On the low-Burnett-Kroll theorem for soft-photon emission

The amplitude for radiation of a soft photon is determined by the amplitude of the corresponding nonradiative process. The first two terms in an expansion in powers of the photon momentum are so determined. In the same approximation, according to Burnett and Kroll, the radiative intensity is determined by the nonradiative intensity in the case of unpolarized initial particles and unobserved final spins. They derived this result for the case of particles of spin 0 and 1/2, and conjectured its general validity. That conjecture is verified. (K.A.)