## Category Archives: New publications

### Visit to Heinrich-Heine-Universitaet Duesseldorf

In July, Anitha Thillaisundaram visited the University of Duesseldorf and made significant progress on her joint paper “The normal Hausdorff spectrum of pro-p groups” with Benjamin Klopsch and Amaia Zugadi-Reizabal. Her other joint paper “Maximal subgroups and irreducible representations of generalised multi-edge spinal groups” with Benjamin Klopsch has been accepted by the Proceedings of the Edinburgh Mathematical Society.

### New paper accepted in Journal of Number Theory

The paper by Sandro Mattarei and Roberto Tauraso, From generating series to polynomial congruenceshas been accepted for publication in Journal of Number Theory.

(You may find the final version in preprint form at https://arxiv.org/pdf/1703.02322.pdf.)

Abstract: Consider an ordinary generating function $\sum_{k=0}^{\infty}c_kx^k$, of an integer sequence of some combinatorial relevance, and assume that it admits a closed form $C(x)$. Various instances are known where the corresponding truncated sum $\sum_{k=0}^{q-1}c_kx^k$, with $q$ a power of a prime $p$, also admits a closed form representation when viewed modulo $p$. Such a representation for the truncated sum modulo $p$ frequently bears a resemblance with the shape of $C(x),$ despite being typically proved through independent arguments. One of the simplest examples is the congruence $\sum_{k=0}^{q-1}\binom{2k}{k}x^k\equiv(1-4x)^{(q-1)/2}\pmod{p}$ being a finite match for the well-known generating function $\sum_{k=0}^\infty\binom{2k}{k}x^k= 1/\sqrt{1-4x}$. We develop a method which allows one to directly infer the closed-form representation of the truncated sum from the closed form of the series for a significant class of series involving central binomial coefficients. In particular, we collect various known such series whose closed-form representation involves polylogarithms ${\rm Li}_d(x)=\sum_{k=1}^{\infty}x^k/k^d$, and after supplementing them with some new ones we obtain closed-forms modulo $p$ for the corresponding truncated sums, in terms of finite polylogarithms $\pounds_d(x)=\sum_{k=1}^{p-1}x^k/k^d$.

### Call for new Kourovka problems in Group Theory

Evgeny Khukhro announced a start for preparation of the new 19th edition of the Kourovka Notebook” — the famous collection of unsolved problems in Group Theory and related areas. This publication originated in Novosibirsk in 1965, since then gained world-wide popularity and now includes more than 1000 problems by about 400 authors from all over the world. Nowadays it is published on the web but still retains discrete issues appearing every 3–4 years, with more frequent updates online.

For more than 50 years the “Kourovka Notebook” has served as a unique means of communication for researchers in Group Theory and nearby fields of mathematics. Maybe the most striking illustration of its success is the fact that more than 3/4 of the problems from the first issue have now been solved! (Of course, it is often easier to propose a new problem than to solved an old one…)

Everybody is welcome to propose new problems to be included in the new edition. Problems may “belong” to those who propose them, or otherwise. In the latter case, one can indicate the author(s) of the problem (if different from the person proposing), or simply that this is a “well-known problem”. In order that the progress would be “measured” and seen, the preference is usually given to concrete questions that admit “yes” or “no” answers.

The Editors also welcome any other comments on, or/and solutions of, existing problems; the current version on Arxiv incorporates all comments so far.

The problems and comments can be sent to any of the Editors (preferably by e-mail):

Evgeny Khukhro khukhro@yahoo.co.uk or Victor Mazurov mazurov@math.nsc.ru

### New paper accepted in Journal of Algebra

The paper by Evgeny Khukhro and Pavel Shumyatsky “Almost Engel compact groups”http://dx.doi.org/10.1016/j.jalgebra.2017.04.021, has just been accepted for publication in Journal of Algebra.

Abstract: We say that a group $G$ is almost Engel if for every $g\in G$ there is a finite set ${\mathscr E}(g)$ such that for every $x\in G$ all sufficiently long commutators $[...[[x,g],g],\dots ,g]$ belong to ${\mathscr E}(g)$, that is, for every $x\in G$ there is a positive integer $n(x,g)$ such that $[...[[x,g],g],\dots ,g]\in {\mathscr E}(g)$ if $g$ is repeated at least $n(x,g)$ times. (Thus, Engel groups are precisely the almost Engel groups for which we can choose ${\mathscr E}(g)=\{ 1\}$ for all $g\in G$.) We prove that if a compact (Hausdorff) group $G$ is almost Engel, then $G$ has a finite normal subgroup $N$ such that $G/N$ is locally nilpotent. If in addition there is a unform bound $|{\mathscr E}(g)|\leq m$ for the orders of the corresponding sets, then the subgroup $N$ can be chosen of order bounded in terms of $m$. The proofs use the Wilson–Zelmanov theorem saying that Engel profinite groups are locally nilpotent.

### New paper

Sandro Mattarei and Roberto Tauraso, From generating series to polynomial congruences, 24 pages, submitted, 2017; arXiv:1703.02322.

Abstract: Consider an ordinary generating function $\sum_{k=0}^{\infty}c_kx^k$, of an integer sequence of some combinatorial relevance, and assume that it admits a closed form $C(x)$. Various instances are known where the corresponding truncated sum $\sum_{k=0}^{q-1}c_kx^k$, with $q$ a power of a prime $p$, also admits a closed form representation when viewed modulo $p$. Such a representation for the truncated sum modulo $p$ frequently bears a resemblance with the shape of $C(x)$, despite being typically proved through independent arguments. One of the simplest examples is the congruence $\sum_{k=0}^{q-1}\binom{2k}{k}x^k\equiv(1-4x)^{(q-1)/2}\pmod{p}$ being a finite match for the well-known generating function $\sum_{k=0}^\infty\binom{2k}{k}x^k= 1/\sqrt{1-4x}$.

We develop a method which allows one to directly infer the closed-form representation of the truncated sum from the closed form of the series for a significant class of series involving central binomial coefficients. In particular, we collect various known such series whose closed-form representation involves polylogarithms $\mathrm{Li}_d(x)=\sum_{k=1}^{\infty}x^k/k^d$, and after supplementing them with some new ones we obtain closed-forms modulo $p$ for the corresponding truncated sums, in terms of finite polylogarithms $\pounds_d(x)=\sum_{k=1}^{p-1}x^k/k^d$.

### New update for “Unsolved problems in group theory (Kourovka Notebook)”

“Kourovka Notebook” is a collection of open problems  in Group Theory proposed by more than 300 mathematicians from all over the world. It has been published every 2-4 years since 1965. This is the 18th edition, which contains 120 new problems and a number of comments on about 1000 problems from the previous editions.

This new update for the current 18th edition has been posted at http://arxiv.org/abs/1401.0300. In addition, for convenience of the readers, all the changes made since the 18th edition first appeared are also listed separately in 18upd-e. We thank all the people who help us keeping Kourovka Notebook up to date.

Evgeny Khukhro khukhro@yahoo.co.uk and Victor Mazurov mazurov@math.nsc.ru, Editors

### Paper accepted for publication

The paper  by Evgeny Khukhro and Pavel Shumyatsky Engel-type subgroups and length parameters of finite groups has been accepted for publication in Israel Journal of Mathematics. The results of the paper have been obtained in collaboration between Evgeny Khukhro of University of Lincoln and Pavel Shumyatsky of University of Brasilia, with Evgeny’s visits to Brasilia supported by CNPq-Brazil grant within the Brazilian Scientific Mobility Program “Ciências sem Fronteiras”.

Abstract: Let $g$ be an element of a finite group $G$. For a positive integer $n$, let $E_n(g)$ be the subgroup generated by all commutators  $[...[[x,g],g],\dots ,g]$ over $x\in G$,  where $g$ is repeated $n$ times. By Baer’s theorem, if $E_n(g)=1$, then  $g$ belongs to the Fitting subgroup $F(G)$. We generalize this theorem in terms of certain  length parameters of $E_n(g)$.  For soluble $G$ we prove that if, for some $n$, the Fitting height of $E_n(g)$ is equal to $k$, then $g$ belongs to the $(k+1)$th Fitting subgroup $F_{k+1}(G)$. For nonsoluble $G$ the results are in terms of  nonsoluble length and generalized Fitting height. The generalized Fitting height $h^*(H)$ of a finite group $H$ is the least number $h$ such that $F^*_h(H)=H$, where  $F^*_0(H)=1$, and $F^*_{i+1}(H)$ is the inverse image of the generalized Fitting subgroup $F^*(H/F^*_{i}(H))$. Let $m$ be the number  of prime factors of $|g|$ counting multiplicities. It is proved that if, for some $n$, the  generalized Fitting height of $E_n(g)$ is equal to $k$, then $g$ belongs to $F^*_{f(k,m)}(G)$, where $f(k,m)$ depends only on $k$ and $m$. The nonsoluble length $\lambda (H)$ of a finite group $H$ is defined as the minimum number of nonsoluble factors in a normal series each of whose factors either is soluble or is a direct product of nonabelian simple groups. It is proved that if  $\lambda (E_n(g))=k$, then $g$ belongs to a normal subgroup whose nonsoluble length is bounded in terms of $k$ and $m$. We also state conjectures of stronger results independent of $m$ and show that these conjectures reduce to a certain question about automorphisms of direct products of finite simple groups.

Full text: https://arxiv.org/abs/1506.00233

### Paper accepted for publication

The paper by E. I. Khukhro and P. Shumyatsky, Almost Engel finite and profinite groups  has been accepted for publication in the International  Journal of Algebra and Computation, ISSN  0218-1967, see also arXiv:1512.06097.

Abstract: Let $g$ be an element of a group $G$. For a positive integer $n$, let $E_n(g)$ be the subgroup generated by all commutators $[...[[x,g],g],\dots, g]$ over $x\in G$, where $g$ is repeated $n$ times. We prove that if $G$ is a profinite group such that for every $g\in G$ there is $n=n(g)$ such that $E_n(g)$ is finite, then $G$ has a finite normal subgroup $N$ such that $G/N$ is locally nilpotent. The proof uses the Wilson–Zelmanov theorem saying that Engel profinite groups are locally nilpotent. In the case of a finite group $G$, we prove that if, for some $n$, $|E_n(g)|\leq m$ for all $g\in G$, then the order of the nilpotent residual $\gamma _{\infty} (G)$ is bounded in terms of $m$.

### New update for “Unsolved problems in group theory (Kourovka Notebook)”

New update for the current 18th edition has been posted at http://arxiv.org/abs/1401.0300 .
In addition, for convenience of the readers, all the changes made since the 18th edition first appeared are also listed separately in 18upd-e . We thank all the people who help us keeping Kourovka Notebook up to date.

Evgeny Khukhro khukhro@yahoo.co.uk and Victor Mazurov mazurov@math.nsc.ru , Editors

### New paper

E. I. Khukhro and P. Shumyatsky, Almost Engel finite and profinite groups, submitted, 2015; arXiv:1512.06097.

Abstract: Let $g$ be an element of a group $G$. For a positive integer $n$, let $E_n(g)$ be the subgroup generated by all commutators $[...[[x,g],g],\dots, g]$ over $x\in G$, where $g$ is repeated $n$ times. We prove that if $G$ is a profinite group such that for every $g\in G$ there is $n=n(g)$ such that $E_n(g)$ is finite, then $G$ has a finite normal subgroup $N$ such that $G/N$ is locally nilpotent. The proof uses the Wilson–Zelmanov theorem saying that Engel profinite groups are locally nilpotent. In the case of a finite group $G$, we prove that if, for some $n$, $|E_n(g)|\leq m$ for all $g\in G$, then the order of the nilpotent residual $\gamma _{\infty} (G)$ is bounded in terms of $m$.