where ipp —* ip^- (with a corresponding change in the sign of the charge), but

298 QUANTUM ELECTRODYNAMICS and the sums over q\... <74 collapse, giving

The r, r' integrals can be done using the familiar substitution (10.21), and pulling out the factors which relate the S- to the .M-matrix gives

and using 77** = (1,0) permit us to write the instantaneous .M-matrix in the following way:

Note that this is similar to Eq. (10.22) except for the sign, which is now + because both the fi~ and e~ have negative charge, and the appearance of v spinors, which ■ AW be discussed below.

The second term, due to //rad. becomes

'RAD

d4x1dix2

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