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Single-Particle Electrodynamics - Assassination Science

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6.4.6 Angular outer integrals<br />

We now turn to the question of performing integrals over the angular coördinates<br />

n d of r d , viz., integrals of the form<br />

∫<br />

d 2 n d f(n d ). (6.40)<br />

Clearly, since the volume of integration of r d is a simple sphere, we may<br />

separate the r d and n d integrals completely by employing spherical coördinates.<br />

(This was not possible for the r s integrals above, since the volume V s is<br />

not, in general, spherical.) Obviously, if the f(n d ) in (6.40) contains an odd<br />

number of factors of n d , the integral will vanish identically, by symmetry.<br />

We are thus left with integrals of the form<br />

∫<br />

d 2 n d n i dn j d ,<br />

∫<br />

d 2 n d n i dn j d nk dn l d,<br />

∫<br />

d 2 n d n i dn j d nk dn l dn m d n n d, (6.41)<br />

and so on. We shall at this point employ clairvoyance for the third and<br />

final time, and predict that the integrals (6.41) exhaust those that we shall<br />

need in the radiation reaction calculations. Now, consideration of three-space<br />

covariance alone requires the integrals (6.41) to be able to be written solely<br />

in terms of the three-covariant quantity δ ij —there being no other covariant<br />

quantity available after the integration has been performed. This, together<br />

with due consideration of the symmetry of the expressions (6.41), already<br />

tells us that the answers must be of the form<br />

∫<br />

d 2 n d n i dn j d = β 2 δ ij ,<br />

∫<br />

d 2 n d n i dn j {<br />

d nk dn l d = β 4 δ ij δ kl + δ ik δ jl + δ il δ jk} ,<br />

∫<br />

d 2 n d n i dn j {<br />

d nk dn l dn m d n n d = β 6 δ ij δ kl δ mn + δ ij δ km δ ln + δ ij δ kn δ lm<br />

252

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