Fundamental solution of the Volkov problem (characteristic   representation)
  The characteristic representation, or Goursat problem, for the Klein-Fock-Gordon equation with Volkov interaction [1] is regarded. It is shown that in this representation the explicit form of the Volkov propagator can be obtained. Using the characteristic representation technique, the Schwinger integral [2] in the Volkov problem can be calculated. 
Alexander A. Borghardt, Darya Ya. Karpenko