arXiv subjects

Will Johnson

Publications and source records attributed to Will Johnson.

At least 19 recordsLinked to original sources

arXiv2026

Weakly o-minimal fields have the exchange property but not generic differentiability

We answer two open questions about weakly o-minimal fields posed by Macpherson, Marker, and Steinhorn: whether weakly o-minimal fields have the exchange property and whether they have generic differentiability. We construct an ordered field $(K,+,\cdot,\le)$ and a function $f : K \to K$ such that the expansion $(K,+,\cdot,\le,f)$ has a weakly o-minimal complete theory but $f$ is nowhere differentiable. In an appendix, we prove that algebraic closure has the exchange property in any weakly o-minimal theory of ordered fields.

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arXiv2026

Definable groups and fields in t-minimal theories

Let $T$ be a theory which is t-minimal, meaning that with respect to some definable topology, a unary definable set $D \subseteq M$ has non-empty interior iff it is infinite. If $K$ is a definable field in $T$, then $K$ is finite or "large" in the sense of Pop: any smooth algebraic curve $C$ over $K$ with at least one $K$-rational point has infinitely many $K$-rational points. We also assign a canonical topology to any abelian definable group $G$ in a t-minimal theory. In the case where the t-minimal theory is "visceral" in the sense of Dolich and Goodrick, meaning that the definable topology is induced by a definable uniformity, we can drop the assumption of abelianity of $G$, and the resulting topology on $G$ is a definable manifold in the style of Acosta L\'opez and Hasson.

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arXiv2025

Topologically 1-based T-minimal Structures

We prove group existence and structure theorems in a general setting of tame topological theories. More precisely, we identify a linear/non-linear dividing line -- called topological 1-basedness -- among the class of t-minimal theories with the independent neighborhood property. This is a wide class including all visceral theories, as well as all dense weakly o-minimal and C-minimal theories (even those where exchange fails). Now assume $\mathcal M$ is highly saturated and t-minimal with the independent neighborhood property. We show that if $\mathcal M$ is non-trivial and topologically 1-based, it admits a type-definable abelian group $(G,+)$ with $G$ an open subset of $M$. Moreover, we can ensure that $G$ is a topological group with the subspace topology inherited from $M$; and in this case, we show that the induced structure on $G$ satisfies an appropriate topological analog of the Hrushovski-Pillay classification of 1-based stable groups.

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arXiv2025

Largeness and generalized t-henselianity

Let $K$ be a countable field. Then $K$ is large in the sense of Pop if and only if it admits a field topology which is "generalized t-henselian" (gt-henselian) in the sense of Dittmann, Walsberg, and Ye, meaning that the implicit function theorem holds for polynomials. Moreover, the \'etale open topology can be characterized in terms of the gt-henselian topologies on $K$: a subset $U \subseteq K^n$ is open in the \'etale open topology if and only if it is open with respect to every gt-henselian topology on $K$.

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arXiv2025

Large implies henselian

Fix a field $K$. We show that $K$ is large if and only if some elementary extension of $K$ is the fraction field of a henselian local domain which is not a field. The proof uses a new result about the \'etale-open topology over $K$: if $K$ is not separably closed and $V \to W$ is an \'etale morphism of $K$-varieties then $V(K) \to W(K)$ is a local homeomorphism in the \'etale-open topology. This, in turn, follows from results comparing the \'etale-open topology on $V(K)$ and the finite-closed topology on $V(K)$, newly introduced in this paper. We show that the \'etale-open topology refines the finite-closed topology when $K$ is perfect, and that the finite-closed topology refines the \'etale-open topology when $K$ is bounded. It follows that these two topologies agree in many natural examples. On the other hand, we construct several examples where these two differ, which allows us to answer a question of Lampe.

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arXiv2025

A note on one-variable theorems for NSOP

We give an example of an SOP theory $T$, such that any $L(M)$-formula $\varphi(x,y)$ with $|y|=1$ is NSOP. We show that any such $T$ must have the independence property. We also give a simplified proof of Lachlan's theorem that if every $L$-formula $\varphi(x,y)$ with $|x|=1$ is NSOP, then $T$ is NSOP.

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arXiv2025

Translating between NIP integral domains and topological fields

We prove that definable ring topologies on NIP fields are closely connected to NIP integral domains. More precisely, we show that up to elementary equivalence, any NIP topological field arises from an NIP integral domain. As an application, we prove several results about definable ring topologies on NIP fields, including the following. Let $K$ be an NIP field or expansion of a field. Let $\tau$ be a definable ring topology on $K$. Then $\tau$ is a field topology, and $\tau$ is locally bounded. If $K$ has characteristic $p$ or finite dp-rank, then $\tau$ is "generalized t-henselian" in the sense of Dittman, Walsberg, and Ye, meaning that the implicit function theorem holds for polynomials. If $K$ has finite dp-rank, then $\tau$ must be a topology of "finite breadth" (a $W_n$-topology). Using these techniques, we give some reformulations of the conjecture that NIP local rings are henselian.

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arXiv2024

The classification of dp-minimal integral domains

We classify dp-minimal integral domains, building off the existing classification of dp-minimal fields and dp-minimal valuation rings. We show that if R is a dp-minimal integral domain, then R is a field or a valuation ring or arises from the following construction: there is a dp-minimal valuation overring O extending R, a proper ideal I in O, and a finite subring S in O/I such that R is the preimage of S in O.

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arXiv2024

Generic differentiability and $P$-minimal groups

We prove generic differentiability in $P$-minimal theories, strengthening an earlier result of Kuijpers and Leenknegt. Using this, we prove Onshuus and Pillay's $P$-minimal analogue of Pillay's conjectures on o-minimal groups. Specifically, let $G$ be an $n$-dimensional definable group in a highly saturated model $M$ of a $P$-minimal theory. Then there is an open definable subgroup $H \subseteq G$ such that $H$ is compactly dominated by $H/H^{00}$, and $H/H^{00}$ is a $p$-adic Lie group of the expected dimension. Additionally, the generic differentiability theorem immediately implies a classification of interpretable fields in $P$-minimal theories, by work of Halevi, Hasson, and Peterzil.

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arXiv2024

Visceral theories without assumptions

Let $T$ be a theory with a definable topology. $T$ is t-minimal in the sense of Mathews if every definable set in one variable has finite boundary. If $T$ is t-minimal, we show that there is a good dimension theory for definable sets, satisfying properties similar to dp-rank in dp-minimal theories, with one key exception: the dimension of $\operatorname{dom}(f)$ can be less than the dimension of $\operatorname{im}(f)$ for a definable function $f$. Using the dimension theory, we show that any definable field in a t-minimal theory is perfect. We then specialize to the case where $T$ is visceral in the sense of Dolich and Goodrick, meaning that $T$ is t-minimal and the definable topology comes from a definable uniformity (i.e., a definable uniform structure). We show that almost all of Dolich and Goodrick's tame topology theorems for visceral theories hold without their additional assumptions of definable finite choice (DFC) and no space-filling functions (NSFF). Lastly, we produce an example of a visceral theory with a space-filling curve, answering a question of Dolich and Goodrick.

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arXiv2024

C-minimal fields have the exchange property

We show that C-minimal fields (i.e., C-minimal expansions of ACVF) have the exchange property, answering a question of Haskell and Macpherson. Additionally, we strengthen some theorems of Cubides Kovacsics and Delon on C-minimal fields. First, we show that definably complete C-minimal fields of characteristic 0 have generic differentiability. Second, we show that if the induced structure on the residue field is a pure ACF, then polynomial boundedness holds. In fact, polynomial boundedness can only fail if there are unexpected definable automorphisms of the multiplicative group of the residue field.

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arXiv2023

One-dimensional subgroups and connected components in non-abelian $p$-adic definable groups

We generalize two of our previous results on abelian definable groups in $p$-adically closed fields to the non-abelian case. First, we show that if $G$ is a definable group that is not definably compact, then $G$ has a one-dimensional definable subgroup which is not definably compact. This is a $p$-adic analogue of the Peterzil-Steinhorn theorem for o-minimal theories. Second, we show that if $G$ is a group definable over the standard model $\mathbb{Q}_p$, then $G^0 = G^{00}$. As an application, definably amenable groups over $\mathbb{Q}_p$ are open subgroups of algebraic groups, up to finite factors. We also prove that $G^0 = G^{00}$ when $G$ is a definable subgroup of a linear algebraic group, over any model.

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arXiv2023

Curve-excluding fields

If $C$ is a curve over $\mathbb{Q}$ with genus at least $2$ and $C(\mathbb{Q})$ is empty, then the class of fields $K$ of characteristic 0 such that $C(K) = \varnothing$ has a model companion, which we call $C\mathrm{XF}$. The theory $C\mathrm{XF}$ is not complete, but we characterize the completions. Using $C\mathrm{XF}$, we produce examples of fields with interesting combinations of properties. For example, we produce (1) a model-complete field with unbounded Galois group, (2) an infinite field with a decidable first-order theory that is not ``large'' in the sense of Pop, (3) a field that is algebraically bounded but not ``very slim'' in the sense of Junker and Koenigsmann, and (4) a pure field that is strictly NSOP$_4$, i.e., NSOP$_4$ but not NSOP$_3$. Lastly, we give a new construction of fields that are virtually large but not large.

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arXiv2023

Dp-finite and Noetherian NIP integral domains

We prove some results on NIP integral domains, especially those that are Noetherian or have finite dp-rank. If $R$ is an NIP Noetherian domain that is not a field, then $R$ is a semilocal ring of Krull dimension 1, and the fraction field of $R$ has characteristic 0. Assuming the henselianity conjecture (on NIP valued fields), $R$ is a henselian local ring. Additionally, we show that integral domains of finite dp-rank are henselian local rings. Finally, we lay some groundwork for the study of Noetherian domains of finite dp-rank, and we classify dp-minimal Noetherian domains.

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arXiv2022

Around definable types in $p$-adically closed fields

We prove some technical results on definable types in $p$-adically closed fields, with consequences for definable groups and definable topological spaces. First, the code of a definable $n$-type (in the field sort) can be taken to be a real tuple (in the field sort) rather than an imaginary tuple (in the geometric sorts). Second, any definable type in the real or imaginary sorts is generated by a countable union of chains parameterized by the value group. Third, if $X$ is an interpretable set, then the space of global definable types on $X$ is strictly pro-interpretable, building off work of Cubides Kovacsics, Hils, and Ye. Fourth, global definable types can be lifted (in a non-canonical way) along interpretable surjections. Fifth, if $G$ is a definable group with definable f-generics ($dfg$), and $G$ acts on a definable set $X$, then the quotient space $X/G$ is definable, not just interpretable. This explains some phenomena observed by Pillay and Yao. Lastly, we show that interpretable topological spaces satisfy analogues of first-countability and curve selection. Using this, we show that all reasonable notions of definable compactness agree on interpretable topological spaces, and that definable compactness is definable in families.

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arXiv2022

A note on geometric theories of fields

Let $T$ be a complete theory of fields, possibly with extra structure. Suppose that model-theoretic algebraic closure agrees with field-theoretic algebraic closure, or more generally that model-theoretic algebraic closure has the exchange property. Then $T$ has uniform finiteness, or equivalently, it eliminates the quantifier $\exists^\infty$. It follows that very slim fields in the sense of Junker and Koenigsmann are the same thing as geometric fields in the sense of Hrushovski and Pillay. Modulo some fine print, these two concepts are also equivalent to algebraically bounded fields in the sense of van den Dries. From the proof, one gets a one-cardinal theorem for geometric theories of fields: any infinite definable set has the same cardinality as the field. We investigate whether this extends to interpretable sets. We show that positive dimensional interpretable sets must have the same cardinality as the field, but zero-dimensional interpretable sets can have smaller cardinality. As an application, we show that any geometric theory of fields has an uncountable model with only countably many finite algebraic extensions.

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arXiv2022

Abelian groups definable in $p$-adically closed fields

Recall that a group $G$ has finitely satisfiable generics ($fsg$) or definable $f$-generics ($dfg$) if there is a global type $p$ on $G$ and a small model $M_0$ such that every left translate of $p$ is finitely satisfiable in $M_0$ or definable over $M_0$, respectively. We show that any abelian group definable in a $p$-adically closed field is an extension of a definably compact $fsg$ definable group by a $dfg$ definable group. We discuss an approach which might prove a similar statement for interpretable abelian groups. In the case where $G$ is an abelian group definable in the standard model $\mathbb{Q}_p$, we show that $G^0 = G^{00}$, and that $G$ is an open subgroup of an algebraic group, up to finite factors. This latter result can be seen as a rough classification of abelian definable groups in $\mathbb{Q}_p$.

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arXiv2022

Topologizing interpretable groups in $p$-adically closed fields

We consider interpretable topological spaces and topological groups in a $p$-adically closed field $K$. We identify a special class of "admissible topologies" with topological tameness properties like generic continuity, similar to the topology on definable subsets of $K^n$. We show every interpretable set has at least one admissible topology, and every interpretable group has a unique admissible group topology. We then consider definable compactness (in the sense of Fornasiero) on interpretable groups. We show that an interpretable group is definably compact if and only if it has finitely satisfiable generics (fsg), generalizing an earlier result on definable groups. As a consequence, we see that fsg is a definable property in definable families of interpretable groups, and that any fsg interpretable group defined over $\mathbb{Q}_p$ is definably isomorphic to a definable group.

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