Calculations in external fields in QCD. An operator method

A convenient formalism is developed which enables one to find coefficients gluon operators in expansion of colorless correlation functions. The method generalizes Schwinger's technique for calculations in external electromagnetic fields. All principal elements and calculational devices are explained using concrete examples. The problem concerning the determinant of Dirac operator for heavy quarks is treated in detail. The answer for the quark loop is obtained as an expansion in powers of Gsub(μνsup(a))/msup(2), where Gsub(μνsup(a)) is the strength tensor of the gluon field and m is the quark mass. All terms up to G4 are taken into account