Sobolev Maps to the Circle: From the Perspective of Analysis, Geometry, and Topology(96 Progress in Nonlinear Differential Equations and Their Applications)

The theory of real-valued Sobolev functions is a classical part of analysis and has a wide range of applications in pure and applied mathematics. By contrast, the study of manifold-valued Sobolev maps is relatively new. The incentive to explore these spaces arose in the last forty years from geometry and physics.  This monograph is the first to provide a unified, comprehensive treatment of Sobolev maps to the circle, presenting numerous results obtained by the authors and others. Many surprising connections to other areas of mathematics are explored, including the Monge-Kantorovich theory in optimal transport, items in geometric measure theory, Fourier series, and non-local functionals occurring, for example, as denoising filters in image processing. Numerous digressions provide a glimpse of the theory of sphere-valued Sobolev maps.  Each chapter focuses on a single topic and starts with a detailed overview, followed by the most significant results, and rather complete proofs. The “Complements and Open Problems” sections provide short introductions to various subsequent developments or related topics, and suggest newdirections of research.  Historical perspectives and a comprehensive list of references close out each chapter.  Topics covered include lifting, point and line singularities, minimal connections and minimal surfaces, uniqueness spaces, factorization, density, Dirichlet problems, trace theory, and gap phenomena. Sobolev Maps to the Circle will appeal to mathematicians working in various areas, such as nonlinear analysis, PDEs, geometric analysis, minimal surfaces, optimal transport, and topology.  It will also be of interest to physicists working on liquid crystals and the Ginzburg-Landau theory of superconductors.

Table of Contents:
Lifting in $W^{1,p}$.- The Geometry of $J(u)$ and $\Sigma(u)$ in 2D; Point Singularities and Minimal Connections.- The Geometry of $J(u)$ and $\Sigma(u)$ in 3D (and higher); Line Singularities and Minimal Surfaces.- A Digression: Sphere-Valued Maps.- Lifting in Fractional Sobolev Spaces and in $VMO$.- Uniqueness of Lifting and Beyond.- Factorization.- Applications of the Factorization.- Estimates of Phases: Positive and Negative Results.- Density.- Traces.- Degree.- Dirichlet Problems, Gaps, Infinite Energies.- Domains with Topology.- Appendices.

Review :
“A nice and unique feature of the book that every chapter contains a section on ‘Complements and open problems’. All in all, I find this text very well written and it would definitely be recommended to graduate students and researchers who would like to learn about the fine properties of Sobolev functions valued in the circle.” (Alpár R. Mészáros, zbMATH 1501.46001, 2023) “This monograph offers a rigorous discussion to a fascinating topic of mathematics with multiple relevant applications to various fields. Also for this reason it is highly recommended both for mathematicians and physicists working in the various fields involved in the theory of Sobolev maps to the circle … . the book may also be useful for Ph.D students who are interested in the topic, and the content of the individual chapters could be used in advanced courses and seminars.” (Antonia Chinnì, Mathematical Reviews, February, 2023)