Pseudo-Riemannian geometry is, to a large extent, the study of the Levi-Civita connection, which is the unique torsion-free connection compatible with the metric structure. There are, however, other affine connections which arise in different contexts, such as conformal geometry, contact structures, Weyl structures, and almost Hermitian geometry. In this book, we reverse this point of view and instead associate an auxiliary pseudo-Riemannian structure of neutral signature to certain affine connections and use this correspondence to study both geometries. We examine Walker structures, Riemannian extensions, and Kähler--Weyl geometry from this viewpoint. This book is intended to be accessible to mathematicians who are not expert in the subject and to students with a basic grounding in differential geometry. Consequently, the first chapter contains a comprehensive introduction to the basic results and definitions we shall need---proofs are included of many of these results to make it as self-contained as possible. Para-complex geometry plays an important role throughout the book and consequently is treated carefully in various chapters, as is the representation theory underlying various results. It is a feature of this book that, rather than as regarding para-complex geometry as an adjunct to complex geometry, instead, we shall often introduce the para-complex concepts first and only later pass to the complex setting. The second and third chapters are devoted to the study of various kinds of Riemannian extensions that associate to an affine structure on a manifold a corresponding metric of neutral signature on its cotangent bundle. These play a role in various questions involving the spectral geometry of the curvature operator and homogeneous connections on surfaces. The fourth chapter deals with Kähler--Weyl geometry, which lies, in a certain sense, midway between affine geometry and Kähler geometry. Another feature of the book is that we have tried wherever possibleto find the original references in the subject for possible historical interest. Thus, we have cited the seminal papers of Levi-Civita, Ricci, Schouten, and Weyl, to name but a few exemplars. We have also given different proofs of various results than those that are given in the literature, to take advantage of the unified treatment of the area given herein.
Eduardo Garcia-Rio is a Professor of Mathematics and a member of the Institute of Mathematics of the University of Santiago de Compostela (Spain). He received his Ph.D. degree in 1992 from the University of Santiago de Compostela and is a member of the editorial board of the Journal of Geometric Analysis. His research specialities are Differential Geometry and Mathematical Physics.
Peter Gilkey is a Professor of Mathematics and a member of the Institute of Theoretical Science at the University of Oregon (USA). He is a fellow of the American Mathematical Society and is a member of the editorial board of Results in Mathematics, Differential Geometry and Applications, and the International Journal of Geometric Methods to Mathematical Physics. He received his Ph.D. in 1972 from Harvard University under the direction of L. Nirenberg. His research specialties are Differential Geometry, Elliptic Partial Differential Equations, and Algebraic topology. He has published more than 230 research articles and books.
Stana Nikcevic is a Professor of Mathematics at the University of Belgrade (Serbia). She received her Ph.D. from the University of Belgrade at the Mathematical Faculty and has been working at the University since 1974. During this period, she also sporadically worked at the University of Banja Luka (Bosnia and Hercegovina) and at the Mathematical Faculty in Kragujevac (Serbia). Her research mainly focuses on Differential Geometry. She has maintained international cooperation and has gone on short visits to the TU Berlin, Charles University Prague, Universitate Pierre et Marie Curie (Paris VI), University of Oregon (USA), and University of Santiago de Compostela (Spain).
Ramon Vazquez-Lorenzo is a member of the research group in Riemannian Geometry at the Department of Geometry and Topology of the University of Santiago de Compostela (Spain). He is a member of the Spanish Research Network on Relativity and Gravitation.He received his Ph.D.in 1997 from the University of Santiago de Compostela. His research focuses mainly on Differential Geometry with special emphasis on the study of the curvature and the algebraic properties of curvature operators in the Lorentzian and in higher signature settings. He has published more than 45 research articles and books.
