GRAPHS AND GROUPS, REPRESENTATIONS AND RELATIONS (G2R2-2018)

1 Aljohani Mohammed, Taibah University, Saudi Arabia
2 Baykalov Anton, The University of Auckland, New Zealand
3 Berikkyzy Zhanar, University of California, USA
4 Cho Eun-Kyung, Pusan National University, South Korea
5 Chen Huye, China Three Gorges University, China
6 Churikov Dmitry, Novosibirsk State University, Russia
7 Dogra Riya, Shiv Nadar University, India
8 El Habouz Youssef, University ibn Zohr, Morocco
9 Evans Rhys, Queen Mary University of London, UK
10 Fu Zhuohui, Northwestern Polytechnical University,China
11 Jin Wanxia, Northwestern Polytechnical University,China
12 Kim Jan, Pusan National University, South Korea
13 Kaushan Kristina, Novosibirsk State University, Russia
14 Khomyakova Ekaterina, Novosibirsk State University, Russia
15 Konstantinov Sergey, Novosibirsk State University, Russia
16 Kwon Young Soo, Yeungnam University, Korea
17 Lin Boyue, Northwestern Polytechnical University,China
18 Mattheus Sam, Vrije Universiteit Brussel, Belgium
19 Mednykh Ilya, Novosibirsk State University, Russia
20 Morales Ismael, Autonomous University of Madrid, Spain
21 Puri Akshay A, Shiv Nadar University, India
22 Qian Chengyang, Shanghai Jiao Tong University, China
23 Ryabov Grigory, Novosibirsk State University, Russia
24 Song Mengmeng, Northwestern Polytechnical University,China
25 Sotnikova Ev, Sobolev Institute of Mathematics, Russia
26 Smith Dorian, USA
27 Vuong Bao, Novosibirsk State University, Russia
28 Wang Guanhua, Northwestern Polytechnical University,China
29 Wang Hui, Northwestern Polytechnical University,China
30 Wang Jingyue, Northwestern Polytechnical University,China
31 Xiong Yanzhen, Shanghai Jiao Tong University, China
32 Xu Zeying, Shanghai Jiao Tong University, China
33 Yang Zhuoke, Moscow Institute of Physics and Technology, Russia
34 Yin Yukai, Northwestern Polytechnical University,China
35 Yu Tinzoe, Hebei Normal University, China
36 Zhang Yue, Northwestern Polytechnical University,China
37 Zhao Da, Shanghai Jiao Tong University, China
38 Zhao Yupeng, Northwestern Polytechnical University,China
39 Zhu Yan, Shanghai University, China
40 Zhu Yinfeng, Shanghai Jiao Tong University, China

There is an alternative cultural program on the period of August, 3-5, for participants are desirous to take a 3-days Altai tour. The tour fee is 13200 Rub (except for the train from Novosibirsk to Biysk and back). The trip covers the most beautiful places of Altai region. All information and even a detailed map you can find via the link/
If you wish to travel to Altai please contact the G2R2-organizers by email: g2@math.nsc.ru.

The course considers some applications of group theory to geometry in dimensions 2 and 3. The main theme will be the study of the automorphism groups of compact Riemann surfaces, especially those surfaces uniformised by subgroups of finite index in triangle groups. By Belyi's Theorem, these are the compact Riemann surfaces which, when regarded as complex algebraic curves, can be defined over an algebraic number field. As such, they give a faithful representation of the absolute Galois group (the automorphism group of the field of algebraic numbers), a group of great complexity and importance in algebraic geometry. These surfaces include the Hurwitz surfaces, those attaining Hurwitz's upper bound of 84(g-1) for the size of the automorphism group of a compact Riemann surface of genus g>1. The course will consider the corresponding Hurwitz groups, the finite quotients of the (2,3,7) triangle group, and it will conclude with a brief look at the corresponding situation in dimension 3, where the normaliser of the Coхeter group [3,5,3] plays a similar role.

Outline of the course:
Lecture 1. Riemann surfaces and Fuchsian groups.
Lecture 2. Compact Riemann surfaces and their automorphism groups.
Lecture 3. Triangle groups and their quotients.
Lecture 4. Maps and hypermaps on surfaces.
Lecture 5. Dessins d'enfants, and Belyi's Theorem.
Lecture 6. The absolute Galois group, and its action on dessins.
Lecture 7. Hurwitz groups and surfaces.
Lecture 8. Hyperbolic 3-manifolds with large symmetry groups.

Bibliography
1. M. D. E. Conder, An update on Hurwitz groups, Groups, Complexity and Cryptology 2 (2010) 25-49.
2. E. Girondo and G. González-Diez, Introduction to Compact Riemann Surfaces and Dessins d'Enfants, LMS Student Texts 79, Cambridge University Press, 2012.
3. G. A. Jones, Bipartite graph embeddings, Riemann surfaces and Galois groups, Discrete Math. 338 (2015) 1801-1813.
4. G. A. Jones and D. Singerman, Complex Functions: An Algebraic and Geometric Viewpoint, Cambridge University Press, 1986.
5. G. A. Jones and J. Wolfart, Dessins d'Enfants on Riemann Surfaces, Springer, 2016. 6. G. A. Jones, Highly Symmetric Maps and Dessins, Matej Bel University, 2015.

A covering between two graphs is a graph epimorphism which is locally bijective. Although the concept of coverings of topological spaces was well known in algebraic topology for a long time, the systematic combinatorial approach to graph coverings is related to the solution of the Heawood map colouring problem by Ringel and Youngs. Nowadays the concept of graph coverings forms an integral part of graph theory and has found dozen of applications, in particular, as a strong construction technique. The aim of the course is to explain foundations of the combinatorial theory of graph coverings and its extension to branched coverings between 1-dimensional orbifolds.

In the course we shall follow the attached plan.
Part 1. Graphs and Groups: graphs with semi-edges and their fundamental groups; actions of groups on graphs; highly symmetrical graphs; subgroup enumeration in some finitely generated groups; enumeration of conjugacy classes of subgroups.
Part 2. Graph coverings and voltage spaces: graph coverings and two group actions on a fibre; voltage spaces; permutation voltage space; Cayley voltage space, Coset voltage space; equivalence of coverings and T-reduced voltage spaces; enumeration of coverings.
Part 3. Applications of graph coverings: regular graphs with large girth; large graphs of given degree and diameter; nowhere-zero flows and coverings; 3-edge colourings of cubic graphs; Heawood map coloring problem.
Part 4. Lifting automorphism problem: classical approach; lifting of graph automorphisms in terms of voltages; lifting problem - case of abelian CT(p); case of elementary abelian CT(p).
Part 5. Branched coverings of graphs: definition and basic properties; Riemann Hurwitz theorem for graphs; Laplacian of a graph and the Matrix-Tree Theorem; Jacobians and harmonic morphisms; graphs of groups, uniformisation.

Bibliography
1. Gross, Jonathan L.; Tucker, Thomas W. Topological graph theory, Wiley-Interscience Series in Discrete Mathematics and Optimization. A Wiley-Interscience Publication. John Wiley & Sons, Inc., New York, 1987. xvi+351 pp. ISBN: 0-471-04926-3
2. A. Mednykh, R. Nedela, Harmonic morphisms of graphs, Part I: Graph Coverings, Matej Bel University, 2015.

In these lectures, we first introduce the theory of permutation representations of finite groups. The existence of a canonical basis of a permutation module makes it different from a general module, leading to numerical invariants such as Krein parameters. Krein parameters are an analogue of tensor product coefficients for irreducible representations, as seen by Scott's theorem. We then discuss multiplicity-free permutation representations in detail, giving a motivation to a more general concept of commutative association schemes. Lack of a group in the definition leads to slight discrepancy in theory, and a long standing conjecture about splitting fields.

Outline of the course:
Lecture 1. Transitive permutation groups and orbitals
Lecture 2. Permutation modules and the centralizer algebra
Lecture 3. Spherical functions and eigenvalues
Lecture 4. The holomorph of a group
Lecture 5. Krein parameters and Scott's theorem
Lecture 6. Association schemes as an abstract centralizer algebra
Lecture 7. Eigenmatrices of association schemes
Lecture 8. Splitting fields of association schemes

Bibliography
1. E. Bannai and T. Ito. Algebraic Combinatorics I: Association Schemes, Benjamin/Cummings, Menlo Park, 1984.
2. A. Munemasa. Splitting fields of association schemes, J. Combin. Theory, Ser.A, 57 (1991), 157-161.

The goal of my lectures is to give an introduction to the theory of coherent configurations with the main focus on a particular case of association schemes. The closely related objects like Schur rings and table algebras will be presented too. In my lectures I will talk about the structure and representation theories of association schemes. A connection between coherent configurations and permutation groups known as Galois correspondence will be discussed too. Some classical results and new developments in this area with their applications will be presented. I also will remind and discuss some open problems in this area.

Outline of the course:
Lectures 1-2-3. Coherent configurations. Association schemes. Schur rings. Table algebras (main definitions and basic properties)
Lecture 4. Galois correspondence between coherent configurations and permutation groups. Schurian coherent configurations and 2-closed permutation groups.
Lecture 5-6. Representation theory of coherent configurations (the semisimple case). Frame number. Applications of representation theory.
Lecture 7. Structure theory of association schemes. Closed subsets, quotients, normal and strongly normal closed subsets.
Lecture 8. Primitive association schemes.

Bibliography
1. Z. Arad, E. Fisman, M. Muzychuk, Generalized table algebras, Israel J. of Mathematics, 114(1999), pp. 29-60.
2. A. Hanaki and K. Uno, Algebraic structure of association schemes of prime order, J. Algebr. Comb. 23 (2006), pp.189–195.
3. D. Higman, Coherent algebras, Linear Algebra and Its Applications, 93 (1987), pp. 209-239.
4. B. Wiesfeiler, On construction and identification of graphs, LNM 558, Springer, 1974
5. P.-H. Zieschang, Algebraic approach to association schemes, LNM 1628, Springer, 1996.