Günter Harder – författare

This book studies the interplay between the geometry and topology of locally symmetric spaces, and the arithmetic aspects of the special values of L-functions.The authors study the cohomology of locally symmetric spaces for GL(N) where the cohomology groups are with coefficients in a local system attached to a finite-dimensional algebraic representation of GL(N). The image of the global cohomology in the cohomology of the Borel–Serre boundary is called Eisenstein cohomology, since at a transcendental level the cohomology classes may be described in terms of Eisenstein series and induced representations. However, because the groups are sheaf-theoretically defined, one can control their rationality and even integrality properties. A celebrated theorem by Langlands describes the constant term of an Eisenstein series in terms of automorphic L-functions. A cohomological interpretation of this theorem in terms of maps in Eisenstein cohomology allows the authors to study the rationality properties of the special values of Rankin–Selberg L-functions for GL(n) x GL(m), where n + m = N. The authors carry through the entire program with an eye toward generalizations.This book should be of interest to advanced graduate students and researchers interested in number theory, automorphic forms, representation theory, and the cohomology of arithmetic groups.

This book studies the interplay between the geometry and topology of locally symmetric spaces, and the arithmetic aspects of the special values of L-functions.The authors study the cohomology of locally symmetric spaces for GL(N) where the cohomology groups are with coefficients in a local system attached to a finite-dimensional algebraic representation of GL(N). The image of the global cohomology in the cohomology of the Borel–Serre boundary is called Eisenstein cohomology, since at a transcendental level the cohomology classes may be described in terms of Eisenstein series and induced representations. However, because the groups are sheaf-theoretically defined, one can control their rationality and even integrality properties. A celebrated theorem by Langlands describes the constant term of an Eisenstein series in terms of automorphic L-functions. A cohomological interpretation of this theorem in terms of maps in Eisenstein cohomology allows the authors to study the rationality properties of the special values of Rankin–Selberg L-functions for GL(n) x GL(m), where n + m = N. The authors carry through the entire program with an eye toward generalizations.This book should be of interest to advanced graduate students and researchers interested in number theory, automorphic forms, representation theory, and the cohomology of arithmetic groups.

This book discusses the mathematical interests of Joachim Schwermer, who throughout his career has focused on the cohomology of arithmetic groups, automorphic forms and the geometry of arithmetic manifolds.

The objects of concern in this book are the sheaf-cohomology groups of locally symmetric spaces attached to arithmetically defined groups Γ, contained in an algebraic group. The sheaves are associated to Γ-modules over the ring of integers or finite extension rings thereof. There is an action of a Hecke algebra on these cohomology groups. Accordingly, they decompose into indecomposable pieces which are of great interest in number theory and algebraic geometry. Upon extending the coefficient systems to complex vector spaces, an individual indecomposable component corresponds to a space of automorphic forms, thereby, giving rise to an attached L-function. For example, the Ramanujan Delta function provides the first case of such a correspondence. Up to division by a carefully chosen period, these L-functions take rational (algebraic) values at certain critical arguments. In various examples, it is discussed how the numerator and denominator ideals of these normalised values shed some light on the integral structure of the cohomology groups as a module under the Hecke algebra. In particular, results concern the denominator of cohomology classes which are represented by Eisenstein series, analytically constructed beforehand. In very special cases, values of the Riemann zeta-function play a decisive role. Within the discussion of these number-theoretic aspects of the cohomology groups, questions of a computational nature unfold. These may lead, by means of experiments, to a better understanding of the general integral structure of these groups. This introduction to the cohomology of arithmetic groups and the associated theory of automorphic forms and special values of L-functions focuses on number theoretic aspects and questions. It is intended for graduate students and researchers in the field of arithmetic as well as in automorphic forms and differential geometry.

This book discusses the mathematical interests of Joachim Schwermer, who throughout his career has focused on the cohomology of arithmetic groups, automorphic forms and the geometry of arithmetic manifolds. To mark his 66th birthday, the editors brought together mathematical experts to offer an overview of the current state of research in these and related areas. The result is this book, with contributions ranging from topology to arithmetic. It probes the relation between cohomology of arithmetic groups and automorphic forms and their L-functions, and spans the range from classical Bianchi groups to the theory of Shimura varieties. It is a valuable reference for both experts in the fields and for graduate students and postdocs wanting to discover where the current frontiers lie.

The aim of this book is to show that Shimura varieties provide a tool to construct certain interesting objects in arithmetic algebraic geometry. These objects are the so-called mixed motives: these are of great arithmetic interest. They can be viewed as quasiprojective algebraic varieties over Q which have some controlled ramification and where we know what we have to add at infinity to compactify them. The existence of certain of these mixed motives is related to zeroes of L-functions attached to certain pure motives. This is the content of the Beilinson-Deligne conjectures which are explained in some detail in the first chapter of the book. The rest of the book is devoted to the description of the general principles of construction (Chapter II) and the discussion of several examples in Chapter II-IV. In an appendix we explain how the (topological) trace formula can be used to get some understanding of the problems discussed in the book. Only some of this material is really proved: the book also contains speculative considerations, which give some hints as to how the problems could be tackled.Hence the book should be viewed as the outline of a programme and it offers some interesting problems which are of importance and can be pursued by the reader. In the widest sense the subject of the paper is number theory and belongs to what is called arithmetic algebraic geometry. Thus the reader should be familiar with some algebraic geometry, number theory, the theory of Liegroups and their arithmetic subgroups. Some problems mentioned require only part of this background knowledge.

This book grew out of three series of lectures given at the summer school on "Modular Forms and their Applications" at the Sophus Lie Conference Center in Nordfjordeid in June 2004. The first series treats the classical one-variable theory of elliptic modular forms. The second series presents the theory of Hilbert modular forms in two variables and Hilbert modular surfaces. The third series gives an introduction to Siegel modular forms and discusses a conjecture by Harder. It also contains Harder's original manuscript with the conjecture. Each part treats a number of beautiful applications.

This second volume introduces the concept of shemes, reviews some commutative algebra and introduces projective schemes. The finiteness theorem for coherent sheaves is proved, here again the techniques of homological algebra and sheaf cohomology are needed. In the last two chapters, projective curves over an arbitrary ground field are discussed, the theory of Jacobians is developed, and the existence of the Picard scheme is proved. Finally, the author gives some outlook into further developments- for instance etale cohomology- and states some fundamental theorems.

This book and the following second volume is an introduction into modern algebraic geometry.

This book and the following second volume is an introduction into modern algebraic geometry. In the first volume the methods of homological algebra, theory of sheaves, and sheaf cohomology are developed. These methods are indispensable for modern algebraic geometry, but they are also fundamental for other branches of mathematics and of great interest in their own.In the last chapter of volume I these concepts are applied to the theory of compact Riemann surfaces. In this chapter the author makes clear how influential the ideas of Abel, Riemann and Jacobi were and that many of the modern methods have been anticipated by them. For this second edition the text was completely revised and corrected. The author also added a short section on moduli of elliptic curves with N-level structures. This new paragraph anticipates some of the techniques of volume II.