FeynAmpDenominator[ PropagatorDenominator[ ... ], PropagatorDenominator[ ... ], ...] is the head of the denominators of the propagators, i.e., FeynAmpDenominator[x] is the representation of 1/x .
See also: FAD, FeynAmpDenominatorSimplify.
The FeynCalc Book | FeynAmp | FeynAmpDenominatorCombine |