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A method for calculating coefficient functions of the operator productexpansion, which was previously derived for the non-singlet case, isgeneralized for the singlet coefficient functions. The resulting formuladefines coefficient functions entirely in terms of corresponding singletcomposite operators without applying to elementary (quark and gluon) fields.Both "diagonal" and "non-diagonal" gluon coefficient functions in the productexpansion of two electromagnetic currents are calculated in QCD. Theirrenormalization properties are studied.

OPE coefficient functions in terms of composite operators only. Singlet case