Individual leading symbols

At a nonzero future null separation, the massless retarded line has its nonzero delta-type normal singularity. The frozen scalar boundary-value lines have nonzero simple-pole normal parts on the selected conormal sides. The massive line has the same principal normal pole; its less singular terms do not change that individual principal symbol. These are properties of the already frozen numerical lines, not choices of replacement propagators.

For the present Fourier convention, denote their nonzero individual leading normal amplitudes by

The selected time-ordered orientation is the copied one. A simultaneous reversal of Fourier convention changes the signs of all selected normal frequencies, not the argument.