The Feynman-Schwinger representation in QCD
  The proper time path integral representation is derived explicitly for Green's functions in QCD. After an introductory analysis of perturbative properties, the total gluonic field is separated in a rigorous way into a nonperturbative background and valence gluon part. For nonperturbative contributions the background perturbation theory is used systematically, yielding two types of expansions,illustrated by direct physical applications. As an application, we discuss the collinear singularities in the Feynman-Schwinger representation formalism. Moreover, the generalization to nonzero temperature is made and expressions for partition functions in perturbation theory and nonperturbative background are explicitly written down. 
Yu.A.Simonov (ITEP, Moscow and Jefferson Lab, USA) and J.A. Tjon (U.   Utrecht, U. Groningen and U. Maryland)