This book presents a functional approach to the construction, use and approximation of Green's functions and their associated ordered exponentials. After a brief historical introduction, the author discusses new solutions to problems involving particle production in crossed laser fields and non-constant electric fields. Applications to problems in potential theory and quantum field theory are covered, along with new approximations for the treatment of color fluctuations in high-energy QCD scattering.

This Advanced Study Institute (ASI) brought together two distinct ."schools of approach" to Quantum Electrodynamics (QED) in the presence of intense, external, electromagnetic fields, in an effort to lay a joint foundation for a needed theoretical explanation of the sharp e+ e- "resonances" observed in the scattering of very heavy IOns. These (GSI/Darmstadt) experiments, whose history, latest reconfirmations, and most recent data were presented in three opening sessions (Bokemeyer, Koenig), show a smooth background of positron (e+) production, as a function of e+ kinetic energy. Superimposed upon this background are four very sharp peaks, of narrow widths ( 30 KeV) and of clear experimental significance ('" 5 standard deviations). Most ofthese peaks correspond to sharp, essentially back-to-back electron-positron emission in the ions' center of mass. Following the approach of "supercritical" potential theory (SPT), where the total ionic charge unit Z satisfies Z > 137, it has been possible to provide a detailed and apparently correct understanding of the smooth e+ e- background; a coherent description of different facets of this approach, emphasizing the nature of the charged, supercritical vacuum, was described by the authors responsible for the invention of SPT (Greiner, Muller, Rafelski). In addition, predictions for related phenomena were outlined by other lecturers using the SPT approach (Bawin, Soff, SsJrensen)."

This book presents a functional approach to the construction, use and approximation of Green's functions and their associated ordered exponentials. After a brief historical introduction, the author discusses new solutions to problems involving particle production in crossed laser fields and non-constant electric fields. Applications to problems in potential theory and quantum field theory are covered, along with approximations for the treatment of color fluctuations in high-energy QCD scattering, and a model for summing classes of eikonal graphs in high-energy scattering problems. The book also presents a variant of the Fradkin representation which suggests a new non-perturbative approximation scheme, and provides a qualitative measure of the error involved in each such approximation. Covering the basics as well as more advanced applications, this book is suitable for graduate students and researchers in a wide range of fields, including quantum field theory, fluid dynamics and applied mathematics.