arXiv subjects

Jason Bell

Publications and source records attributed to Jason Bell.

At least 19 recordsLinked to original sources

arXiv2026

A Gaschütz Lemma for linear algebraic groups with applications to the Riemann-Hilbert correspondence

The classical Gaschütz Lemma states that for a surjective morphism $G\to H$ of finite groups, generators $h_1,\ldots,h_d$ of $H$ can be lifted to generators $g_1,\ldots,g_d$ of $G$, provided that $G$ can be generated by $d$ elements. We prove an analog of this statement for linear algebraic groups. This has applications to the algebraic-geometric theory of linear differential equations: We show that every regular singular differential embedding problem over the rational function field has a regular singular solution. We also establish a purely algebraic variant of the Riemann-Hilbert correspondence on the punctured projective line that works over any countable algebraically closed field of characteristic zero.

math.AG↗

arXiv2026

An undecidable problem in dynamics

Given a variety $X$ with a self-map $ϕ$ and subvarieties $Y$ and $Z$ of $X$, we consider the set of $n$ for which $ϕ^n(Y)$ intersects $Z$ nontrivially. Extending a beautiful construction of Lee and Nam, we show this gives rise to an undecidable problem for a fixed ambient variety $X$ and morphism $ϕ$. We further use this construction to give a counterexample to a conjecture of the author, Satriano, and Sierra.

math.DS↗

arXiv2026

Filtered deformations of three-variable polynomial algebras

We classify all filtered deformations of polynomial rings in three variables. We use our characterization to answer a special case of a question of Etingof by showing that if $F$ is an algebraically closed field of positive characteristic and $A$ is an $F$-algebra with a filtration by finite dimensional subspaces whose associated graded ring is a polynomial ring in three variables then $A$ satisfies a polynomial identity.

math.RA↗

arXiv2026

Wild automorphisms and compound isotriviality

Inspired by the model theory of difference fields in characteristic zero, a class of automorphisms of an algebraic variety, here called compound fundamental isotrivial, is introduced. These are algebraic dynamical systems that are built up via a finite sequence of equivariant fibrations from (possibly nonautonomous) algebraic dynamics which trivialise after base extension over themselves. Every wild automorphism of an abelian variety is compound fundamental isotrivial. Conversely, it is shown that the only irreducible projective varieties admitting a wild automorphism that is compound fundamental isotrivial are the abelian varieties. That is, the wild automorphism conjecture of Reichstein, Rogalski, and Zhang is here proven for compound fundamental isotrivial dynamics. Along the way, a counterexample to the naive generalisation of the conjecture to the nonautonomous setting of $σ$-varieties is provided.

math.AG↗

arXiv2026

Frequencies of subwords in words of linear subword complexity

Using a method of Balková--Pelantová, we show that if ${\bf w}$ is a right-infinite word over a finite alphabet, then for each nonnegative integer $N$ there are at most $3(p_{\bf w}(N+1)-p_{\bf w}(N))+1$ distinct upper (and likewise lower and ordinary when they exist) frequencies for length-$(N+1)$ subwords of ${\bf w}$, where $p_{\bf w}(n)$ is the subword complexity function of $n$. In particular, this gives a uniform upper bound when ${\bf w}$ has linearly bounded subword complexity. We provide examples showing that whenever $f(n)$ is a weakly increasing function tending to infinity, there is a word ${\bf w}$ such that the number of subwords of length $n$ is $O(nf(n))$ and for which the limit supremum of the number of distinct upper frequencies of length-$N$ subwords of ${\bf w}$ as $N\to\infty$ is infinite.

cs.FL↗

arXiv2026

A differential analogue of the wild automorphism conjecture

A differential analogue of the conjecture of Reichstein, Rogalski, and Zhang in algebraic dynamics is here established: if $X$ is a projective variety over an algebraically closed field of characteristic zero which admits a global algebraic vector field $v:X\to TX$ such that $(X,v)$ has no proper invariant subvarieties then $X$ is an abelian variety. Vector fields on abelian varieties with this property are also examined. Some of the analysis works in the more general context of $D$-varieties over differential fields: projective $D$-varieties without proper $D$-subvarieties are homogeneous. But the main theorem does not extend: an example of a $D$-variety structure on the projective line without proper $D$-subvarieties is given.

math.AG↗

arXiv2026

A Dichotomy for $k$-automatic expansions of Presburger Arithmetic

Let $k\ge 2$ and let $X$ be a subset of the natural numbers that is $k$-automatic and not eventually periodic. We show that the following dichotomy holds: either all $k$-automatic subsets are definable in the expansion of Presburger arithmetic in which we adjoin the predicate $X$, or $(\mathbb{N},+,X)$ has the same definable sets as $(\mathbb{N},+,k^{\mathbb{N}})$.

math.LO↗

arXiv2026

Mahler series with multiplicative coefficient sequences

We prove that every Mahler series, over a field of characteristic $0$, with multiplicative coefficients is regular in the sense of Allouche and Shallit. We also obtain an explicit characterization of such series. This yields a joint extension of the characterization of rational series with multiplicative coefficients (by Bézivin and Bell--Bruin--Coons) and of multiplicative automatic sequences (by Konieczny--Lemańczyk--Müllner). Both of these results are used in our characterization, so we do not obtain new proofs of these special cases.

math.NT↗

arXiv2026

On the Weierstrass Preparation Theorem over General Rings

We study rings over which an analogue of the Weierstrass preparation theorem holds for power series. We show that a commutative ring $R$ admits a factorization of every power series in $R[[x]]$ as the product of a polynomial and a unit if and only if $R$ is isomorphic to a finite product of complete local principal ideal rings. We also characterize Noetherian rings $R$ for which this factorization holds under the weaker condition that the coefficients of the series generate the unit ideal: this occurs precisely when $R$ is isomorphic to a finite product of complete local Noetherian integral domains. Beyond this, we investigate the failure of Weierstrass-type preparation in finitely generated rings and prove a general transcendence result for zeros of $p$-adic power series, producing a large class of power series over number rings that cannot be written as a polynomial times a unit. Finally, we show that for a finitely generated infinite commutative ring $R$, the decision problem of determining whether an integer power series (with computable coefficients) factors as a polynomial times a unit in $R[[x]]$ is undecidable.

math.AC↗

arXiv2025

Sparse regular subsets of the reals

This paper concerns the expansion of the real ordered additive group by a predicate for a subset of $[0,1]$ whose base-$r$ representations are recognized by a Büchi automaton. In the case that this predicate is closed, a dichotomy is established for when this expansion is interdefinable with the structure $(\mathbb{R},<,+,0,r^{-\mathbb{N}})$ for some $r \in \mathbb{N}_{>1}$. In the case that the closure of the predicate has Hausdorff dimension less than $1$, the dichotomy further characterizes these expansions of $(\mathbb{R},<,+,0,1)$ by when they have NIP and NTP$_2$, which is precisely when the closure of the predicate has Hausdorff dimension $0$.

math.LO↗

arXiv2024

Maximal dimensional subalgebras of general Cartan type Lie algebras

Let $\Bbbk$ be a field of characteristic zero and let $\mathbb{W}_n = \operatorname{Der}(\Bbbk[x_1,\cdots,x_n])$ be the $n^{\text{th}}$ general Cartan type Lie algebra. In this paper, we study Lie subalgebras $L$ of $\mathbb{W}_n$ of maximal Gelfand-Kirillov (GK) dimension, that is, with $\operatorname{GKdim}(L) = n$. For $n = 1$, we completely classify such $L$, proving a conjecture of the second author. As a corollary, we obtain a new proof that $\mathbb{W}_1$ satisfies the Dixmier conjecture, in other words, $\operatorname{End}(\mathbb{W}_1) \setminus \{0\} = \operatorname{Aut}(\mathbb{W}_1)$, a result first shown by Du. For arbitrary $n$, we show that if $L$ is a GK-dimension $n$ subalgebra of $\mathbb{W}_n$, then $U(L)$ is not (left or right) noetherian.

math.RA↗

arXiv2024

Enveloping algebras of derivations of commutative and noncommutative algebras

Let $\Bbbk$ be a field of characteristic zero. Motivated by the fundamental question of whether it is possible for the universal enveloping algebra of an infinite-dimensional Lie algebra to be noetherian, we study Lie algebras of derivations of associative algebras. The main result of this paper is that the universal enveloping algebra of the Lie algebra of derivations of a finitely generated $\Bbbk$-algebra is not noetherian. This extends a result of Sierra and Walton on the Witt algebra, as well as a result of the second author on Krichever-Novikov algebras. We highlight that the result applies to derivations of both commutative and noncommutative algebras without restriction on their growth.

math.RA↗

arXiv2024

Effective isotrivial Mordell-Lang in positive characteristic

The isotrivial Mordell-Lang theorem of Moosa and Scanlon describes the set $X\capΓ$ when $X$ is a subvariety of a semiabelian variety $G$ over a finite field $\mathbb{F}_q$ and $Γ$ is a finitely generated subgroup of $G$ that is invariant under the $q$-power Frobenius endomorphism $F$. That description is here made effective, and extended to arbitrary commutative algebraic groups $G$ and arbitrary finitely generated $\mathbb{Z}[F]$-submodules $Γ$. The approach is to use finite automata to give a concrete description of $X\cap Γ$. These methods and results have new applications even when specialised to the case when $G$ is an abelian variety over a finite field, $X\subseteq G$ a subvariety defined over a function field $K$, and $Γ=G(K)$. As an application of the automata-theoretic approach, a dichotomy theorem is established for the growth of the number of points in $X(K)$ of bounded height. As an application of the effective description of $X\capΓ$, decision procedures are given for the following three diophantine problems: Is $X(K)$ nonempty? Is it infinite? Does it contain an infinite coset?

math.NT↗

arXiv2024

Invariant rational functions under rational transformations

Let $X$ be an algebraic variety equipped with a dominant rational self-map $ϕ:X\to X$. A new quantity measuring the interaction of $(X,ϕ)$ with trivial dynamical systems is introduced; the stabilised algebraic dimension of $(X,ϕ)$ captures the maximum number of new algebraically independent invariant rational functions on the cartesian product of $(X, ϕ)$ and $(Y, ψ)$, as $(Y,ψ)$ ranges over all algebraic dynamical systems. It is shown that this birational invariant agrees with the maximum dimension of a dominant equivariant rational image $(X',ϕ')$ where $ϕ'$ is part of an algebraic group action on $X'$. As a consequence, it is deduced that if some cartesian power of $(X,ϕ)$ admits a nonconstant invariant rational function, then already the second cartesian power does.

math.AG↗

arXiv2024

Consecutive Power Occurrences in Sturmian Words

We show that every Sturmian word has the property that the distance between consecutive ending positions of cubes occurring in the word is always bounded by $10$ and this bound is optimal, extending a result of Rampersad, who proved that the bound $9$ holds for the Fibonacci word. We then give a general result showing that for every $e \in [1,(5+\sqrt{5})/2)$ there is a natural number $N$, depending only on $e$, such that every Sturmian word has the property that the distance between consecutive ending positions of $e$-powers occurring in the word is uniformly bounded by $N$.

math.CO↗

arXiv2023

Filtered deformations of commutative algebras of Krull dimension two

Let $F$ be an algebraically closed field of positive characteristic and let $R$ be a finitely generated $F$-algebra with a filtration with the property that the associated graded ring of $R$ is an integral domain of Krull dimension two. We show that under these conditions $R$ satisfies a polynomial identity, answering a question of Etingof in the affirmative in a special case.

math.RA↗

arXiv2023

Birational maps with transcendental dynamical degree

We give examples of birational selfmaps of $\mathbb{P}^d, d \geq 3$, whose dynamical degree is a transcendental number. This contradicts a conjecture by Bellon and Viallet. The proof uses a combination of techniques from algebraic dynamics and diophantine approximation.

math.DS↗

arXiv2023

Duality of Lattices Associated to Left and Right Quotients

We associate lattices to the sets of unions and intersections of left and right quotients of a regular language. For both unions and intersections, we show that the lattices we produce using left and right quotients are dual to each other. We also give necessary and sufficient conditions for these lattices to have maximal possible complexity.

cs.FL↗