The Fesenko groups have finite width

Finitely generated infinite simple groups of infinite commutator width

It is shown that there exists a finitely generated infinite simple group of infinite commutator width, and that the commutator width of a finitely generated infinite boundedly simple group can be arbitrarily large. Besides, such groups can be constructed with decidable word and conjugacy problems.

Generating groups by conjugation-invariant sets

Let S be a generating set of a group G. We say that G has FINITE WIDTH relative to S if G=(S\cup S^{-1})^k for a suitable natural number k. We say that a group G is a group of FINITE C-WIDTH if G has finite width with respect to all conjugation-invariant generating sets. We give a number of examples of groups of finite C-width, and, in particular, we prove that the commutator subgroup F' of Thompson's group F is a group of finite C-width. We also study the behaviour of the class of all groups of finite C-width under some group-theoretic constructions; it is established, for instance, that this…

Finitely generated infinite simple groups of infinite square width and vanishing stable commutator length

It is shown that there exist finitely generated infinite simple groups of infinite commutator width and infinite square width on which there exists no stably unbounded conjugation-invariant norm, and in particular stable commutator length vanishes. Moreover, a recursive presentation of such a group with decidable word and conjugacy problems is constructed.

Cayley Graph Expanders and Groups of Finite Width

We present new infinite families of expander graphs of vertex degree 4, which is the minimal possible degree for Cayley graph expanders. Our first family defines a tower of coverings (with covering indices equals 2) and our second family is given as Cayley graphs of finite groups with very short presentations with only 2 generators and 4 relations. Both families are based on particular finite quotients of a group G of infinite upper triangular matrices over the ring M(3,F2). We present explicit vector space bases for the finite abelian quotients of the lower exponent-2 groups of G by upper tri…

Infinite-dimensional general linear groups are groups of universally finite width

Recently George Bergman proved that the symmetric group of an infinite set possesses the following property which we call by the {\it universality of finite width}: given any generating set $X$ of the symmetric group of an infinite set $Ω,$ there is a uniform bound $k \in \N$ such that any permutation $σ\in \text{Sym}(Ω)$ is a product of at most $k$ elements of $X \cup X^{-1},$ or, in other words, $\text{Sym}(Ω)=(X^{\pm 1})^k.$ Bergman also formulated a sort of general conjecture stating that `the automorphism groups of structures that can be put together out of many isomorphic copies of thems…

Lie Methods in Growth of Groups and Groups of Finite Width

In the first, mostly expository, part of this paper, a graded Lie algebra is associated to every group G given with an N-series of subgroups. The asymptotics of the Poincare series of this algebra give estimates on the growth of the group G. This establishes the existence of a gap between polynomial growth and growth of type $e^{\sqrt n}$ in the class of residually-p groups, and gives examples of finitely generated p-groups of uniformly exponential growth. In the second part, we produce two examples of groups of finite width and describe their Lie algebras, introducing a notion of Cayley graph…

On the widths of finite groups (I)

Let $G$ be a finite group, $π(G)$ be the set of prime divisors dividing the order of $G$ and $π_e(G)$ (spectrum) denote the set of element orders of $G$. We define $w_o(G)$ = $|π(G)|$ the width of order of $G$ and $w_s(G)$ = max$\{|π(k)| | k \in π_e(G)\}$ the width of spectrum of $G$. In this paper, we discuss the cases of $w_o(G)$ and $w_s(G)$ are small, prove several new results and give a survey about the two widths of groups.

可达性与孪生宽度

Accessibility and Twin-width

研究有限生成群中有限孪生宽度与可达性的关系，通过结合威尔克斯构造及博内等人的结果，证明存在有限生成群$\Gamma$，其有有限一致孪生宽度但不可达，相应凯莱图也如此。

Palindromic words in simple groups

A palindrome is a word that reads the same left-to-right as right-to-left. We show that every simple group has a finite generating set $X$, such that every element of it can be written as a palindrome in the letters of $X$. Moreover, every simple group has palindromic width $pw(G,X)=1$, where $X$ only differs by at most one Nielsen-transformation from any given generating set. On the contrary, we prove that all non-abelian finite simple groups $G$ also have a generating set $S$ with $pw(G,S)>1$. As a by-product of our work we also obtain that every just-infinite group has finite palindromic wi…

On the p-width of finite simple groups

In this paper we measure how efficiently a finite simple group $G$ is generated by its elements of order $p$, where $p$ is a fixed prime. This measure, known as the $p$-width of $G$, is the minimal $k\in \mathbb{N}$ such that any $g\in G$ can be written as a product of at most $k$ elements of order $p$. Using primarily character theoretic methods, we sharply bound the $p$-width of some low rank families of Lie type groups, as well as the simple alternating and sporadic groups.

拟随机性与均匀孪生宽度

Quasirandomness and Uniform Twin-Width

本文研究有限群的拟随机度与均匀孪生宽度的关系，证明前者给出后者的多项式下界、最小忠实复表示次数给出线性上界，非阿贝尔有限单群中二者多项式等价，还构造了特定有限表现群并确定三个Thompson群的均匀孪生宽度。

Bounded generation and commutator width of Chevalley groups: function case

We prove that Chevalley groups over polynomial rings $\mathbb F_q[t]$ and over Laurent polynomial $\mathbb F_q[t,t^{-1}]$ rings, where $\mathbb F_q$ is a finite field, are boundedly elementarily generated. Using this we produce explicit bounds of the commutator width of these groups. Under some additional assumptions, we prove similar results for other classes of Chevalley groups over Dedekind rings of arithmetic rings in positive characteristic. As a corollary, we produce explicit estimates for the commutator width of affine Kac--Moody groups defined over finite fields. The paper contains als…

Twin-width VII: groups

Twin-width is a recently introduced graph parameter with applications in algorithmics, combinatorics, and finite model theory. For graphs of bounded degree, finiteness of twin-width is preserved by quasi-isometry. Thus, through Cayley graphs, it defines a group invariant. We prove that groups which are abelian, hyperbolic, ordered, solvable, or with polynomial growth, have finite twin-width. Twin-width can be characterised by excluding patterns in the self-action by product of the group elements. Based on this characterisation, we propose a strengthening called uniform twin-width, which is sta…

Commutator width in the first Grigorchuk group

Let $G$ be the first Grigorchuk group. We show that the commutator width of $G$ is $2$: every element $g\in [G,G]$ is a product of two commutators, and also of six conjugates of $a$. Furthermore, we show that every finitely generated subgroup $H\leq G$ has finite commutator width, which however can be arbitrarily large, and that $G$ contains a subgroup of infinite commutator width. The proofs were assisted by the computer algebra system GAP.

单宽子群、通用元素和有限群的lambda数

Uni-width subgroups, universal elements, and lambda number of finite groups

本文研究了有限群的单宽子群、通用元素和lambda数，证明了幂图存在非恒等通用元素的条件，并给出了lambda数的最优界。

On the width of transitive sets: bounds on matrix coefficients of finite groups

We say that a finite subset of the unit sphere in $\mathbf{R}^d$ is transitive if there is a group of isometries which acts transitively on it. We show that the width of any transitive set is bounded above by a constant times $(\log d)^{-1/2}$. This is a consequence of the following result: If $G$ is a finite group and $ρ: G \rightarrow \mbox{U}_d(\mathbf{C})$ a unitary representation, and if $v \in \mathbf{C}^d$ is a unit vector, there is another unit vector $w \in \mathbf{C}^d$ such that \[ \sup_{g \in G} |\langle ρ(g) v, w \rangle| \leq (1 + c \log d)^{-1/2}.\] These results answer a questi…