Reminding!
With the aim of safiety of all the participants we kindly ask to avoid any political discussions during our meetings.
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Aims and Scope
This seminar continues 2020 Ural Workshop on Group Theory and Combinatorics. The seminar covers modern aspects of group theory (including questions of actions of groups on combinatorial objects), graph theory, some combinatorial aspects of topology and optimization theory, and related topics and aims to support communications between specialists on Group Theory and Combinatorics all over the World.
The seminar will be held on Tuesdays, usually one time in 2 weeks, with possible some exceptions. The list of talks can be found below.
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Declarations
The situation in the World in the recent times is very complicate. However, we are going to keep the working of the Ural Seminar on Group Theory and Combinatorics as non-commercial and political-free meetings where people are able to share Math do not keeping in mind their nacionality, political positions, religion, gender, colour, and so on.
The official position of the Russian Academy of Sciences can be found in their official website. From our side, we declare that the participation in the Ural Seminar on Group Theory and Combinatorics cannot be considered directly as a declaration of any political or social position, except commitment to regular Humanitarian values and involving to Mathematical research. And with the aim of safiety of all the participants we kindly ask to avoid any political discussions during our meetings.
The only aim of the Ural Seminar on Group Theory and Combinatorics is to support communications between specialists on Group Theory and Combinatorics all over the world. We started in the tricky pandemic time and are going to continue in the same frame. Any specialist can register for free on the seminar website and attend the lectures online or see the records of the lectures. All the activities of speakers are for free and are aimed at develop of Mathematical research only.
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Scientific Committee
Chair: Natalia Maslova (N.N. Krasovskii Institute of Mathematics and Mechanics UB RAS and Ural Federal University)
Alexander Makhnev (N.N. Krasovskii Institute of Mathematics and Mechanics UB RAS and Ural Federal University) Danila Revin (Sobolev Institute of Mathematics SB RAS, Novosibirsk, Russia and N.N. Krasovskii Institute of Mathematics and Mechanics UB RAS, Yekaterinburg, Russia)
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Organizers
Chair: Natalia Maslova (N.N. Krasovskii Institute of Mathematics and Mechanics UB RAS and Ural Federal University)
Ivan Belousov (N.N. Krasovskii Institute of Mathematics and Mechanics UB RAS and Ural Federal University)
Mikhail Golubiatnikov (N.N. Krasovskii Institute of Mathematics and Mechanics UB RAS and Ural Federal University)
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Registration
To attend the seminar please register for free via this website. You do not need to register again if you was a participant of 2020 Ural Workshop on Group Theory and Combinatorics, to login to the seminar website you can use we the same login and password as for the workshop website.
In your registration form, you are welcome to give us some information on your mathematical interests.
We kindly ask invited speakers to register via this website to be available for mailings!
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Contributed talks
The option of 20-minutes contributed talks is available for participants from all over the World. We can shedulle some such talks after the talk of the main speaker. If somebody wants to give a contributed talk, please contct Natalia Maslova via butterson[at]mail.ru.
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April 23, 2024
Time: 4 p.m. by Yekaterinburg Speaker: Long Miao (Hohai University, Nanjing, China) Topic: Some new progress on the second maximal subgroups of finite groups Abstract. In this talk, we will introduce some new properties of subgroups and use these properties to consider the influence of second maximal subgroups on the structure of finite groups. Particularly, we obtained a new characterization of some special class of nonsolvable groups.
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March 26, 2024
Time: 4 p.m. by Yekaterinburg
Speaker: Ilya Gorshkov (Sobolev Institute of Mathematics SB RAS, Novosibirsk, Russia)
Topic: The structure of finite groups with restrictions on conjugate graph
Abstract. Consider a finite group $G$. For $g\in G$, denote by $g^G$ the conjugacy class of $G$ containing $g$, and by $|g^G|$ the size of $g^G$. Let $N(G)$ be the set of conjugacy class sizes of $G$ excluding $1$. We can define a graph structure on the set $N(G)$ in many ways. In [I. Gorshkov, Towards Thompson's conjecture for alternating and symmetric groups. J. Group Theory 19:2 (2016), 331--336] the conjugate graph $\Gamma(G)$ of a finite group $G$ was introduced. The set of vertices of the graph $\Gamma(G)$ is $N(G)\setminus \{1\}$, the vertices $a$ and $b$ are connected by an oriented edge from $a$ to $b$ if $a$ divides $b$ and $N(G)$ does not contain a number $d\neq a,b$ such that $a$ divides $d$ and $d$ divides $b$. In the talk we will discuss results on groups with restrictions on the conjugate graph.
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March 12,2024
Time: 12:00 by Yekaterinburg
Speaker: Marston Conder (University of Auckland, Auckland, New Zealand)
Topic: Some observations about normal quotients of symmetric graphs
Abstract. In her abstract for a talk at the (online) Ural Workshop on Group Theory and Combinatorics in August 2023, Cheryl Praeger mentioned a suggestion made by Caiheng Li in a 2001 paper (in the Bulletin of the London Math. Society) that `non-basic' $2$-arc-transitive graphs of prime-power order that occur as normal covers of smaller $2$-arc-transitive graphs might be rare and difficult to construct. This seemed suspicious to me, so I set myself the challenge of finding examples. That turned out to be remarkably easy. In this talk I'll briefly explain what is meant by $s$-arc-transitivity, normal quotients and normal covers, and then describe three infinite families of graphs that refute Caiheng's suggestion. One obvious family is that of the cycles of proper prime-power order $q>7$. Another is the (infinite) family of all $2$-arc-regular $3$-valent graphs of $2$-power-order that admit arc-reversing automorphisms of order $2$. The third is the family of all $n$-cube graphs $Q_n$ for even $n>2$. Also I'll explain how the family of $n$-cube graphs $Q_n$ for odd $n > 3$ contradicts one of the main theorems in Caiheng's 2001 paper.
Some of this is joint work with Primo{\v z} Poto{\v c}nik (Ljubljana)
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February 27, 2024
Time: 4 p.m. by Yekaterinburg
Speaker: Marta Morigi (Università di Bologna, Bologna, Italy)
Topic: The commuting probability for the Sylow subgroups of finite and profinite groups
Abstract can be found here.
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February 13, 2024
Time: 4 p.m. by Yekaterinburg
Speaker: Tatsuro Ito (Kanazawa University, Kanazawa, Japan and Anhui University, Hefei, China)
Topic: Modular T-algebras of trees
Abstract can be found here.
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January 30, 2024
Time: 4 p.m. by Yekaterinburg
Speaker: Alexander Makhnev (N.N. Krasovskii Institute of Mathematics and Mechanics UB RAS and Ural Federal University, Yekaterinburg, Russia)
Topic: On $AT4$-graphs, Shilla graphs, and distance-regular graphs of degree 44
Abtsract. Intersection arrays of $AT4$-graphs with $q\le 4$ and Shilla graphs with $b=6$ were enumerated. We prove that some Shilla graphs with $b\le 6$ such that $c_2$ does not divide $b_2$ do not exist. There are 7 feasible intersection arrays of distance-regular graphs of degree 44. We prove distance-regular graphs with 5 of these arrays do not exist.
Time: 5 p.m. by Yekaterinburg
Speaker: Maxim Mornev (EPFL SB MATH TN, Lausanne, Switzerland)
Topic: Local monodromy of Drinfeld modules
Abtsract. Compared with algebraic varieties the local monodromy of Drinfeld modules appears to be hopelessly complex: The image of the wild inertia subgroup under Tate module representations is generally infinite. Nonetheless I have recently discovered that Tate modules of Drinfeld modules are ramified in a limited way: The image of a sufficiently deep ramification subgroup is trivial. I plan to discuss this theorem and related results including ones from a forthcoming article with Pink. The talk is aimed at a general audience.
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December 19, 2023
Time: 4 p.m. by Yekaterinburg Speaker: Jack Koolen (School of Mathematical Sciences, University of Science and Technology of China, Hefei, China) Topic: Distance-regular graphs whose $q$-distance matrix has exactly one positive eigenvalue Abstract. The $q$ distance matrix $D_q$ of a connected graph is defined by $(D_q)_{xy} = 1 + \frac{1}{q} + \cdots + \frac{1}{q^{d-1}}$ when $d(x, y) = d \geq 1$ and 0 otherwise. In this talk we study this matrix for the class of distance-regular and show that if it has only one positive eigenvalue it must have certain nice properties. I also show that this happens for many infinite families of distance-regular graphs. This is based on joint work with Sakander Hayat, Mamonn Abdullah and Brhane Gebremichel.
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December 5, 2023
Time: 4 p.m. by Yekaterinburg
Speaker: Elena V. Konstantinova (China Three Gorges Mathematical Research Center, CTGU, Yichang, China, Sobolev Institute of Mathematics and Novosibirsk State University, Novosibirsk, Russia)
Topic: Revisiting the sequence reconstruction problem
Abstract can be found here.
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June 6, 2023
Time: 4 p.m. by Yekaterinburg
A series of three contributed talks.
4 p.m. Viachaslau Murashka (Francisk Skorina Gomel State University, Gomel, Belarus), On the question of T. Hawkes and arithmetic graphs of finite groups
4:30 p.m. Ruslan Skuratovsky (...), Permutational wreath product of symmetric groups and its normal subgroups
5 p.m. Boris Durakov (Siberian Federal University, Krasnoyarsk, Russia), On infinite groups with involutions saturated with finite Frobenius groups
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May 23, 2023
Time: 4 p.m. by Yekaterinburg
Speaker: Gülin Ercan (Middle East Technical University, Ankara, Turkey)
Topic: Noncoprime fixed point free action of a nilpotent group
Abstract can be found here.
The slides can be found here.
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April 25, 2023
Time: 4 p.m. by Yekaterinburg Speaker: Long Miao (Hohai University, Nanjing, China) Topic: Finite groups with non-nilpotent maximal subgroups of even order Abstract. Starting from the p-solvable groups, some new class of nonsolvable groups are given through commutator subgroups and Frattini subgroups of Sylow subgroups of the chief factors. And some new information about nonsolvable groups is obtained by characterizing them with second maximal subgroups from non-nilpotent maximal subgroups of even order.
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April 11, 2023
Time: 4 p.m. by Yekaterinburg Speaker: Sergey Gorchinskiy (Steklov Mathematical Institute of Russian Academy of Sciences, Moscow, Russia) Topic: Height pairing for points on surfaces Abstract. The famous Abel-Jacobi theorem describes an algebraic condition for the vanishing of integrals of holomorphic forms along paths on a Riemann surface, i.e., on a complex curve. We will recall this and describe what problems occur for complex surfaces. This leads to an invariant of a surface - a certain abelian group, which is one of key objects in algebraic geometry related to algebraic cycles. The main result of the talk consists in an explicit construction of non-zero elements in this group based on a new pairing between points on surfaces, which generalizes the height pairing for points on curves. This is a joined work with Dmitri Krekov.
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March 28, 2023
Time: 4 p.m. by Yekaterinburg Speaker: Long Miao (Hohai University, Nanjing, China) Topic: Finite groups with non-nilpotent maximal subgroups of even order Abstract. Starting from the p-solvable groups, some new class of nonsolvable groups are given through commutator subgroups and Frattini subgroups of Sylow subgroups of the chief factors. And some new information about nonsolvable groups is obtained by characterizing them with second maximal subgroups from non-nilpotent maximal subgroups of even order. The talk is postponed to April 25 by a technical reason.
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March 14, 2023
Time: 4 p.m. by Yekaterinburg
Speaker: Cristina Acciarri (University of Modena and Reggio Emilia, Italy, and University of Brasilia, Brazil)
Topic: On profinite groups with restricted centralizers of $\pi$-elements
Abstract. A group $G$ is said to have restricted centralizers if for each $g$ in $G$ the centralizer $C_G(g)$ either is finite or has finite index in $G$. This notion was introduced by A. Shalev, who proved that a profinite group in which all centralizers are restricted is virtually abelian, that is, has an abelian subgroup of finite index. In this talk we will consider a much more general situation, where the restrictedness condition is imposed only on $\pi$-elements of a profinite group $G$, for an arbitrary set of primes $\pi$.
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February 28, 2023
Time: 4 p.m. by Yekaterinburg
Topic: 2-modular representations of the Conway simple groups as binary codes
Abstract can be found here.
Contributed talks:
5 p.m. Olga Dashkova (MSU Branch in Sevastopol, Sevastopol, Russia) Ranks of a group
5:25 p.m. Irina Sokhor (Francisk Skorina Gomel State University, Gomel, Belarus) Finite groups with formational subnormal subgroups of bounded exponent
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February 14, 2023
Time: 4 p.m. by Yekaterinburg
Speaker: Alexey Galt (Sobolev Institute of Mathematics SB RAS, Novosibirsk, Russia)
Topic: On splitting of the normalizer of a maximal torus in groups of Lie type
Abstract can be found here.
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December 20, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Jin Guo (Hainan University, Hainan, China)
Topic: Nonsimple collections of cells whose polyomino ideals are prime
Abstract can be found here.
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December 6, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Carmine Monetta (Department of Mathematics, University of Salerno, Fisciano (SA), Italy)
Topic: Graphs encoding the structure of a finite group
Abstract can be found here.
Contributed talks:
5 p.m. Viktor Panshin (Sobolev Institute of Mathematics SB RAS and Novosibirsk State University, Novosibirsk, Russia) On characterization by Gruenberg-Kegel graph of finite simple exceptional groups of Lie type
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November 22, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Ilya Gorshkov (Sobolev Institute of Mathematics SB RAS, Novosibirsk, Russia)
Topic: The structure of finite groups with restrictions on the set of conjugacy classes
Abstract can be found here.
The slides can be found here.
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November 8, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Alexander Makhnev (N.N. Krasovskii Institute of Mathematics and Mechanics UB RAS and Ural Federal University, Yekaterinburg, Russia)
Topic: Classification strongly regular graphs with \lambda=0 and \mu=2
This is joint work with Mikhail Golubyatnikov
Abtsract. There is a potentially infinite series of intersection arrays of distance-regular graphs \Gamma=\Gamma(a,\mu) of diameter 5: (b + \mu, b + \mu -1, b, \mu,1; 1, \mu, b, b+\mu-1, b+\mu), where b=b_2(\Gamma) = (a^2-1)\mu/4. Antipodal quotient of \Gamma is a strongly regular graph with parameters ((a^2\mu + 2a - \mu + 6)(a^2\mu - 2a - \mu + 6)\mu)/16, (a^2 - 1)\mu^2/4 + \mu, 0, \mu). Thus, only in the case \mu \in \{2, 4, 6\} we have an infinite series of intersection arrays of \Gamma. In this talk, we prove that a distance-regular graph with \mu=2 and intersection array (a^2+1, a^2, a^2-1, 2, 1; 1, 2, a^2-1, a^2, a^2+1) exists only if a \in \{2, 3\}. In particular, a strongly regular graph with parameters ((a^4+3a^2+4)/2, a^2+1, 0, 2) does not exist for a>3.
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October 25, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Mercede Maj (Dipartimento di Matematica Universit`a di Salerno, Fisciano (Salerno), Italy)
Topic: Detecting properties of a finite group through the study of somefunctions on element orders
Abstract can be found here.
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October 11, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Michael Khachay (N.N. Krasovskii Institute of Mathematics and Mechanics UB RAS and Ural Federal University, Yekaterinburg, Russia)
Topic: Towards a polynomial time approximation of a symmetric routing problems within fixed ratios
Abstract. In this talk, we will discuss the first fixed-ratio polynomial time approximation algorithms are proposed for a series of asymmetric routing combinatorial optimization problems including the Steiner Cycle Problem (SCP), Rural Postman Problem (RPP), Generalized Traveling Salesman Problem (GTSP), Capacitated Vehicle Routing Problem with Unsplittable Customer Demands (CVRP-UCD), and Prize Collecting Traveling Salesman Problem (PCTSP). The presented results are shared the common property, all of them rely on polynomial-timel cost-preserving reduction to appropriate settings of the Asymmetric Traveling Salesman Problem (ATSP) and the seminal (22+ε)-approximation algorithm for this classical problem proposed by O. Svensson and V. Traub in 2019.
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June 7, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Gang Chen (School of Mathematics and Statistics, Central China Normal University, Wuhan, China)
Topic: Schur rings over some classes of groups
Abstract. An Schur ring A over a group G is called a traditional Schur ring if it is either a discrete Schur ring, a trivial Schur ring, a Schur ring of tensor product type, an orbit Schur ring, or a Schur ring of wedge product type. A group is called traditional if any Schur ring over it is traditional. Let $Z$ denote the infinite cyclic group of the set of integers with respect to addition of numbers. In this talk, it is proved that $Z \times Z_p$ is traditional, where $p$ is a prime with $p \le 13$; neither $Z \times Z$ nor $Z \times Z_2 \times Z_2$ is traditional. Finallly, Schur rings over alternating group on five letters which contain the set of 3-cycles as an S-set are classfied.
Contributed talks:
5 p.m. Ivan Timofeenko (Siberian Federal University, Krasnoyarsk, Russia), Generation of the Chevalley groups $E_l$ over the ring of integers by three involutions two of which commute
5:25 p.m. Yinfeng Zhu (Shanghai Jiao Tong University, Shanghai, China; Imperial College London, London, UK), Automata with an almost constant rank property on words
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May 24, 2022
Time: 4 p.m. by Yekaterinburg
A series of five contributed talks. The speakers are presented in the alphabet ordering.
4 p.m. Boris Durakov (Siberian Federal University, Krasnoyarsk, Russia) On periodic groups of 2-rank one
4:25 p.m. Andrei Kukharev (Siberian Federal University, Krasnoyarsk, Russia) Simple groups with Brauer trees of principal blocks in the shape of a star
4:50 p.m. Nikolai Minigulov (Krasovskii Institute of Mathematics and Mechanics UB RAS, Yekaterinburg, Russia) On finite groups which Gruenberg-Kegel graphs are isomorphic to the paw
5:15 p.m. Dmitry Panasenko (Chelyabinsk State University, Chelyabinsk, Russia) Vertex connectivity of some classes of divisible design graphs
5:40 p.m. Ruslan Skuratovsky (...) Verbal width by set of squares in alternating group A_n and Mathieu groups, squareness criterions in the group in PSL_2(F_p) and SL_2(F_p)
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April 26, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Yaokun Wu (Shanghai Jiao Tong University, Shanghai, China)
Topic: Hurwitz primitivity and \v{C}ern\'{y} function
Abstract. For each positive integer $m$, we use $[m]$ for the set of first $m$ positive integers. Let $\mathcal{A} = (A_1, \ldots, A_m)$ be an $m$-tuple of nonnegative $n\times n$ matrices. For each word $\alpha$ over $[m]$, say $\alpha=\alpha_1\cdots \alpha_s$, we write $\mathcal{A}_{\alpha}$ for the product $A_{\alpha_1}\cdots A_{\alpha_s} $. We call $\mathcal{A}$ primitive if $\mathcal{A}_{\alpha} > 0$ for a nonempty word $\alpha$ over $[m]$. We call $\mathcal{A}$ Hurwitz primitive provided there exists a nonnegative integer vector $\tau=(\tau(1),\ldots,\tau(m))$ such that for each $x,y\in [n]$ there exists a nonempty word $\alpha^{x,y}$ over $[m]$ such that $\mathcal{A}_{\alpha^{x,y}}(x,y)>0$ and the number of occurrence of $i$ in $\alpha^{x,y}$ is $\tau(i)$ for each $i \in [m]$. The $m$-tuple $\tau$ satisfying the above property is named a Hurwitz primitive vector of $\mathcal{A}$.
Let $\mathsf{NZ}_1$ denote the set of nonnegative matrices without zero rows and let $\mathsf{NZ}_2$ denote the set of nonnegative matrices without zero rows/columns. We give a unified combinatorial proof for the Protasov-Vonyov characterization (2012) of primitive $\mathsf{NZ}_2$-matrix tuples and the Protasov characterization (2013) of Hurwitz primitive $\mathsf{NZ}_1$-matrix tuples. By establishing a connection with synchronizing automata, for any Hurwitz primitive $m$-tuple $\mathcal{A}$ of $n\times n$ $\mathsf{NZ}_1$-matrices we give an $O(n^3m^2)$-time algorithm to find a Hurwitz primitive vector $\tau$ of $\mathcal{A}$ such that $\sum_{i\in [m]}\tau (i) = O(n^3)$. For any given $m$-tuple of $n\times n$ $\mathsf{NZ}_2$-matrices, we present an $O(n^2m)$-time algorithm to test whether or not it is primitive. We also report results on ergodic and Hurwitz ergodic matrix tuples.
This talk is based on a joint paper with Yinfeng Zhu, the current version of which can be found here.
Contributed talks:
5 p.m. Viktor Panshin (Sobolev Institute of Mathematics SB RAS and Novosibirsk State University, Novosibirsk, Russia) On recognition of $A_6 \times A_6$ by the set of conjugacy class sizes
5:25 p.m. Mikhail Golubiatnikov (Krasovskii Institute of Mathenatics and Mechanics UB RAS ans Ural Federal University, Yekaterinburg, Russia) Deza graphs related to intersections of conjugate subgroups in groups SL2(q)s
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April 12, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Fedor Dudkin (Sobolev Institute of Mathematics SB RAS, Novosibirsk, Russia)
Topic: Recent results on generalized Baumslag-Solitar groups
Abstract. A finitely generated group G acting on a tree with all vertex and edge stabilizers are infinite cyclic groups is called a generalized Baumslag–Solitar group (GBS-group). These groups turned out to be of interest because of their geometric, algorithmic, and group-theoretic properties. They have been actively studied during the last twenty years. Our goal is to tell about some recent results on GBS groups: outer automorphism group, description of the centralizer dimension, the problem of universal equivalence, K-residuality, connection with knot groups. Some open problems will be discussed at the end of the talk.
Contributed talks:
5 p.m. Dmitry Churikov (Sobolev Institute of Mathematics SB RAS, Novosibirsk, Russia) The minimal size of a generating set for primitive 3/2-transitive groups
5:25 p.m. Boris Durakov (Siberian Federal University, Krasnoyarsk, Russia) On infinite groups saturated with finite Frobenius groups of even orders
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March 29, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Grigory Ryabov (Sobolev Institute of Mathematics SB RAS, Novosibirsk, Russia)
Topic: On Cayley representations of central Cayley graphs over almost simple groups
Joint work with J. Guo, W. Guo, and A. Vasil'ev Abstract. A Cayley graph over a group $G$ is said to be central if its connection set is a normal subset of $G$. We prove that every central Cayley graph over a simple group $G$ has at most two pairwise nonequivalent Cayley representations over $G$ associated with the subgroups of $Sym(G)$ induced by left and right multiplications of $G$. We also provide an algorithm which, given a central Cayley graph $\Gamma$ over an almost simple group $G$ whose socle is of a bounded index, finds the full set of pairwise nonequivalent Cayley representations of $\Gamma$ over $G$ in time polynomial in size of $G$
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March 15, 2022
Time: 4 p.m. by Yekaterinburg
Topic: Graphs with a symmetrical Euler cycle
Joint work with: Jiyong Chen, Cai Heng Li, and Shu-Jiao Song
Abstract. This work was motivated by our wish to understand arc-transitive embeddings of graphs in surfaces, where the graphs might have more than one edge between adjacent vertices. So the graphs should be undirected, of finite valency with no loops, and admitting a subgroup of automorphisms which is transitive on arcs (incident vertex-edge pairs). Apart from some degenerate cases, the boundary of a face in such an embedding is a sequence of pairwise distinct edges which form a cycle in the graph, and the stabiliser of this cycle admits a cyclic group of automorphisms which is either regular or bi-regular on edges. Such a cycle is called a symmetrical Euler cycle of its edge-induced subgraph. We were curious: what kinds of graphs (with multiple edges allowed) admit a sequencing of all their edges into a symmetrical Euler cycle? And secondly (but this is a bit beyond the lecture), what kinds of symmetrical Euler cycles arise in arc-transitive maps? Answering the first question involved developing a group theoretic model for edge-transitive graphs. It was the basic tool (though not sufficient) we used to classify all graphs with a symmetrical Euler cycle.
Contributed talks:
5 p.m. Alexey S. Vasil'ev (Krasovskii Institute of Mathematics and Mechanics UB RAS, Yekaterinburg, Russia and Sobolev Institute of Mathematics and Mechanics SB RAS, Novosibirsk, Russia) Relatively maximal odd-index subgroups of symmetric groups
5:25 p.m. Artem Kravchuk (Novosibirsk State University, Novosibirsk, Russia) Spectrum of the Transposition graph
5:50 p.m. Ruslan Skuratovsky (...) Normal subgroups of iterated wreath products of symmetric groups and alternating with symmetric groups
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February 15, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Alexander Makhnev (N.N. Krasovskii Institute of Mathematics and Mechanics UB RAS and Ural Federal University, Yekaterinburg, Russia)
Topic: On distance-regular graphs $\Gamma$ of diameter 3 such that $\Gamma_3$ is strongly regular
Abstract. If $\Gamma$ is a simple graph, then define $\Gamma_3$ to be a graph with the same vertex set as $\Gamma$ in which two different vertices are adjacent if and only if they are at distance $3$ in $\Gamma$. If $\Gamma$ be a distance-regular graph with the second large eigenvalue $\theta_2=-1$, then the complement to $\Gamma_3$ is a pseudo-geometric graph for $pG_{c_3}(k,b_1/c_2)$ (Makhnev-Nirova). We investigate distance-regular graphs $\Gamma$ for which the complement to $\Gamma_3$ are pseudo-geometric for: 1) a net $pG_{t}(s,t)$ (Makhnev-Guo-Golubyatnikov); 2) a dual $2$-disign $pG_{t+1}(s,t)$ (Belousov-Makhnev); 3) a generalized quadrangle (Makhnev-Nirova). Moreover, we investigate distance-regular graphs $\Gamma$ with $\Gamma_3$ strongly regular without triangles (Belousov-Makhnev-Paduchikh).
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February 1, 2022
Time: 4 p.m. by Yekaterinburg
Speaker: Vladislav Kabanov (N.N. Krasovskii Institute of Mathematics and Mechanics UB RAS and Ural Federal University, Yekaterinburg, Russia)
Topic: New construction of strongly regular graphs with parameters of symplectic graphs
Abstract. A. Abiad and W.H. Haemers in [1] used Godsil-McKay switching to obtain strongly regular graphs with the same parameters as Sp(2d,2) for all d at least 3. F. Ihringer in [2] provided a general construction of strongly regular graphs from the collinearity graph of a finite classical polar spaces of rank at least 3 over a finite field of order q. Recently, A.E. Brouwer, F. Ihringer and W.M. Kantor in [3] described a switching operation on collinearity graphs of polar spaces to obtain graphs that satisfy the 4-vertex condition if the original graph belongs to a symplectic polar space.
In this talk we present new construction of strongly regular graphs with parameters of the complement of symplectic graphs. For our construction, we use new construction of divisible design graphs and do not use any switching.
[1] A. Abiad and W.H. Haemers, Switched symplectic graphs and their 2-ranks, Des. Codes Cryptogr., 81 (2016) 35-41. [2] F. Ihringer, A switching for all strongly regular collinearity graphs from polar spaces, J. Algebr. Comb., 46 (2017) 263-274. [3] A. E. Brouwer, F. Ihringer, W. M. Kantor, Strongly regular graphs satisfying the 4-vertex condition, arXiv:2107.00076v1 [math.CO] [4] W. H. Haemers, H. Kharaghani, M. Meulenberg, Divisible design graphs, J. Combinatorial Theory A, 118 (2011) 978-992.
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January 25, 2022
Time: 4 p.m. by Yekaterinburg
Topic: Diagonal structures and beyond
Abstract. Diagonal structures have been used in group theory since the mid-twentieth century. Recent work uses them in various combinatorial contexts, including Latin squares, Hamming graphs, folded cubes, and other graphs. All of these depend on the theory of the partial order on partitions of the same set, so the first part of this talk describes this theory. The second part tells the story of some statisticians who developed part of this theory, not always using the words "partition" or "partial order", and not usually talking to pure mathematicians. The next two parts describe diagonal semi-lattices and diagonal graphs. The final section generalizes both of these in a way analogous to generalizing a Latin square to a set of mutually orthogonal Latin squares.
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December 21, 2021
Time: 4 p.m. by Yekaterinburg Speaker: Long Miao (Hohai University, Yangzhou University, Yangzhou, China) Topic: Some new ideas on the class of nonsolvable groups Abstract. Starting from the p-solvable groups, some new class of nonsolvable groups are given through the chief factors of Sylow subgroups, commutator subgroups and Frattini subgroups of some nonsolvable groups. And some new information about nonsolvable groups is obtained by characterizing them with second maximal subgroups.
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December 7, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Vladimir Trofimov (Krasovskii Institute of Mathematics and Mechanics UB RAS, and Ural Federal University, Yeketerinburg, Russia)
Topic: Symmetrical extensions of graphs
Abstract. A connected graph $\Sigma$ is a symmetrical extention of a graph $\Gamma$ by a graph $\Delta$ if there are a vertex-transitive group $G$ of autumorphisms of $\Sigma$ and imprirmitivity system $\Sigma$ of $G$ on the vertex set of $\Sigma$ such that the quotient graph $\Sigma/\sigma$ is isomorphic to $\Gamma$ and blocks of $\sigma$ generate in $\Sigma$subgraphs isomorphic to $\Delta$. Symmetrical extensions of graphs are of interest for group theory, graph theory, topology, but also for crystallography and physics. In the talk the following question is discussed. Let $\Gamma$ be an infinite locally finite graph and $\Delta$ be a finite graph. Are there only finitely many (pairwise non-isomorphic) symmetrical extensions of $\Gamma$ by $\Delta$?Although in gene ral the question is answered in the negative, in some important cases of$\ Gamma$ and $\Delta$ the answer to the question is positive.
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November 23, 2021
Time: 4 p.m. by Yekaterinburg Speaker: Peter J. Cameron (University of St Andrews, UK) Topic: Generalizing EPPO groups by means of graphs Abstract. EPPO groups are finite groups in which all elements have prime power order. They were introduced by Higman in the 1950s, and the simple EPPO groups found by Suzuki in the 1960s, but the complete classification is more recent. There are two characterizations of EPPO groups in terms of graphs: they are the groups whose Gruenberg-Kegel graph has no edges, and also groups whose power graph and enhanced power graph coincide. Based on these and similar ideas, I propose several problems involving classes of groups widere than EPPO groups, and give a few preliminary results.
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November 9, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Anatoly Kondrat'ev (Krasovskii Institute of Mathematics and Mechanics UB RAS, Ural Federal University, and Ural Mathematical Center, Yeketerinburg, Russia)
Topic: On finite groups with given properties of Gruenberg-Kegel graphs.
Abstract. The Gruenberg-Kegel graph (or the prime graph) of a finite group G is a (labelled) graph in which the vertices are the prime divisors of the order of G, and two distinct vertices p and q are adjacent in this graph if and only if G contains an element of order pq. This graph is a fundamental arithmetical invariant of a finite group which have numerous applications. This talk is devoted to some problems and results on the study of finite groups with given properties of their Gruenberg-Kegel graphs.
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October 26, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Jinbao Li (Department of Mathematics, Chongqing University of Arts and Sciences, Chongqing, China)
Topic: On weaker quantitative characterization of finite nonabelian simple groups.
Abstract. In the past forty years, several kinds of quantitative characterizations of finite groups especially finite simple groups have been investigated by many mathematicians, such as quantitative characterizations by group order and element orders, by element orders alone, by the set of sizes of conjugacyclasses, by dimensions of irreducible characters, by the set of orders of maximal abelian subgroups. In this talk, we will introduce some weaker quantitative characterizations of finite nonabeliansimple groups by their orders together with some special quantitative properties such as the largestelement orders, the largest conjugacy class sizes and the number of prime-order elements.
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October 12, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Andrei Mamontov (Sobolev Institute of Mathematics SB RAS and Novosibirsk State University, Novosibirsk, Russia)
Topic: On periodic groups with a given spectrum.
Abstract. The spectrum of a periodic group is the set of its element order. A periodic group is called a group with a dense spectrum, or $OC_n$-group, if its spectrum consists of all integers from 1 to some fixed number $n$. In the talk we discuss periodic $OC_n$-groups ($n\leq 7$).
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September 28, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Valeriy G. Bardakov (Sobolev Institute of Mathematics SB RAS and Novosibirsk State University, Novosibirsk, Russia)
Topic: Quandles: Algebraic theory and applications
Abstract. Quandle is a non-empty set with one binary algebraic operations which satisfies to three axioms. At first they arrive in Knot Theory, but now Quandle Theory is a part of Abstract algebra like Group Theory or Ring Theory. On my talk I give a definition and examples of quandles, explain their connection with groups, give a geometric interpretation of quandles, describe some interesting classes of quandles. We discuss connection of quandles with Knot Theory and with set-theoretic solutions of the Yang-Baxter equation. Further I introduce some properties of quandles: residually finiteness, orderability, and formulate results on quandles which have these properties.
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September 14, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Lev Kazarin (P.G. Demidov Yaroslavl State University, Yaroslavl, Russia)
Topic: Conjugacy class sizes and factorizations of finite groups
Abstract. The aim of the talk is to give a short survey concerning recent progress in the study of groups with factorizations and its relation with the structure of groups with an information on the sizes of a conjugacy classes of groups.
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June 8, 2021
Time: 4 p.m. by Yekaterinburg
Topic: Recent results on the eigenvalue 1 problem for representations offinite groups of Lie type
Abstract. In this talk I shall discuss some aspects of the problem of determining unisingular iredicible representations of finite simple groups of Lie type over fields of describing characteristic. Some recent results will be exposed and commented. A representation $\phi$ of a group $G$ is called unisingular if 1 is an eigenvalue of $\phi(g)$ for every $g\in G$.
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May 25, 2021
Time: 2 p.m. by Yekaterinburg
Speaker: Gareth Jones (University of Southampton, Southampton, UK)
Topic: Paley, Carlitz and the Paley graphs
Abstract. Anyone who seriously studies algebraic graph theory or finite permutation groups will, sooner or later, come across the Paley graphs and their automorphism groups. The most frequently cited sources for these are respectively Paley's 1933 paper for their discovery, and Carlitz's 1960 paper for their automorphism groups. It is remarkable that neither of those papers uses the concepts of graphs, groups or automorphisms. Indeed, one cannot find these three terms, or any synonyms for them, in those papers: Paley's paper is entirely about the construction of what are now called Hadamard matrices, while Carlitz's is entirely about permutations of finite fields.
The aim of this talk is to explain how this strange situation came about, by describing the background to these two papers and how they became associated with the Paley graphs. This involves links with other branches of mathematics, such as matrices, number theory, block designs, coding theory, finite geometry, polytopes and group theory, reaching back to 1509, with important contributions from Coxeter and Todd, Sachs, and Erd\H os and R\'enyi. I will also briefly cover some recent developments concerning surface embeddings of Paley graphs. A preprint is available at https://arxiv.org/abs/1702.00285.
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May 11, 2021
Time: 4 p.m. by Yekaterinburg
Topic: Constructing tetravalent half-arc-transitive graphs
Abstract. Half-arc-transitive graphs are a fascinating topic, which connects graph theory, Riemann surfaces and group theory. Although fruitful results have been obtained over the last half a century, it is still challenging to construct half-arc-transitive graphs with prescribed vertex stabilizers. In this talk, I'll focus on the tetravalent case, giving new constructions of half-arc-transitive graphs with various vertex stabilizers. This sheds light on the larger problem of which groups can be the vertex stabilizer of a tetravalent half-arc-transitive graph.
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April 27, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Jack Koolen (The University Science and Technology of China, Hefei, China)
Topic: Improving Neumaier's Theorem on strongly regular graphs
Abstract. In this talk I will discuss a Theorem of Neumaier and some recent improvements.
This is based on joint work with Gary Greaves and Jongyook Park.
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April 13, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Stephen Glasby (The University of Western Australia, Perth, Australia)
Topic: Recognizing classical groups
Abstract. Dr Who has been captured by the evil Celestial Toymaker. In order to be released, Dr Who must recognize a large (finite) classical group G
(known only to the Toymaker) in under 5 minutes. The elements of G are
encoded as strings of 0s and 1s, and so are not familiar dxd matrices
over GF(q) preserving a certain non-degenerate form. Dr Who is allowed to
1. choose random elements,
2. multiply elements,
3. invert elements, and
4. test the order of elements of G,
in order to (constructively and quickly) recognize G.
I shall first explain why the Toymaker's problem is central to computational
group theory, and why a quick solution is highly desirable. In so
doing, we will briefly review some key ideas for matrix group recognition
before reducing the Toymaker's problem to the following geometric problem.
Given two (small-dimensional) non-degenerate subspaces U, U' of a
symplectic/unitary/orthogonal space V, what is the probability that
the subspace U + U' is non-degenerate and of dimension dim(U) + dim(U')?
(The sum U + U' is usually not perpendicular.)
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March 30, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Alexander Buturlakin (Sobolev Institute of Mathematics of the Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia)
Topic: Structure of locally finite groups and some classes of subgroups
Abstract. We study the interplay between the structure of a locally finite group and some properties of its subgroups. Three classes of subgroups are considered: centralizers, cyclic subgroups, and Hall subgroups. We describe the structure of a locally finite group in which the lengths of chains of nested centralizers are finite and uniformly bounded. We finish a description of the spectra (the sets of orders of elements) of all finite simple groups and study the algorithmic aspect of the problem of recognition of finite simple groups by their spectra. Finally, we give a criteria for the existence of a solvable Hall subgroup in a finite group.
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March 16, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Pablo Spiga (Department of Mathematics and Applications, University of Milano-Bicocca, Italy)
Topic: A generalization of Sims conjecture for finite primitive groups and two point stabilizers
Abstract. In this talk we first discuss the classic Sims' conjecture on finite primitive groups. Then, we propose a refinement of Sims conjecture and we present some modest progress towards the proof of this refinement.
By analysing this refinement, when dealing with primitive groups of diagonal type, we construct a finite primitive group G on X and two distinct points x,y in X with G_x\cap G_y normal in G_x and G_x\cap G_y \ne 1, where G_x and G_y are the stabilizers of x and y in G. In particular, this example gives an answer to a question raised independently by Peter Cameron and by Anatily Fomin.
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March 9, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Ilia Ponomarenko (St.Petersburg Department of V.A.Steklov Institute of Mathematics of RAS, St.Petersburg, Russia)
Topic: The 3-closure of a solvable permutation group is solvable
Based on joint work with E.A. O'Brien, A.V. Vasil'ev, and E.P. Vdovin
Abstract. Let m be a positive integer and let V be a finite set. The m-closure of G<Sym(V)is the largest permutation group on V having the same orbits as G in itsinduced action on the Cartesian product V^m. The 1-closure and 2-closure of asolvable permutation group need not be solvable. We prove that the m-closureof a solvable permutation group is always solvable for m>2.
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February 16, 2021
Time: 4 p.m. by Yekaterinburg
Topic: Maximality of Seidel matrices and switching roots of graphs
Abstract. In this talk, we discuss maximality of Seidel matrices with a fixed largest eigenvalue and fixed rank. We present a classification of maximal Seidel matrices of largest eigenvalue 3, which gives a classification of maximal equiangular lines in a Euclidean space with angle arccos(1/3). This may sound like a problem which has already been completed in 1970's by Seidel and others. However, maximality of equiangular lines with a fixed rank seems to be considered only recently. The use of a switching root, newly introduced in our work, facilitates the classification and puts the problem in the context of root systems in a canonical manner. Motivated by the maximality of the exceptional root system E_8, we define strong maximality of a Seidel matrix, and show that every Seidel matrix achieving the absolute bound is strongly maximal. Thus, the Seidel matrix of order 276 coming from the McLaughlin graph is strongly maximal. This is based on joint work with Meng-Yue Cao, Jack H. Koolen and Kiyoto Yoshino.
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February 2, 2021
Time: 4 p.m. by Yekaterinburg
Speaker: Gareth Jones (University of Southampton, Southampton, UK)
Topic: Primitive permutation groups of prime degree
Abstract. The study of transitive permutation groups of prime degree can be traced back two and a half centuries, through Burnside and Galois, to the work of Lagrange on polynomials of prime degree. It is sometimes asserted that the groups of prime degree are now completely known, as a consequence of the classification of finite simple groups: apart from a few interesting but easily-understood exceptions, there are infinite families of affine, alternating and symmetric groups, together with various projective groups related to $PSL_n(q)$, all acting naturally in those cases when their natural degree is prime. Although true, this assertion ignores an apparently difficult number-theoretic problem, namely whether or not there exist infinitely many primes equal to the natural degree $(q^n-1)/(q-1)$ of $PSL_n(q)$. Such primes are also relevant to alternative versions of Waring's problem. In joint work with Sasha Zvonkin I shall present heuristic arguments and computational evidence to support a conjecture that for each prime $n\ge 3$ there are infinitely many primes of this form, even if one considers only prime values of $q$.
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January 19, 2021
Time: 4 p.m. by Yekaterinburg
Topic: Finite edge-transitive Cayley graphs, quotient graphs and Frattini groups
Joint work with Behnam Khosravi, Institute of Advanced Studies in Basic Sciences, Zanjan, Iran
Abstract. The edge-transitivity of a Cayley graph is most easily recognisable if the subgroup of “affine maps” preserving the graph structure is itself edge-transitive. These are the so-called normal edge-transitive Cayley graphs. Each of them determines a set of quotients which are themselves normal edge-transitive Cayley graphs and are built from a very restricted family of groups (direct products of simple groups). We address the questions: how much information about the original Cayley graph can we retrieve from this set of quotients? And can we ever reconstruct the original Cayley graph from them: if so, then how? Our answers to these questions involve a type of “relative Frattini subgroup” determined by the Cayley graph, which has similar properties to the Frattini subgroup of a finite group – I’ll discuss this and give some examples. It raises many new questions about Cayley graphs.
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December 22, 2020
Time: 2 p.m. by Yekaterinburg
Topic: Some applications of finite group theory in the design of experiments
Abstract. Group theory is used in (at least) two different ways in the design of experiments.
The first is in randomization, the process by which an initial design is turned into the actual layout for the experiment by applying a permutation of the experimental units, chosen at random from a certain group of permutations. Which group? What properties should it have?
The second is in design construction. The set of treatments is identified with a finite Abelian group, and the blocks are all translates of one or more initial blocks. The characters of this group form its dual group: they are the eigenvectors of the matrix that we need to consider to see how good the proposed design is.
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December 8, 2020
Date: December 8, 2020 Time: 2 p.m. by Yekaterinburg Speaker: Peter J. Cameron (University of St Andrews, UK) Topic: Graphs on groups: old and new connections Abstract. Several graphs defined on the vertex set of a group have been studied. Theseinclude the commuting graph, introduced by Brauer and Fowler in 1955, the powergraph (Kelarev and Quinn 1999) and the enhanced power graph (Aalipour et al.2017). It turns out that there are connections with other topics in grouptheory, including the Gruenberg--Kegel graph and Schur covers, as well asapplications in computational group theory. I will discuss some of thesethings, including the most recent, a graph which lies between the enhancedpower graph and the commuting graph.
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