Originally Posted by

**dungnguyen**
The complete electron propagator is S=[p+m -sigma]^-1 .

where the higher order corrections to the propagator is to replace the electron mass m by m-sigma.Here sigma is a function of p and m.Then they saids: so the effect of these corrections on matrix elements of the operator psi bar.psi between one electron states of four momentum p is to multiply them by a factor:

F(p)=1-(d sigma(p,m)/dm) at m=0.Then I do not understand whyS=[p+m-sigma]^-1=([p+m]^-1)(1-(d sigma/dm))