Expansion of the Retardation Time

only the term linear in ti2 requires special attention in order to maintain a consistent order of 0(c ). For this purpose, we need to study the Taylor expansion of fi2 in Eq. (3.217), for which we first need to expand the distance 

Unfortunately, we encounter a new fi2 on the right-hand side. But since this term will finally be of order 0(c ), we may approximate fi2 uih)/ and arrive at the final low-order expansion for fi2/ 

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