Calculations in external fields in quantum chromodynamics. Technical review

Abstract We review the technique of calculation of operator expansion coefficients. The main emphasis is put on gluon operators which appear in expansion of n ‐point functions induced by colourless quark currents. Two convenient schemes are discussed in detail: the abstract operator method and the method based on the Fock‐Schwinger gauge for the vacuum gluon field. We consider a large number of instructive examples important from the point of view of physical applications.