G2R2

August 06-19, 2018 - Novosibirsk, Russia

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G2R2-Summer School is presented by four courses each of which contains eight 50-minutes lectures.
G2R2-Summer School Timetable see here.

Lecturers and Minicourses

 
Gareth Davis

Gareth Jones

Title: Groups and symmetry in low dimensional geometry and topology (12 hours)

Description: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 Coxeter group [3,5,3] plays a similar role.

Outline of the course:

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

Bibliography:

1. M. D. E. Conder, An update on Hurwitz groups, Groups, Complexity and Cryptology 2 (2010) 25-49.
2. E. Girondo and G. Gonzalez-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.

Evan Davis

Gareth Jones

University of Southampton, UK

Group and symmetry in low dimensional geometry and topology

•••
Horace Dediu

Akihiro Munemasa

Title: Permutation representations of finite groups and association schemes (12 hours)

Description: 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 (pdf)
Lecture 2. Permutation modules and the centralizer algebra (pdf)
Lecture 3. Spherical functions and eigenvalues (pdf)
Lecture 4. The holomorph of a group (pdf)
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

List of problems

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.

Akihiro Munemasa

Tohoku University, Japan

Permutation representations of finite groups and association schemes

•••
Horace Dediu
Fraser Speirs

Mikhail Muzychuk

Title: Coherent configurations and association schemes: structure theory and linear representations (12 hours)

Description: 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:

Lecture 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.

Lecture 1
Lecture 2

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.

Mikhail Muzychuk

Ben-Gurion University of the Negev, Israel

Coherent configurations and association schemes: structure theory and linear representations

•••
Fraser Speirs
Elliot Jay Stocks

Roman Nedela

Title: Graph coverings and harmonic morphisms between graphs (12 hours)

Description: 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.

Outline of the course:

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); 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 and uniformisation.

Graph Coverings 1 (pdf)
Graph Coverings 2 (pdf)
Graph Coverings 3 (pdf)
Graph Coverings 4 (pdf)
Graph Coverings 5 (pdf)
Graph Coverings 6 (pdf)
Graph Coverings 7 (pdf)

Bibliography
1. A. Mednykh, R. Nedela, Harmonic morphisms of graphs, Part I: Graph Coverings, Matej Bel University, 2015.

Roman Nedela

University of West Bohemia, Czech Republic

Graph coverings and harmonic morphisms between graphs

•••
Elliot Jay Stocks

G2R2-Siberian Summer School

G2R2-summer school is included into the program of Siberian Summer Schools of Novosibirsk State University. In the frame of this program, G2R2-summer school is extended by a cultural program including local tours and a day trip to Altai. Novosibirsk State University awards 10 scholarships for this Summer School Program. At the end of the G2R2-summer school students will either pass a written examination or present a talk at the G2R2-conference. Everyone will be given a certificate of attendance. Please visit G2R2-Siberian Summer School to get more information.

List of Siberian Summer School Participants

Aljohani Mohammed, Taibah University, Saudi Arabia

Baykalov Anton, The University of Auckland, New Zealand

Berikkyzy Zhanar, University of California, USA

Cho Eun-Kyung, Pusan National University, South Korea

Chen Huye, China Three Gorges University, China

Churikov Dmitry, Novosibirsk State University, Russia

Dogra Riya, Shiv Nadar University, India

El Habouz Youssef, University ibn Zohr, Morocco

Evans Rhys, Queen Mary University of London, UK

Fu Zhuohui, Northwestern Polytechnical University, China

Jin Wanxia, Northwestern Polytechnical University, China

Kim Jan, Pusan National University, South Korea

Kaushan Kristina, Novosibirsk State University, Russia

Khomyakova Ekaterina, Novosibirsk State University, Russia

Konstantinov Sergey, Novosibirsk State University, Russia

Kwon Young Soo, Yeungnam University, Korea

Lin Boyue, Northwestern Polytechnical University, China

Mattheus Sam, Vrije Universiteit Brussel, Belgium

Mednykh Ilya, Novosibirsk State University, Russia

Morales Ismael, Autonomous University of Madrid, Spain

Puri Akshay A., Shiv Nadar University, India

Qian Chengyang, Shanghai Jiao Tong University, China

Ryabov Grigory, Novosibirsk State University, Russia

Song Mengmeng, Northwestern Polytechnical University, China

Sotnikova Ev, Sobolev Institute of Mathematics, Russia

Smith Dorian, USA

Vuong Bao, Novosibirsk State University, Russia

Wang Guanhua, Northwestern Polytechnical University, China

Wang Hui, Northwestern Polytechnical University, China

Wang Jingyue, Northwestern Polytechnical University, China

Xiong Yanzhen, Shanghai Jiao Tong University, China

Xu Zeying, Shanghai Jiao Tong University, China

Yang Zhuoke, Moscow Institute of Physics and Technology, Russia

Yin Yukai, Northwestern Polytechnical University, China

Yu Tinzoe, Hebei Normal University, China

Zhang Yue, Northwestern Polytechnical University, China

Zhao Da, Shanghai Jiao Tong University, China

Zhao Yupeng, Northwestern Polytechnical University, China

Zhu Yan, Shanghai University, China

Zhu Yinfeng, Shanghai Jiao Tong University, China

Important dates

G2R2-Siberian Summer School: August 1-22, 2018

Enjoy the Art of Mathematics with us!