $K^m \to \mathbb{Q}^m_p$, and $X \subseteq K^m$ be $K$-definable. Delon has shown that $\mathbb{Q}^m_p \cap X$ is $\mathbb{Q}_p$-definable. Yao has shown that $\dim \mathbb{Q}^m_p \cap X \leq \dim X$ and $\dim \mathrm{st}(V^n \cap X) \leq \dim X$. We give new $\mathrm{NIP}$-theoretic proofs of these results and show that both inequalities hold in much more general settings. We also prove the analogous results for the expansion $\mathbb{Q}^{\mathrm{an}}_p$ of $\mathbb{Q}_p$ by all analytic functions $\mathbb{Z}^n_p \to \mathbb{Q}_p$. As an application we show that if $(X_k)_{k \in \mathbb{N}}$ is a sequence of elements of an $\mathbb{Q}^{\mathrm{an}}_p$-definable family of subsets of $\mathbb{Q}^m_p$ which converges in the Hausdroff topology to $X \subseteq \mathbb{Q}^m_p$ then $X$ is $\mathbb{Q}^{\mathrm{an}}_p$-definable and $\dim X \leq \limsup_{k \to \infty} \dim X_k$.

math.LO↗
arXiv2020

Dp-minimal expansions of $(\mathbb{Z},+)$ via dense pairs via Mordell-Lang

This is a contribution to the classification problem for dp-minimal expansions of $(\mathbb{Z},+)$. Let $S$ be a dense cyclic group order on $(\mathbb{Z},+)$. We use results on "dense pairs" to construct uncountably many dp-minimal expansions of $(\mathbb{Z},+,S)$. These constructions are applications of the Mordell-Lang conjecture and are the first examples of "non-modular" dp-minimal expansions of $(\mathbb{Z},+)$. We canonically associate an o-minimal expansion $\mathcal{R}$ of $(\mathbb{R},+,\times)$, an $\mathcal{R}$-definable circle group $\mathbb{H}$, and a character $\mathbb{Z} \to \mathbb{H}$ to a "non-modular" dp-minimal expansion of $(\mathbb{Z},+,S)$. We also construct a "non-modular" dp-minimal expansion of $(\mathbb{Z},+,\mathrm{Val}_p)$ from the character $\mathbb{Z} \to \mathbb{Z}^\times_p$, $k \mapsto \mathrm{exp}(pk)$.

math.LO↗
arXiv2020

Externally definable quotients and NIP expansions of the real ordered additive group

Let $\mathcal{R}$ be an $\mathrm{NIP}$ expansion of $(\mathbb{R},<,+)$ by closed subsets of $\mathbb{R}^n$ and continuous functions $f : \mathbb{R}^m \to \mathbb{R}^n$. Then $\mathcal{R}$ is generically locally o-minimal. It follows that if $X \subseteq \mathbb{R}^n$ is definable in $\mathcal{R}$ then the $C^k$-points of $X$ are dense in $X$ for any $k \geq 0$. This follows from a more general theorem on $\mathrm{NIP}$ expansions of locally compact groups, which itself follows from a result on quotients of definable sets by equivalence relations which are externally definable and $\bigwedge$-definable. We also show that $\mathcal{R}$ is strongly dependent if and only if $\mathcal{R}$ is either o-minimal or $(\mathbb{R},<,+,α\mathbb{Z})$-minimal for some $α> 0$.

math.LO↗
arXiv2020

Generalizing a theorem of Bès and Choffrut

Bès and Choffrut recently showed that there are no intermediate structures between $(\mathbb{R},<,+)$ and $(\mathbb{R},<,+,\mathbb{Z})$. We prove a generalization: if $\mathcal{R}$ is an o-minimal expansion of $(\mathbb{R},<,+)$ by bounded subsets of Euclidean space then there are no intermediate structures between $\mathcal{R}$ and $(\mathcal{R},\mathbb{Z})$. It follows there are no intermediate structures between $(\mathbb{R},<,+,\sin|_{[0,2π]})$ and $(\mathbb{R},<,+,\sin)$.

math.LO↗
arXiv2020

Continuous Regular Functions

Following Chaudhuri, Sankaranarayanan, and Vardi, we say that a function $f:[0,1] \to [0,1]$ is $r$-regular if there is a Büchi automaton that accepts precisely the set of base $r \in \mathbb{N}$ representations of elements of the graph of $f$. We show that a continuous $r$-regular function $f$ is locally affine away from a nowhere dense, Lebesgue null, subset of $[0,1]$. As a corollary we establish that every differentiable $r$-regular function is affine. It follows that checking whether an $r$-regular function is differentiable is in $\operatorname{PSPACE}$. Our proofs rely crucially on connections between automata theory and metric geometry developed by Charlier, Leroy, and Rigo.

cs.LO↗
arXiv2019

Coarse dimension and definable sets in expansions of the ordered real vector space

Suppose $E \subseteq \mathbb{R}$ is nowhere dense. If $(\mathbb{R},<,+,(x \mapsto λx)_{λ\in \mathbb{R} }, E)$ does not define every bounded Borel subset of every $\mathbb{R}^n$ then for every $s > 0$ we have $$ | \{ k \in \mathbb{Z}, -m \leq k \leq m - 1 : [k,k+1] \cap E \neq \emptyset \} | < m^s $$ for sufficiently large $m \in \mathbb{N}$. Then there is an $n \in \mathbb{N}$ and a linear $T : \mathbb{R}^n \to \mathbb{R}$ such that $T(E^n)$ is dense. It follows that if $E$ is in addition nowhere dense then $(\mathbb{R},<,+,0,(x \mapsto λx)_{λ\in \mathbb{R}}, E)$ defines every bounded Borel subset of every $\mathbb{R}^n$.

math.LO↗
arXiv2018

Expansions of the real field by discrete subgroups of Gl$_n(\mathbb{C})$

Let $Γ$ be an infinite discrete subgroup of Gl$_n(\mathbb{C})$. Then either $(\mathbb{R}, <, +, \cdot, Γ)$ is interdefinable with $(\mathbb{R}, <, +, \cdot, λ^\mathbb{Z})$ for some $λ\in \mathbb{R}$, or $(\mathbb{R}, < , +, \cdot, Γ)$ defines the set of integers. When $Γ$ is not virtually abelian, the second case holds.

math.LO↗
arXiv2017

Wild theories with o-minimal open core

Let $T$ be a consistent o-minimal theory extending the theory of densely ordered groups and let $T'$ be a consistent theory. Then there is a complete theory $T^*$ extending $T$ such that $T$ is an open core of $T^*$, but every model of $T^*$ interprets a model of $T'$. If $T'$ is NIP, $T^*$ can be chosen to be NIP as well. From this we deduce the existence of an NIP expansion of the real field that has no distal expansion.

math.LO↗
arXiv2017

How to avoid a compact set

A first-order expansion of the $\mathbb{R}$-vector space structure on $\mathbb{R}$ does not define every compact subset of every $\mathbb{R}^n$ if and only if topological and Hausdorff dimension coincide on all closed definable sets. Equivalently, if $A \subseteq \mathbb{R}^k$ is closed and the Hausdorff dimension of $A$ exceeds the topological dimension of $A$, then every compact subset of every $\mathbb{R}^n$ can be constructed from $A$ using finitely many boolean operations, cartesian products, and linear operations. The same statement fails when Hausdorff dimension is replaced by packing dimension.

math.LO↗
arXiv2016

Interpreting the monadic second order theory of one successor in expansions of the real line

We give sufficient conditions for a first order expansion of the real line to define the standard model of the monadic second order theory of one successor. Such an expansion does not satisfy any of the combinatorial tameness properties defined by Shelah, such as $\textrm{NIP}$ or even $\textrm{NTP}_2$. We use this to deduce the first general results about definable sets in $\textrm{NTP}_2$ expansions of $(\mathbb{R},<,+)$.

math.LO↗
arXiv2016

Isometric embeddings of snowflakes into finite-dimensional Banach spaces

We consider a general notion of snowflake of a metric space by composing the distance by a nontrivial concave function. We prove that a snowflake of a metric space $X$ isometrically embeds into some finite-dimensional normed space if and only if $X$ is finite. In the case of power functions we give a uniform bound on the cardinality of $X$ depending only on the power exponent and the dimension of the vector space.

math.MG↗