Menny Aka
Two finitely generated groups have the same set of finite quotients if and only if their profinite completions are isomorphic. Consider the map which sends (the isomorphism class of) an S-arithmetic group to (the isomorphism class of) its profinite completion. We show that for a wide class of S-arithmetic groups, this map is finite to one, while the the fibers are of unbounded size.
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In this note we refine examples by Aka from arithmetic to S-arithmetic groups to show that the vanishing of the $i$-th $\ell^2$-Betti number is not a profinite invariant for all $i \ge 2$.
In this note we compute the fundamental domain of the action on the corresponding Bruhat-Tits tree of the arithmetic subgroup of PGL2 with respect to an arbitrary place of the rational function field.
Classes of Subgroups of a finite group with isomorphic quotient groups
本文研究有限群中具有同构商群的子群类构成的偏序集QIso(G)，针对阿贝尔群及几类非阿贝尔亚循环群展开分析，得出QIso(G)的同构性质及相关群类的刻画结论。
Identification of non-isomorphic 2-groups with dihedral central quotient and isomorphic modular group algebras
本文将非同构有限p-群具有同构模群代数的否定解嵌入到具有二面体中心商的二生成有限2-群类中，解决了该类所有群的原始问题，发现了新否定解并揭示了其特殊性。
We determine the minimal volume of arithmetic hyperbolic orientable n-dimensional orbifolds (compact and non-compact) for every odd dimension n>3. Combined with the previously known results it solves the minimal volume problem for arithmetic hyperbolic n-orbifolds in all dimensions.
Let $\rG$ be a reductive group over a commutative ring $k$. In this article, we prove that the adjoint quotient $\adqG$ is stable under base change. Moreover, if $\rG$ has a maximal torus $\rT$, then the adjoint quotient of the torus $\rT$ by its Weyl group will be isomorphic to $\adqG$. Then we focus on the semisimple simply connected group $\rG$ of the constant type. In this case, $\adqG$ is isomorphic to the Weil restriction $\underset{\rD/\spec k}{\prod}\aff^{1}_\rD$, where $\rD$ is the Dynkin scheme of $\rG$. Then we prove that for such $\rG$, the Steinberg's cross-section can be defined …
It was previously shown by Grunewald and Lubotzky that the automorphism group of a free group, $\text{Aut}(F_n)$, has a large collection of virtual arithmetic quotients. Analogous results were proved for the mapping class group by Looijenga and by Grunewald, Larsen, Lubotzky, and Malestein. In this paper, we prove analogous results for the automorphism group of a right-angled Artin group for a large collection of defining graphs. As a corollary of our methods we produce new virtual arithmetic quotients of $\text{Aut}(F_n)$ for $n \geq 4$ where $k$th powers of all transvections act trivially fo…
On the isomorphism problem for central extensions II
本文研究中心扩张的同构问题，在商群为单群或纯非阿贝尔群等新情形下，给出了两个中心扩张同构的充分必要条件，并刻画了保持商群不变及核群与商群同构时的同构问题。
Torsion-free nilpotent groups of small Hirsch length with isomorphic finite quotients
本文刻画了 Hirsch 长度不超过 5 的有限生成无挠幂零群中具有相同有限商同构类的群集合，并证明当 Hirsch 长度至少为 4 时，这类集合的基数可任意大。
Let F be a global field, and let S be a finite set of places of F containing all archimedean places. Consider the product X of the symmetric spaces and Bruhat-Tits buildings for PGL_d of the completions of F at archimedean and non-archimedean places in S, respectively. We construct compactifications of the quotient of X by S-arithmetic subgroups of PGL_d(F). The constructions make delicate use of reductive Borel-Serre spaces for archimedean places and polyhedral and seminorm compactifications at nonarchimedean places. We also briefly discuss a few potential applications of our compacifications…
To every $Q$-irreducible representation $r$ of a finite group $H$, there corresponds a simple factor $A$ of $Q[H]$ with an involution $τ$. To this pair $(A,τ)$, we associate an arithmetic group $Ω$ consisting of all $(2g-2)\times (2g-2)$ matrices over a natural order of $A^{op}$ which preserve a natural skew-Hermitian sesquilinear form on $A^{2g-2}$. We show that if $H$ is generated by less than $g$ elements, then $Ω$ is a virtual quotient of the mapping class group $Mod(Σ_g)$, i.e. a finite index subgroup of $Ω$ is a quotient of a finite index subgroup of $\Mod(Σ_g)$. This shows that the mapp…
The normal Farb growth of a group quantifies how well-approximated the group is by its finite quotients. We show that any S-arithmetic subgroup of a higher rank Chevalley group G has normal Farb growth n^dim(G).
Special subvarieties of non-arithmetic ball quotients and Hodge Theory
本文证明非算术球商中极大全测地子簇的无穷性蕴含格的算术性，并借助周期域与不可能相交理论建立了相应的 Ax-Schanuel 猜想。
Let $K$ be the function field of a curve $C$ over a field $\mathbb{F}$ of either odd or zero characteristic. Following the work by Serre and Mason on $\mathrm{SL}_2$, we study the action of arithmetic subgroups of $\mathrm{SU}(3)$ on its corresponding Bruhat-Tits tree associated to a suitable completion of $K$. More precisely, we prove that the quotient graph "looks like a spider", in the sense that it is the union of a set of cuspidal rays (the "legs"), parametrized by an explicit Picard group, that are attached to a connected graph (the "body"). We use this description in order to describe t…
We prove that there exists a countable metrizable topological group $G$ such that every countable metrizable group is isomorphic to a quotient of $G$. The completion $H$ of $G$ is a Polish group such that every Polish group is isomorphic to a quotient of $H$.
We show that the abstract commensurator of an S-arithmetic subgroup of a solvable algebraic group over Q is isomorphic to the Q-points of an algebraic group, and compare this with examples of nonlinear abstract commensurators of S-arithmetic groups in positive characteristic. In particular, we include a description of the abstract commensurator of the lamplighter group.
We prove that various arithmetic quotients of the unit ball in $\mathbb{C}^n$ are Mordellic, in the sense that they have only finitely many rational points over any finitely generated field extension of $\mathbb{Q}$. In the previously known case of compact hyperbolic complex surfaces, we give a new proof using their Albanese in conjunction with some key results of Faltings, but without appealing to the Shafarevich conjecture. In higher dimension, our methods allow us to solve an alternative of Ullmo and Yafaev. Our strongest result uses in addition Rogawski's theory and establishes the Mordell…
Baer characterized capable finite abelian groups (a group is capable if it is isomorphic to the quotient of some group by its center) by a condition on the size of the factors in the invariant factor decomposition (the group must be noncyclic and the top two invariant factors must be equal). We provide a different characterization, given in terms of a condition on the lattice of subgroups. Namely, a finite abelian group G is capable if and only if there exists a family {H_i} of subgroups of G with trivial intersection, such that the union generates G and all the quotients G/H_i have the same e…
Arithmetic subgroups of Chevalley group schemes over function fields I: quotients of the Bruhat-Tits building by $\{P\}$-arithmetic subgroups
本文研究函数域上 Chevalley 群概形的 A-点算术子群在 Bruhat-Tits 建筑上的作用，证明其轨道空间可描述为闭连通 CW-复形与由 Picard 群和群秩参数化的扇形腔室的粘合，并刻画了可嵌入轨道空间的子扇形面。
Let m and n be any integers with n>m>=2. Using just the entropy function it is possible to define a partial order on S_mn (the symmetric group on mn letters) modulo a subgroup isomorphic to S_m x S_n. We explore this partial order in the case m=2, n=3, where thanks to the outer automorphism the quotient space is actually isomorphic to a parabolic quotient of S_6. Furthermore we show that in this case it has a fairly simple algebraic description in terms of elements of the group ring.
Finite quotients, arithmetic invariants, and hyperbolic volume
本文研究闭双曲3维流形的有限完备化如何决定算术不变量，证明有限完备化同构可对应Zariski稠密表示，并在p进Borel调节子单射猜想下推出不变迹域、四元数代数、协体积与算术性均相同。
In this paper we produce many examples of thin subgroups of special linear groups that are isomorphic to the fundamental groups of non-arithmetic hyperbolic manifolds. Specifically, we show that the non-arithmetic lattices in $\mathrm{SO}(n,1)$ constructed by Gromov and Piatetski-Shapiro can be embedded into $\mathrm{SL}_{n+1}(\mathbb{R})$ so that their images are thin subgroups
This is the first of two papers in which we introduce and study two bivariate zeta functions associated to unipotent group schemes over rings of integers of number fields. One of these zeta functions encodes the numbers of isomorphism classes of irreducible complex representations of finite dimensions of congruence quotients of the associated group and the other one encodes the numbers of conjugacy classes of each size of such quotients. In this paper, we show that these zeta functions satisfy Euler factorizations and almost all of their Euler factors are rational and satisfy functional equati…
We prove that an irreducible lattice in a semisimple algebraic group is virtually isomorphic to an arithmetic lattice if and only if it admits a faithful self-similar action on a rooted tree of finite valency.
Profinite Non-Rigidity of Arithmetic Lattices and the Kähler Property
本文研究算术格的非有限刚性，刻画了允许非同构实形式的高秩李群绝对Dynkin类型，并证明有限呈现剩余有限群的Kähler性质不由其有限完备化决定。
Spectrum of weighted adjacency operator on a non-uniform arithmetic quotient of $PGL_3$
本文研究了 $PGL(3,\mathbb{F}_q[t])$ 商复形上加权邻接算子的自守谱，证明了其非平凡近似特征值集合包含同时谱与内摆线，并从组合角度重新证明了该商不是 Ramanujan 复形。
Spherical and affine Artin groups, finite quotients, and the isomorphism problem
本文证明球面和仿射Artin群的最小非交换商与相应Coxeter群的同构，并推出不可约球面Artin群由其有限商群决定。
Vertex volumes, lattice-minima tails, and height zeta functions for the standard arithmetic quotient of $\operatorname{PGL}_d$
该研究针对 $\text{PGL}_d$ 的标准算术商，基于建筑理论计算顶点体积，引入同伦不变归一性格极小值高度，确定其可积阈值与尖点尾估计，推导高度泽塔函数的收敛域与解析延拓性质，明确计算了 $d=3,4,5$ 时的有理函数。
Eisenstein series on arithmetic quotients of rank 2 Kac--Moody groups over finite fields
研究在有限域上秩2 Kac-Moody 群的算术商上定义并证明Eisenstein系列的收敛性与meromorphic继续性
Let $p$ be a fixed prime number and let $R$ denote a uniserial $p$-adic space group of dimension $d_x=(p-1)p^{x-1}$ and with cyclic point group of order $p^x$. In this short note we prove that all the quotients of $R$ of size bigger than or equal to $p^{d_x+x}$ have isomorphic mod $p$ cohomology groups. In particular, we show that the cohomology groups of sufficiently large quotients of the unique maximal class pro-$p$ group are isomorphic as $\F_p$-modules.
For every odd prime $p$ and every integer $n\geq 12$ there is a Heisenberg group of order $p^{5n/4+O(1)}$ that has $p^{n^2/24+O(n)}$ pairwise nonisomorphic quotients of order $p^{n}$. Yet, these quotients are virtually indistinguishable. They have isomorphic character tables, every conjugacy class of a non-central element has the same size, and every element has order at most $p$. They are also directly and centrally indecomposable and of the same indecomposability type. The recognized portions of their automorphism groups are isomorphic, represented isomorphically on their abelianizations, an…
We show that an arithmetic function which satisfies some weak multiplicativity properties and in addition has a non-decreasing or $\log$-uniformly continuous normal order is close to a function of the form $n\mapsto n^c$. As an application we show that a finitely generated, residually finite, infinite group, whose normal growth has a non-decreasing or a $\log$-uniformly continuous normal order is isomorphic to $(\mathbb{Z}, +)$.
We show that up to commensurability there are only finitely many cocompact arithmetic Kleinian groups generated by rotations. This implies, in particular, that there exist only finitely many conjugacy classes of cocompact two generated arithmetic Kleinian groups. The proof of the main result is based on a generalized Gromov--Guth inequality and bounds for the hyperbolic and tube volumes of the quotient orbifolds.
Let X=G/H be the quotient of a connected reductive algebraic C-group G defined over the field of complex numbers C by a finite subgroup H. We describe the topological fundamental group of the homogeneous space X, which is nonabelian when H is nonabelian. Further, we construct an example of a homogeneous space X and an automorphism s of C such that the topological fundamental groups of X and of the conjugate variety sX are not isomorphic.
Let $M = \mathbb{B}^2 / Γ$ be a smooth ball quotient of finite volume with first betti number $b_1(M)$ and let $\mathcal{E}(M) \ge 0$ be the number of cusps (i.e., topological ends) of $M$. We study the growth rates that are possible in towers of finite-sheeted coverings of $M$. In particular, $b_1$ and $\mathcal{E}$ have little to do with one another, in contrast with the well-understood cases of hyperbolic $2$- and $3$-manifolds. We also discuss growth of $b_1$ for congruence arithmetic lattices acting on $\mathbb{B}^2$ and $\mathbb{B}^3$. Along the way, we provide an explicit example of a l…
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