Aims and Scope
This seminar continues 2020 Ural Workshop on Group Theory and Combinatorics. The seminar aims to cover 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.
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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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)
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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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December 8, 2020
Date: December 8, 2020
Time: 2 p.m. by Yekaterinburg
Speaker: Peter 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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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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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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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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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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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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