arXiv subjects

Benjamin Klopsch

Publications and source records attributed to Benjamin Klopsch.

At least 19 recordsLinked to original sources

arXiv2026

Representation growth of quasi-semisimple profinite groups

The representation zeta function of a profinite group $G$ encodes the distribution of continuous irreducible complex representations of $G$ as a function of the dimension. Its abscissa of convergence $\alpha(G)$ describes the polynomial degree of representation growth of $G$. Within the class of quasi-semisimple profinite groups, we characterise those of polynomial representation growth (PRG) and we prove that whether such a group $G$ has PRG or not only depends on its semisimple part $G/\mathrm{Z}(G)$. Moreover, we show that, for quasi-semisimple profinite groups $G$ that have uniformly bounded Lie ranks, the degree of growth satisfies $\alpha(G) = \alpha(G/\mathrm{Z}(G))$. We provide a technique to produce, for any prescribed positive real number $\varrho$, quasi-semisimple profinite groups $G$ with PRG of degree $\alpha(G) = \varrho$. Our method allows for considerable flexibility regarding the inclusion of finite simple groups of Lie type as composition factors of $G$. Furthermore, we can arrange for the groups $G$ of prescribed representation growth to be profinite completions of suitable finitely generated discrete groups $\Gamma$ so that the group $\Gamma$ has the same representation zeta function as $G$.

math.GR↗

arXiv2025

Normal subgroups of non-torsion multi-EGS groups

We study the distribution of normal subgroups in non-torsion, regular branch multi-EGS groups and show that the congruence completions of such groups have bounded finite central width. In particular, we show that the profinite completion of the Fabrykowski--Gupta group acting on the $p$-adic tree has central width 2 for every odd prime $p$. The methods used also apply to the family of Sunic groups, which closely resemble the Grigorchuk group.

math.GR↗

arXiv2025

On the Hausdorff spectra of free pro-$p$ groups and certain $p$-adic analytic groups

We establish that finitely generated non-abelian direct products $G$ of free pro-$p$ groups have full Hausdorff spectrum with respect to the lower $p$-series $\mathcal{L}$. This complements similar results with respect to other standard filtration series and a recent theorem showing that the Hausdorff spectrum $\text{hspec}^\mathcal{L}(G)$ of a $p$-adic analytic pro-$p$ group $G$ is discrete and consists of at most $2^{\dim(G)}$ rational numbers. The latter also left some room for improvement regarding the upper bound. Indeed, for finitely generated nilpotent pro-$p$ groups $G$ we obtain the stronger assertion that the cardinality of the Hausdorff spectrum is at most the analytic dimension of $G$. Moreover, we produce a corresponding result when the $p$-adic analytic pro-$p$ group $G$ is just infinite, which holds not just for the lower $p$-series but for arbitrary filtration series. Finally, we show that, if $G$ is a countably based pro-$p$ group with an open subgroup mapping onto the free abelian pro-$p$ group $\mathbb{Z}_p \oplus \mathbb{Z}_p$, then for every prescribed finite set $\{0,1\} \subseteq X \subseteq [0,1]$ there is a filtration series $\mathcal{S}$ such that $\text{hspec}^\mathcal{S}(G) = X$; in particular, $|\text{hspec}^{\mathcal{S}}(G)|$ is unbounded, as $\mathcal{S}$ runs through all filtration series of $G$ with $|\text{hspec}^{\mathcal{S}}(G)| < \infty$.

math.GR↗

arXiv2024

The lower $p$-series of analytic pro-$p$ groups and Hausdorff dimension

Let $G$ be a $p$-adic analytic pro-$p$ group of dimension $d$. We produce an approximate series which descends regularly in strata and whose terms deviate from the lower $p$-series in a uniformly bounded way. This brings to light a new set of rational invariants, canonically associated to $G$, that yield the aforementioned uniform bound and that restrict the possible values for the Hausdorff dimensions of closed subgroups of $G$ with respect to the lower $p$-series. In particular, the Hausdorff spectrum of $G$ with respect to the lower $p$-series is discrete and consists of at most $2^d$ rational numbers.

math.GR↗

arXiv2023

Finite axiomatizability of the rank and the dimension of a pro-$\pi$ group

The Pr\"ufer rank $\mathrm{rk}(G)$ of a profinite group $G$ is the supremum, across all open subgroups $H$ of $G$, of the minimal number of generators $\mathrm{d}(H)$. It is known that, for any given prime $p$, a profinite group $G$ admits the structure of a $p$-adic analytic group if and only if $G$ is virtually a pro-$p$ group of finite rank. The dimension $\dim G$ of a $p$-adic analytic profinite group $G$ is the analytic dimension of $G$ as a $p$-adic manifold; it is known that $\dim G$ coincides with the rank $\mathrm{rk}(U)$ of any uniformly powerful open pro-$p$ subgroup $U$ of $G$. Let $\pi$ be a finite set of primes, let $r \in \mathbb{N}$ and let $\mathbf{r} = (r_p)_{p \in \pi}, \mathbf{d} = (d_p)_{p \in \pi}$ be tuples in $\{0, 1, \ldots,r\}$. We show that there is a single sentence $\sigma_{\pi,r,\mathbf{r},\mathbf{d}}$ in the first-order language of groups such that for every pro-$\pi$ group $G$ the following are equivalent: (i) $\sigma_{\pi,r,\mathbf{r},\mathbf{d}}$ holds true in the group $G$, that is, $G \models \sigma_{\pi,r,\mathbf{r},\mathbf{d}}$; (ii) $G$ has rank $r$ and, for each $p \in \pi$, the Sylow pro-$p$ subgroups of $G$ have rank $r_p$ and dimension $d_p$. Loosely speaking, this shows that, for a pro-$\pi$ group $G$ of bounded rank, the precise rank of $G$ as well as the ranks and dimensions of the Sylow subgroups of $G$ can be recognized by a single sentence in the first-order language of groups.

math.GR↗

arXiv2022

The structure of groups with all proper quotients virtually nilpotent

Just infinite groups play a significant role in profinite group theory. For each $c \geq 0$, we consider more generally JNN$_c$F profinite (or, in places, discrete) groups that are Fitting-free; these are the groups $G$ such that every proper quotient of $G$ is virtually class-$c$ nilpotent whereas $G$ itself is not, and additionally $G$ does not have any non-trivial abelian normal subgroup. When $c = 1$, we obtain the just non-(virtually abelian) groups without non-trivial abelian normal subgroups. Our first result is that a finitely generated profinite group is virtually class\nbd$c$ nilpotent if and only if there are only finitely many subgroups arising as the lower central series terms $\gamma_{c+1}(K)$ of open normal subgroups $K$ of $G$. Based on this we prove several structure theorems. For instance, we characterize the JNN$_c$F profinite groups in terms of subgroups of the above form $\gamma_{c+1}(K)$. We also give a description of JNN$_c$F profinite groups as suitable inverse limits of virtually nilpotent profinite groups. Analogous results are established for the family of hereditarily JNN$_c$F groups and, for instance, we show that a Fitting-free JNN$_c$F profinite (or discrete) group is hereditarily JNN$_cF$ if and only if every maximal subgroup of finite index is JNN$_c$F. Finally, we give a construction of hereditarily JNN$_c$F groups, which uses as an input known families of hereditarily just infinite groups.

math.GR↗

arXiv2022

Free polynilpotent groups and the Magnus property

Motivated by a classic result for free groups, one says that a group $G$ has the Magnus property if the following holds: whenever two elements generate the same normal subgroup of $G$, they are conjugate or inverse-conjugate in $G$. It is a natural problem to find out which relatively free groups display the Magnus property. We prove that a free polynilpotent group of any given class row has the Magnus property if and only if it is nilpotent of class at most $2$. For this purpose we explore the Magnus property more generally in soluble groups, and we produce new techniques, both for establishing and for disproving the property. We also prove that a free centre-by-(polynilpotent of given class row) group has the Magnus property if and only if it is nilpotent of class at most $2$. On the way, we display $2$-generated nilpotent groups (with non-trivial torsion) of any prescribed nilpotency class with the Magnus property. Similar examples of finitely generated, torsion-free nilpotent groups are hard to come by, but we construct a $4$-generated, torsion-free, class-$3$ nilpotent group of Hirsch length $9$ with the Magnus property. Furthermore, using a weak variant of the Magnus property and an ultraproduct construction, we establish the existence of metabelian, torsion-free, nilpotent groups of any prescribed nilpotency class with the Magnus property.

math.GR↗

arXiv2022

The degree of commutativity of wreath products with infinite cyclic top group

The degree of commutativity of a finite group is the probability that two uniformly and randomly chosen elements commute. This notion extends naturally to finitely generated groups $G$: the degree of commutativity $\text{dc}_S(G)$, with respect to a given finite generating set $S$, results from considering the fractions of commuting pairs of elements in increasing balls around $1_G$ in the Cayley graph $\mathcal{C}(G,S)$. We focus on restricted wreath products the form $G = H \wr \langle t \rangle$, where $H \ne 1$ is finitely generated and the top group $\langle t \rangle$ is infinite cyclic. In accordance with a more general conjecture, we show that $\text{dc}_S(G) = 0$ for such groups $G$, regardless of the choice of $S$. This extends results of Cox who considered lamplighter groups with respect to certain kinds of generating sets. We also derive a generalisation of Cox's main auxiliary result: in `reasonably large' homomorphic images of wreath products $G$ as above, the image of the base group has density zero, with respect to certain types of generating sets.

math.GR↗

arXiv2021

P\'olya-Carlson dichotomy for coincidence Reidemeister zeta functions via profinite completions

We consider coincidence Reidemeister zeta functions for tame endomorphism pairs of nilpotent groups of finite rank, shedding new light on the subject by means of profinite completion techniques. In particular, we provide a closed formula for coincidence Reidemeister numbers for iterations of endomorphism pairs of torsion-free nilpotent groups of finite rank, based on a weak commutativity condition, which derives from simultaneous triangularisability on abelian sections. Furthermore, we present results in support of a P\'olya-Carlson dichotomy between rationality and a natural boundary for the analytic behaviour of the zeta functions in question.

math.GR↗

arXiv2020

A pro-$p$ group with full normal Hausdorff spectra

For each odd prime $p$, we produce a $2$-generated pro-$p$ group $G$ whose normal Hausdorff spectra \[ \mathrm{hspec}_{\trianglelefteq}^{\mathcal{S}}(G) = \{ \mathrm{hdim}_{G}^{\mathcal{S}}(H)\mid H\trianglelefteq_\mathrm{c} G \} \] with respect to five standard filtration series $\mathcal{S}$ - namely the lower $p$-series, the dimension subgroup series, the $p$-power series, the iterated $p$-power series and the Frattini series - are all equal to the full unit interval $[0,1]$. Here $\mathrm{hdim}_G^{\mathcal{S}} \colon \{ X\mid X \subseteq G \} \to[0,1]$ denotes the Hausdorff dimension function associated to the natural translation-invariant metric induced by the filtration series $\mathcal{S}$.

math.GR↗

arXiv2020

Strong conciseness in profinite groups

A group word $w$ is said to be strongly concise in a class $\mathcal{C}$ of profinite groups if, for every group $G$ in $\mathcal{C}$ such that $w$ takes less than $2^{\aleph_0}$ values in $G$, the verbal subgroup $w(G)$ is finite. Detomi, Morigi and Shumyatsky established that multilinear commutator words -- and the particular words $x^2$ and $[x^2,y]$ -- have the property that the corresponding verbal subgroup is finite in a profinite group $G$ whenever the word takes at most countably many values in $G$. They conjectured that, in fact, this should be true for every word. In particular, their conjecture included as open cases power words and Engel words. In the present paper, we take a new approach via parametrised words that leads to stronger results. First we prove that multilinear commutator words are strongly concise in the class of all profinite groups. Then we establish that every group word is strongly concise in the class of nilpotent profinite groups. From this we deduce, for instance, that, if $w$ is one of the group words $x^2$, $x^3$, $x^6$, $[x^3,y]$ or $[x,y,y]$, then $w$ is strongly concise in the class of all profinite groups. Indeed, the same conclusion can be reached for all words of the infinite families $[x^m,z_1,\ldots,z_r]$ and $[x,y,y,z_1,\ldots,z_r]$, where $m \in \{2,3\}$ and $r \ge 1$.

math.GR↗

arXiv2019

Generating pairs of projective special linear groups that fail to lift

The following problem was originally posed by B.H. Neumann and H. Neumann. Suppose that a group $G$ can be generated by $n$ elements and that $H$ is a homomorphic image of $G$. Does there exist, for every generating $n$-tuple $(h_1,\ldots, h_n)$ of $H$, a homomorphism $\vartheta \colon G \to H$ and a generating $n$-tuple $(g_1,\ldots,g_n)$ of $G$ such that $(g_1^\vartheta,\ldots,g_n^\vartheta) = (h_1,\ldots,h_n)$? M.J. Dunwoody gave a negative answer to this question, by means of a carefully engineered construction of an explicit pair of soluble groups. Via a new approach we produce, for $n = 2$, infinitely many pairs of groups $(G,H)$ that are negative examples to the Neumanns' problem. These new examples are easily described: $G$ is a free product of two suitable finite cyclic groups, such as $C_2 \ast C_3$, and $H$ is a suitable finite projective special linear group, such as $\mathrm{PSL}(2,p)$ for a prime $p \ge 5$. A small modification yields the first negative examples $(G,H)$ with $H$ infinite.

math.GR↗

arXiv2019

A pro-p group with infinite normal Hausdorff spectra

Using wreath products, we construct a finitely generated pro-p group G with infinite normal Hausdorff spectrum with respect to the p-power series. More precisely, we show that this normal Hausdorff spectrum contains an infinite interval; this settles a question of Shalev. Furthermore, we prove that the normal Hausdorff spectra of G with respect to other filtration series have a similar shape. In particular, our analysis applies to standard filtration series such as the Frattini series, the lower p-series and the modular dimension subgroup series. Lastly, we pin down the ordinary Hausdorff spectra of G with respect to the standard filtration series. The spectrum of G for the lower p-series displays surprising new features.

math.GR↗

arXiv2019

Zeta functions associated to admissible representations of compact p-adic Lie groups

Let $G$ be a profinite group. A strongly admissible smooth representation $ρ$ of $G$ over $\mathbb{C}$ decomposes as a direct sum $ρ\cong \bigoplus_{π\in \mathrm{Irr}(G)} m_π(ρ) \, π$ of irreducible representations with finite multiplicities $m_π(ρ)$ such that for every positive integer $n$ the number $r_n(ρ)$ of irreducible constituents of dimension $n$ is finite. Examples arise naturally in the representation theory of reductive groups over non-archimedean local fields. In this article we initiate an investigation of the Dirichlet generating function \[ ζ_ρ(s) = \sum_{n=1}^\infty r_n(ρ) n^{-s} = \sum_{π\in \mathrm{Irr}(G)} \frac{m_π(ρ)}{(\dim π)^s} \] associated to such a representation $ρ$. Our primary focus is on representations $ρ= \mathrm{Ind}_H^G(σ)$ of compact $p$-adic Lie groups $G$ that arise from finite dimensional representations $σ$ of closed subgroups $H$ via the induction functor. In addition to a series of foundational results - including a description in terms of $p$-adic integrals - we establish rationality results and functional equations for zeta functions of globally defined families of induced representations of potent pro-$p$ groups. A key ingredient of our proof is Hironaka's resolution of singularities, which yields formulae of Denef-type for the relevant zeta functions. In some detail, we consider representations of open compact subgroups of reductive $p$-adic groups that are induced from parabolic subgroups. Explicit computations are carried out by means of complementing techniques: (i) geometric methods that are applicable via distance-transitive actions on spherically homogeneous rooted trees and (ii) the $p$-adic Kirillov orbit method. Approach (i) is closely related to the notion of Gelfand pairs and works equally well in positive defining characteristic.

math.GR↗

arXiv2019

Hausdorff dimensions in $p$-adic analytic groups

Let $G$ be a finitely generated pro-$p$ group, equipped with the $p$-power series. The associated metric and Hausdorff dimension function give rise to the Hausdorff spectrum, which consists of the Hausdorff dimensions of closed subgroups of $G$. In the case where $G$ is $p$-adic analytic, the Hausdorff dimension function is well understood; in particular, the Hausdorff spectrum consists of finitely many rational numbers closely linked to the analytic dimensions of subgroups of $G$. Conversely, it is a long-standing open question whether the finiteness of the Hausdorff spectrum implies that $G$ is $p$-adic analytic. We prove that the answer is yes, in a strong sense, under the extra condition that $G$ is soluble. Furthermore, we explore the problem and related questions also for other filtration series, such as the lower $p$-series, the Frattini series, the modular dimension subgroup series and quite general filtration series. For instance, we prove, for odd primes $p$, that every countably based pro-$p$ group $G$ with an open subgroup mapping onto 2 copies of the $p$-adic integers admits a filtration series such that the corresponding Hausdorff spectrum contains an infinite real interval.

math.GR↗

arXiv2019

Pro-$p$ groups of positive rank gradient and Hausdorff dimension

Let $G$ be a finitely generated pro-$p$ group of positive rank gradient. Motivated by the study of Hausdorff dimension, we show that finitely generated closed subgroups $H$ of infinite index in $G$ never contain any infinite subgroups $K$ that are subnormal in~$G$ via finitely generated successive quotients. This pro-$p$ version of a well-known theorem of Greenberg generalises similar assertions that were known to hold for non-abelian free pro-$p$ groups, non-soluble Demushkin pro-$p$ groups and other related pro-$p$ groups. The result we prove is reminiscent of Gaboriau's theorem for countable groups with positive first $\ell^2$-Betti number, but not quite a direct analogue. The approach via the notion of Hausdorff dimension in pro-$p$ groups also leads to our main results. We show that every finitely generated pro-$p$ group $G$ of positive rank gradient has full Hausdorff spectrum $\text{hspec}^\mathcal{F}(G) = [0,1]$ with respect to the Frattini series~$\mathcal{F}$. Using different, Lie-theoretic techniques we also prove that finitely generated non-abelian free pro-$p$ groups and non-soluble Demushkin groups $G$ have full Hausdorff spectrum $\text{hspec}^\mathcal{Z}(G) = [0,1]$ with respect to the Zassenhaus series~$\mathcal{Z}$. This resolves a long-standing problem in the subject of Hausdorff dimensions in pro-$p$ groups. The results about full Hausdorff spectra hold more generally for finite direct products of finitely generated pro-$p$ groups of positive rank gradient and for mixed finite direct products of finitely generated non-abelian free pro-$p$ groups and non-soluble Demushkin groups, respectively. Analogously, the aforementioned results with respect to the Frattini series generalise further to Hausdorff dimension functions with respect to arbitrary iterated verbal filtrations. Finally, we determine the normal Hausdorff spectra of such direct products.

math.GR↗

arXiv2017

Maximal subgroups and irreducible representations of generalised multi-edge spinal groups

Let $p\ge 3$ be a prime. A generalised multi-edge spinal group is a subgroup of the automorphism group of a regular $p$-adic rooted tree T that is generated by one rooted automorphism and $p$ families of directed automorphisms, each family sharing a common directed path disjoint from the paths of the other families. This notion generalises the concepts of multi-edge spinal groups, including the widely studied GGS-groups, and the extended Gupta-Sidki groups that were introduced by Pervova. Extending techniques that were developed in these more special cases, we prove: generalised multi-edge spinal groups that are torsion have no maximal subgroups of infinite index. Furthermore we use tree enveloping algebras, which were introduced by Sidki and Bartholdi, to show that certain generalised multi-edge spinal groups admit faithful infinite dimensional irreducible representations over the prime field $\mathbb{Z}/p\mathbb{Z}$.

math.GR↗

arXiv2017

Embedding properties of hereditarily just infinite profinite wreath products

We study infinitely iterated wreath products of finite permutation groups with respect to product actions. In particular, we prove that, for every non-empty class of finite simple groups $\mathcal{X}$, there exists a finitely generated hereditarily just infinite profinite group $W$ with composition factors in $\mathcal{X}$ such that any countably based profinite group with composition factors in $\mathcal{X}$ can be embedded into $W$. Additionally we investigate when infinitely iterated wreath products of finite simple groups with respect to product actions are co-Hopfian or non-co-Hopfian.

math.GR↗