具体描述
"Applications of Group Theory to Combinatorics" contains 11 survey papers from international experts in combinatorics, group theory and combinatorial topology. The contributions cover topics from quite a diverse spectrum, such as design theory, Belyi functions, group theory, transitive graphs, regular maps, and Hurwitz problems, and present the state-of-the-art in these areas. "Applications of Group Theory to Combinatorics" will be useful in the study of graphs, maps and polytopes having maximal symmetry, and is aimed at researchers in the areas of group theory and combinatorics, graduate students in mathematics, and other specialists who use group theory and combinatorics.Jack Koolen teaches at the Department of Mathematics at Pohang University of Science and Technology, Korea. His main research interests include the interaction of geometry, linear algebra and combinatorics, on which he published 60 papers. Jin Ho Kwak is Professor at the Department of Mathematics at Pohang University of Science and Technology, Korea, where he is director of the Combinatorial and Computational Mathematics Center (Com2MaC). He works on combinatorial topology, mainly on covering enumeration related to Hurwitz problems and regular maps on surfaces, and published more than 100 papers in these areas. Ming-Yao Xu is Professor in Department of Mathematics at Peking University, China. The focus in his research is in finite group theory and algebraic graph theory. Ming-Yao Xu published over 80 papers on these topics.
Applications of Group Theory to Combinatorics: A Deep Dive into Abstract Structures and Discrete Worlds This volume explores the profound and often unexpected intersections between the abstract machinery of group theory and the tangible world of combinatorial enumeration and structure. Rather than presenting a survey of existing applications, this book focuses on developing a cohesive framework that leverages group-theoretic concepts—such as symmetry, permutation representation, and character theory—to solve challenging problems in counting, design theory, coding theory, and graph enumeration. The central theme is the translation of combinatorial objects into representations of specific groups, allowing powerful algebraic tools to illuminate otherwise intractable counting problems. The initial chapters lay the groundwork by establishing the necessary algebraic foundations without assuming prior advanced knowledge of representation theory. We begin with a rigorous treatment of permutation groups, focusing intently on the concept of transitivity and the structure of stabilizers. This is crucial, as most combinatorial applications stem from analyzing the action of a group on a set of configurations (e.g., colorings, labelings, or arrangements). We introduce the Orbit-Stabilizer Theorem not just as a counting tool, but as the fundamental principle underpinning the Pólya Enumeration Theorem (PET). The heart of the book lies in the comprehensive treatment of the Pólya Enumeration Theorem (PET) and its generalizations. We move beyond simple cycle index calculations, dedicating substantial sections to the application of PET in analyzing patterns under various symmetry groups. This includes detailed case studies on enumerating necklaces, bracelets, and polyominoes, meticulously deriving the cycle indices for dihedral, cyclic, and full symmetry groups relevant to geometric arrangements. Crucially, we extend the discussion to Exponential Generating Functions (EGFs) within the context of PET, exploring how exponential structures (like rooted trees or functional mappings) necessitate the use of EGFs instead of ordinary generating functions (OGFs), providing a complete picture of how structural constraints influence the appropriate generating function framework. A significant departure from standard texts is the dedicated focus on Burnside's Lemma as a pre-cursor to Character Theory in Enumeration. While Burnside's Lemma often suffices for basic counting under permutation groups, we demonstrate its limitations when dealing with weighted enumeration or structures where the coloring scheme itself imposes algebraic constraints. This motivates the introduction of character theory. We systematically develop the concepts of irreducible representations, characters, and orthogonality relations for finite groups. The connection is forged by showing how the structure of the permutation representation of the group action on the set of colorings can be decomposed into irreducible components. This decomposition provides a far more nuanced count than simple fixed-point counting, particularly useful in advanced topics like enumerating inequivalent labelings of graphs or molecules. The second half of the book shifts focus to more specialized combinatorial domains where group theory plays a structural, rather than purely enumerative, role. Group Theory in Design Theory and Coding: We explore how the concept of automorphism groups defines the equivalence classes of combinatorial designs (such as Steiner systems or block designs). We analyze the search for specific symmetric designs (e.g., Hadamard matrices or projective planes) by treating the incidence structure as a set acted upon by a presumed group of symmetries. The constraint that the structure must be invariant under the group action significantly prunes the search space. Following this, we introduce coding theory, specifically focusing on Group Codes and Cyclic Codes. Here, the group structure is intrinsic to the code construction itself—often leveraging the cyclic group $mathbb{Z}_n$ or related finite fields. We demonstrate how the algebraic properties derived from Fourier transforms over these groups (akin to character theory) lead directly to efficient methods for calculating minimum distances and correcting errors. Symmetry and Graph Theory: A major section is devoted to the enumeration of non-isomorphic graphs, rooted and unrooted, under the action of various automorphism groups. We detail techniques for constructing adjacency matrices and analyzing their spectral properties in relation to the group action. The application of PET to map colorings is expanded to encompass labeling problems on graphs, providing explicit formulas for counting labelings that respect certain symmetries. Furthermore, we delve into the construction of Cayley graphs and vertex-transitive graphs, where the very definition of the graph relies upon the algebraic structure of the underlying group and a chosen set of generators. Analyzing the properties of these graphs—such as connectivity and diameter—becomes inherently linked to the group's internal structure. Advanced Applications: Quantum Information and Algebra: For advanced readers, the final chapters touch upon cutting-edge applications. We explore how tensor products of permutation representations lead to tools applicable in the nascent field of quantum combinatorics, particularly in analyzing non-local correlations via group-theoretic constraints (e.g., in Bell inequalities derived from group actions). Furthermore, we provide a detailed study on the use of Sylow Theorems not for enumeration, but for guaranteeing the existence of specific subgroups within the automorphism groups of highly regular combinatorial structures, which can simplify the structural analysis dramatically. Throughout the text, emphasis is placed on concrete examples rooted in geometry and finite sets, ensuring that the transition from abstract group axioms to practical counting formulas is transparent and robust. The book aims not merely to show that group theory works, but how its foundational concepts of action, invariance, and representation structure provide the essential language for modern combinatorial analysis.