A detailed exposition, with applications to problems in potential theory and quantum field theory.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. Suitable for graduate students and researchers in a wide range of fields, including quantum field theory, fluid dynamics and applied mathematics.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. Suitable for graduate students and researchers in a wide range of fields, including quantum field theory, fluid dynamics and applied mathematics.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.Preface; 1. Introduction; 2. Elementary functional methods; 3. SchwingerFradkin methods; 4. Lals*