Combinatorial Reciprocity Theorems: An Invitation To Enumerative Geometric Combinatorics is a book by Matthias Beck and Raman Sanyal published in 2018 in the Graduate Studies in Mathematics series of the American Mathematical Society. A combinatorial reciprocity is a curious phenomenon: A polynomial, whose evaluations at positive integers count combinatorial objects of some sort, may give the number of combinatorial objects of a different sort when evaluated at negative integers (and suitably normalized). Such combinatorial reciprocity theorems occur in connections with graphs, partially ordered sets, polyhedra, and more. With combinatorial reciprocity theorems as a leitmotif, this book unfolds central ideas and techniques in enumerative and geometric combinatorics. Written as an accessible graduate textbook with almost 300 exercises, numerous illustrations, and pointers to the research literature, topics include concise introductions to partially ordered sets, polyhedral geometry, and rational generating functions, followed by highly original chapters on subdivisions, geometric realizations of posets, and hyperplane arrangements. Excerpts from Reviews "Nearly half a century [after Stanley's paper "Combinatorial reciprocity theorems"], the book under review pays a loving ode to Stanley's paper and extends it to over 300 pages, forming a comprehensive work of reference to combinatorial reciprocity theorems old and new. This book is a cleverly and beautifully woven tapestry, however, and the combinatorial reciprocity theorems make it both possible and necessary to introduce enumerative and geometrical combinatorics more generally. Among the interconnecting subtopics are classes of combinatorial polynomials, partially ordered sets, polytopes and the Euler characteristic, rational generating functions, hyperplane arrangements, and many more. Accessible to undergraduate students--who are helped along by a wealth of interesting problems (though no solutions) at the end of each chapter--and an invaluable work of reference for researchers, this is an impressive and beautifully written book. It shines with clarity and the authors' evident joy, in the beautiful maths at hand and in the authors' delivery of it to the reader." Zentralblatt MATH "This book presents these combinatorial reciprocity theorems and a number of others that have a geometric origin, as is made clear by the subtitle of the book. [...] They rephrase various counting problems in terms of polyhedral geometry, and then prove combinatorial reciprocity phenomena in this setting. The content of this book thus illustrates the fact that geometry can help shed light on a number of problems that have a priori a purely combinatorial nature. [...] All in all, the authors gather numerous topics in a single impressive work, and organize the material in a very engaging and unique fashion." Mathematical Reviews

"First, it is necessary to study the facts, to multiply the number of observations, and then later to search for formulas that connect them so as thus to discern the particular laws governing a certain class of phenomena. In general, it is not until after these particular laws have been established that one can expect to discover and articulate the more general laws that complete theories by bringing a multitude of apparently very diverse phenomena together under a single governing principle."
Augustin Louis Cauchy

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