arXiv subjects

Jonas Deré

Publications and source records attributed to Jonas Deré.

At least 19 recordsLinked to original sources

arXiv2026

Conjugator length in nilpotent groups

For every rational number $α\geq 2$, we construct a 2-step nilpotent group with conjugator length function $\operatorname{CL}(n)\simeq n^α$. We deduce that the nilpotent conjugator length spectrum is dense in $\{0\}\cup[2,\infty)$, even after restricting to groups of nilpotency class at most $2$. Moreover, we show that, despite being a commensurability invariant of nilpotent groups, conjugator length is not a quasi-isometry invariant: for every $m \geq 1$, the cocompact lattices in $\mathsf H(\mathbb R)^m$ realize exactly the growth types $n^2,\ldots,n^{m+1}$. Finally, for every real cubic algebraic number $θ$, we construct a 2-step nilpotent group such that $n^3 \preceq \operatorname{CL}(n) \preceq n^{3+\varepsilon}$ for every $\varepsilon > 0$, with $n^3 \prec \operatorname{CL}(n)$ if and only if at least one real conjugate of $θ$ has unbounded partial quotients. Consequently, determining the conjugator length function for arbitrary 2-step nilpotent groups is at least as hard as settling the bounded-partial-quotient problem for real cubic algebraic numbers with two nonreal conjugates.

math.GR↗

arXiv2026

Automorphisms and monomorphisms of direct products of virtually solvable minimax groups

This paper studies automorphisms and monomorphisms of direct products $Γ=Γ_1\times\cdots\timesΓ_r$ of finitely generated virtually solvable minimax groups, a class containing all virtually polycyclic groups. Under an indecomposability assumption on the $\mathbb Q$-algebraic hulls, we prove that every monomorphism of $Γ$ factorizes uniquely as $φ=θ\cdotζ$, where $θ$ sends each factor into a permuted factor with $\mathbb Q$-isomorphic hull and $ζ$ is central and off-diagonal. Conversely, every such pair defines a monomorphism of $Γ$, and $φ$ is an automorphism if and only if $θ$ is. This indecomposability assumption is sharp: we show it cannot be weakened to direct indecomposability of the factors. The proof proceeds in three steps: first by establishing the corresponding central mixing property for finite-dimensional Lie algebras and algebraic Lie algebras, then for connected linear algebraic groups, and finally by transferring these results to minimax groups via $\mathbb Q$-algebraic hulls. This extends the previously known nilpotent case both from automorphisms to monomorphisms and from finitely generated torsion-free nilpotent groups to the broader class of finitely generated virtually solvable minimax groups. As applications, we characterize co-Hopfian direct products and derive formulas for Reidemeister numbers and Reidemeister spectra.

math.GR↗

arXiv2026

Residual Finiteness Growth in Virtually Nilpotent Groups

The residual finiteness growth $\text{RF}_G: \mathbb{N} \to \mathbb{N}$ of a finitely generated group $G$ is a function that gives the smallest value of the index $[G:N]$ with $N$ a normal subgroup not containing a non-trivial element $g$, in function of the word norm of that element $g$. It has been studied for several classes of finitely generated groups, including free groups, linear groups and virtually abelian groups. This paper shows that if $G$ is virtually nilpotent, then $\text{RF}_G = \log^δ$ for some $δ\in \mathbb{N}\cup\{0\}$, with moreover an explicit formula for $δ$ in terms of Lie algebras. This implies in particular that it is an invariant of the complex Mal'cev completion, leading to the application that residual finiteness growth is a profinite invariant for virtually nilpotent groups.

math.GR↗

arXiv2026

Nice bases for Lie algebras

The concept of a nice basis for a Lie algebra was introduced to study the Ricci curvature on nilpotent Lie groups equipped with a left-invariant metric. Despite the many applications in differential geometry, for example in the construction of Einstein manifolds, very little is known about the existence and number of nice bases on a given Lie algebra. This paper studies this question for three classes of Lie algebras, namely direct sums, almost abelian ones and nilpotent Lie algebras associated to a graph. As an application we compute the number of nice bases for Lie algebras up to dimension $3$, and show that for a general Lie algebra the existence depends on the field over which it is defined. Moreover, for every natural number $n$ we give an indecomposable Lie algebra such that there exists exactly $n$ nice bases up to equivalence.

math.DG↗

arXiv2026

Automorphism groups of solvable groups of finite abelian ranks

This paper gives a new explicit construction of the $\mathbb{Q}$-algebraic hull for virtually solvable groups $Γ$ of finite abelian ranks, taking into account the spectrum $S$ of the group $Γ$. As an application, we make a detailed study of the structure of $Aut(Γ)$ in the finitely generated case and show that a number of natural subgroups are $S$-arithmetic under the condition that $Fitt(Γ)$ is $S$-arithmetic. We then proceed by demonstrating that $Out(Γ)$ has a $S$-arithmetic image in the group of algebraic outer automorphisms of the $\mathbb{Q}$-algebraic hull. We finish by discussing further applications of the $\mathbb{Q}$-algebraic hull towards an open conjecture by Nekrashevych and Pete and topological fixed point theory.

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arXiv2025

Residual Finiteness Growth in Minimax Groups

If $g\in G$ is a non-trivial element in a residually finite group, then there exists by definition a finite group $Q$ and a homomorphism $φ: G \to Q$ such that $φ(g) \neq e$. The residual finiteness growth $\text{RF}_G$ of a finitely generated residually finite group $G$ estimates the size of $Q$ in terms of the word norm $\|g\|$ of the element $g\in G$. This function has been studied for several classes of groups, including free groups, lamplighter groups and nilpotent groups. For finitely generated linear groups $G\leq \text{GL}(m, \mathbb{C})$ this function is known to be bounded by $\text{RF}_G(r) \preceq r^{m^2+1}$, which is quadratic in $m$. This paper establishes an improved bound of the form $\text{RF}_G(r) \preceq r^{4k}$ with $k$ the Prüfer rank of $G$ for certain virtually solvable linear groups, namely minimax groups, a class which includes virtually polycyclic and Baumslag-Solitar groups. Moreover, the upper bound is invariant under taking finite extensions, and also establishes an improved polylogarithmic version for virtually nilpotent groups, generalizing the known exact bound for virtually abelian groups. If the group is not virtually nilpotent, we prove that $\text{RF}_G(r)$ is at least linear, improving a recent result.

math.GR↗

arXiv2025

Residual Finiteness Growth in Two-Step Nilpotent Groups

Given a finitely generated residually finite group $G$, the residual finiteness growth $\text{RF}_G: \mathbb{N} \to \mathbb{N}$ bounds the size of a finite group $Q$ needed to detect an element of norm at most $r$. More specifically, if $g\in G$ is a non-trivial element with $\|g\|_G \leq r$, so $g$ can be written as a product of at most $r$ generators or their inverses, then we can find a homomorphism $ϕ: G \to Q$ with $ϕ(g) \neq e_Q$ and $|Q| \leq \text{RF}_G(r)$. The residual finiteness growth is defined as the smallest function with this property. This function has been bounded from above and below for several classes of groups, including virtually abelian, nilpotent, linear and free groups. However, for many of these groups, the exact asymptotics of $\text{RF}_G$ are unknown (in particular this is the case for a general nilpotent group), nor whether it is a quasi-isometric invariant for certain classes of groups. In this paper, we make a first step in giving an affirmative answer to the latter question for $2$-step nilpotent groups, by improving the polylogarithmic upper bound known in literature, and to show that it only depends on the complex Mal'cev completion of the group. If the commutator subgroup is one- or two-dimensional, we prove that our bound is in fact exact, and we conjecture that this holds in general.

math.GR↗

arXiv2024

Effective conjugacy separability of virtually abelian groups

A natural question for groups $H$ is which data can be detected in its finite quotients. A subset $X \subset H$ is called separable if for all $h\in H \setminus X$, there exists an epimorphism $φ$ to a finite group $Q$ such that $φ(h)\notinφ(X)$. More specifically, a group is said to be conjugacy separable if every conjugacy class is separable. It is known that many classes of groups are conjugacy separable, including virtually free and polycyclic groups. The minimal order of the quotient $Q$, in terms of the complexity of the conjugacy classes under consideration, is captured by the conjugacy separability function $\mathrm{Conj}_H: \mathbb{N} \to \mathbb{N}$. This function is in general ill understood, in fact the only large class of groups for which it is known exactly are the abelian groups. Indeed, in this case $\mathrm{Conj}_H$ is equal to the residual finiteness function, that is the size of quotients needed to separate singletons, and thus logarithmic if the group is infinite. Recent work has described the residual finiteness function for the class of virtually abelian groups, which gives a lower bound for the conjugacy separability function. The main result of this paper is a characterization of $\mathrm{Conj}_H$ for every virtually abelian group $H$. If the corresponding extension is associated with an irreducible representation over $\mathbb{Q}$, we demonstrate that we obtain the same function as the residual finiteness function. However, if the representation is not irreducible, we find an expression that is in some cases strictly larger, which we illustrate with several examples.

math.GR↗

arXiv2024

A note on the existence of the Reidemeister zeta function on groups

Given an endomorphism $φ: G \to G$ on a group $G$, one can define the Reidemeister number $R(φ) \in \mathbb{N} \cup \{\infty\}$ as the number of twisted conjugacy classes. The corresponding Reidemeister zeta function $R_φ(z)$, by using the Reidemeister numbers $R(φ^n)$ of iterates $φ^n$ in order to define a power series, has been studied a lot in the literature, especially the question whether it is a rational function or not. For example, it has been shown that the answer is positive for finitely generated torsion-free virtually nilpotent groups, but negative in general for abelian groups that are not finitely generated. However, in order to define the Reidemeister zeta function of an endomorphism $φ$, it is necessary that the Reidemeister numbers $R(φ^n)$ of all iterates $φ^n$ are finite. This puts restrictions, not only on the endomorphism $φ$, but also on the possible groups $G$ if $φ$ is assumed to be injective. In this note, we want to initiate the study of groups having a well-defined Reidemeister zeta function for a monomorphism $φ$, because of its importance for describing the behavior of Reidemeister zeta functions. As a motivational example, we show that the Reidemeister zeta function is indeed rational on torsion-free virtually polycyclic groups. Finally, we give some partial results about the existence in the special case of automorphisms on finitely generated torsion-free nilpotent groups, showing that it is a restrictive condition.

math.GR↗

arXiv2024

On post-Lie algebras structures coming from simply transitive NIL-affine actions

Given a simply connected solvable Lie group $G$, there always exists NIL-affine action $ρ: G \to \operatorname{Aff}(H)$ on a nilpotent Lie group $H$ such that $G$ acts simply transitively. The question whether this is always possible for $H = \mathbb{R}^n$ abelian was known as Milnor's question, with a negative answer due to a counterexample of Benoist. This counterexample is based on a correspondence between certain affine actions $ρ: G \to \operatorname{Aff}(\mathbb R^n)$ and left-symmetric structures on the corresponding Lie algebra $\mathfrak g$ of $G$, where simply transitive actions correspond exactly to the so-called complete left-symmetric structures. In general however, the question remains open which solvable Lie groups $G$ can act on which nilpotent Lie groups $H$. A natural candidate for a correspondence on the Lie algebra level is the notion of post-Lie algebra structures, which form the natural generalization of left-symmetric structures. In this paper, we show that every simply transitive NIL-affine action of $G$ on a nilpotent Lie group $H$ indeed induces a post-Lie algebra structure on the pair of Lie algebras $(\mathfrak g,\mathfrak h)$. Moreover, we discuss a new notion of completeness for these structures in the case that $\mathfrak h$ is $2$-step nilpotent, equivalent but different from the known definition for $H = \mathbb R^n$. We then show that simply transitive actions exactly correspond to complete post-Lie algebra structures in the $2$-step nilpotent case. However, the questions how to define completeness in higher nilpotency classes remains open, as we illustrate with an example in the $3$-step nilpotent case.

math.DG↗

arXiv2023

Residual Finiteness Growth in Virtually Abelian Groups

A group $G$ is called residually finite if for every non-trivial element $g \in G$, there exists a finite quotient $Q$ of $G$ such that the element $g$ is non-trivial in the quotient as well. Instead of just investigating whether a group satisfies this property, a new perspective is to quantify residual finiteness by studying the minimal size of the finite quotient $Q$ depending on the complexity of the element $g$, for example by using the word norm $\|g\|_G$ if the group $G$ is assumed to be finitely generated. The residual finiteness growth $\text{RF}_G: \mathbb{N} \to \mathbb{N}$ is then defined as the smallest function such that if $\|g\|_G \leq r$, there exists a morphism $φ: G \to Q$ to a finite group $Q$ with $|Q| \leq \text{RF}_G(r)$ and $φ(g) \neq e_Q$. Although upper bounds have been established for several classes of groups, exact asymptotics for the function $\text{RF}_G$ are only known for very few groups such as abelian groups, the Grigorchuk group and certain arithmetic groups. In this paper, we show that the residual finiteness growth of virtually abelian groups equals $\log^k$ for some $k \in \mathbb{N}$, where the value $k$ is given by an explicit expression. As an application, we show that for every $m \geq 1$ and every $1 \leq k \leq m$, there exists a group $G$ containing a normal abelian subgroup of rank $m$ and with $\text{RF}_G \approx \log^k$.

math.GR↗

arXiv2023

A characterization of Anosov rational forms in nilpotent Lie algebras associated to graphs

Anosov diffeomorphisms are an important class of dynamical systems with many peculiar properties. Ever since they were introduced in the sixties, it has been an open question which manifolds can admit such diffeomorphisms, where tori of dimension greater than or equal to two are the typical examples. It is conjectured that the only manifolds supporting an Anosov diffeomorphism are finitely covered by a nilmanifold, a type of manifold closely related to rational nilpotent Lie algebras. In this paper, we study the existence of Anosov diffeomorphisms for a large class of these nilpotent Lie algebras, namely the ones that can be realized as a rational form in a Lie algebra associated to a graph. From a given simple undirected graph, one can construct a complex $c$-step nilpotent Lie algebra, which in general contains different non-isomorphic rational forms, as described by the authors in previous work. We determine precisely which forms correspond to a nilmanifold admitting an Anosov diffeomorphism, leading to the first class of complex nilpotent Lie algebras having several non-isomorphic rational forms and for which all the ones that are Anosov are described. In doing so, we put a new perspective on certain classifications in low dimensions and correct a false result in the literature.

math.DS↗

arXiv2022

Classification of $K$-forms in nilpotent Lie algebras associated to graphs

Given a simple undirected graph, one can construct from it a $c$-step nilpotent Lie algebra for every $c \geq 2$ and over any field $K$, in particular also over the real and complex numbers. These Lie algebras form an important class of examples in geometry and algebra, and it is interesting to link their properties to the defining graph. In this paper, we classify the isomorphism classes of $K$-forms in these real and complex Lie algebras for any subfield $K \subset \mathbb{C}$ from the structure of the graph. As an application, we show that the number of rational forms up to isomorphism is always one or infinite, with the former being true if and only if the group of graph automorphisms is generated by transpositions.

math.DS↗

arXiv2022

Survey on effective separability

Separability for groups refers to the question which subsets of a group can be detected in its finite quotients. Classically, separability is studied in terms of which classes have a certain separability property, and this question is related to algorithmic problems in groups such as the word problem. A more recent perspective tries to study the order of the smallest finite quotient in which one detects the subset under consideration depending on its complexity, measured using the word norm on a finitely generated group. In this survey, we present what is currently known in the field of effective separability and give an overview of the open questions for several classes of groups.

math.GR↗

arXiv2021

Strongly scale-invariant virtually polycyclic groups

A finitely generated group $Γ$ is called strongly scale-invariant if there exists an injective endomorphism $φ: Γ\to Γ$ with the image $φ(Γ)$ of finite index in $Γ$ and the subgroup $\displaystyle \bigcap_{n>0}φ^n(Γ)$ finite. The only known examples of such groups are virtually nilpotent, or equivalently, all examples have polynomial growth. A question by Nekrashevych and Pete asks whether these groups are the only possibilities for such endomorphisms, motivated by the positive answer due to Gromov in the special case of expanding group morphisms. In this paper, we study this question for the class of virtually polycyclic groups, i.e. the virtually solvable groups for which every subgroup is finitely generated. Using the $\mathbb{Q}$-algebraic hull, which allows us to extend the injective endomorphisms of certain virtually polycyclic groups to a linear algebraic group, we show that the existence of such an endomorphism implies that the group is virtually nilpotent. Moreover, we fully characterize which virtually nilpotent groups have a morphism satisfying the condition above, related to the existence of a positive grading on the corresponding radicable nilpotent group. As another application of the methods, we generalize a result of Fel'shtyn and Lee about which maps on infra-solvmanifolds can have finite Reidemeister number for all iterates.

math.GR↗

arXiv2021

On closed manifolds admitting an Anosov diffeomorphism but no expanding map

A few years ago, the first example of a closed manifold admitting an Anosov diffeomorphism but no expanding map was given. Unfortunately, this example is not explicit and is high-dimensional, although its exact dimension is unknown due to the type of construction. In this paper, we present a family of concrete 12-dimensional nilmanifolds with an Anosov diffeomorphism but no expanding map, where nilmanifolds are defined as the quotient of a 1-connected nilpotent Lie group by a cocompact lattice. We show that this family has the smallest possible dimension in the class of infra-nilmanifolds, which is conjectured to be the only type of manifolds admitting Anosov diffeomorphisms up to homeomorphism. The proof shows how to construct positive gradings from the eigenvalues of the Anosov diffeomorphism under some additional assumptions related to the rank, using the action of the Galois group on these algebraic units.

math.DS↗

arXiv2020

Anosov diffeomorphisms on infra-nilmanifolds associated to graphs

Anosov diffeomorphisms on closed Riemannian manifolds are a type of dynamical systems exhibiting uniform hyperbolic behavior. Therefore their properties are intensively studied, including which spaces allow such a diffeomorphism. It is conjectured that any closed manifold admitting an Anosov diffeomorphism is homeomorphic to an infra-nilmanifold, i.e. a compact quotient of a 1-connected nilpotent Lie group by a discrete group of isometries. This conjecture motivates the problem of describing which infra-nilmanifolds admit an Anosov diffeomorphism. So far, most research was focused on the restricted class of nilmanifolds, which are quotients of 1-connected nilpotent Lie groups by uniform lattices. For example, Dani and Mainkar studied this question for the nilmanifolds associated to graphs, which form the natural generalization of nilmanifolds modeled on free nilpotent Lie groups. This paper further generalizes their work to the full class of infra-nilmanifolds associated to graphs, leading to a necessary and sufficient condition depending only on the induced action of the holonomy group on the defining graph. As an application, we construct families of infra-nilmanifolds with cyclic holonomy groups admitting an Anosov diffeomorphism, starting from faithful actions of the holonomy group on simple graphs.

math.DS↗

arXiv2020

Orthogonal bi-invariant complex structures on metric Lie algebras

This paper studies how many orthogonal bi-invariant complex structures exist on a metric Lie algebra over the real numbers. Recently, it was shown that irreducible Lie algebras which are additionally $2$-step nilpotent admit at most one orthogonal bi-invariant complex structure up to sign. The main result generalizes this statement to metric Lie algebras with any number of irreducible factors and which are not necessarily $2$-step nilpotent. It states that there are either $0$ or $2^k$ such complex structures, with $k$ the number of irreducible factors of the metric Lie algebra. The motivation for this problem comes from differential geometry, for instance to construct non-parallel Killing-Yano $2$-forms on nilmanifolds or to describe the compact Chern-flat quasi-Kähler manifolds. The main tool we develop is the unique orthogonal decomposition into irreducible factors for metric Lie algebras with no non-trivial abelian factor. This is a generalization of a recent result which only deals with nilpotent Lie algebras over the real numbers. Not only do we apply this fact to describe the orthogonal bi-invariant complex structures on a given metric Lie algebra, but it also gives us a method to study different inner products on a given Lie algebra, computing the number of irreducible factors and orthogonal bi-invariant complex structures for varying inner products.

math.DG↗