= Polynomial-source photon transport solution
{title2=$\Delta(\tau)=[\Delta_R-B_R]e^{-ik\mu(\tau-\tau_R)}+B(\tau)$}
For $\dot\Delta+ik\mu\Delta=2\mu^2\dot h$ with $\dddot h=0$ and $k\ne0$, a particular solution is $B=-2i\mu\dot h/k+2\ddot h/k^2$. Differentiating it cancels its $\ddot h$ terms and gives the prescribed source. The homogeneous solution transports the initial condition with a direction-dependent Fourier phase. The inverse powers of $k$ need not signal a physical singularity: use the original source integral to take a long-wavelength limit consistently.
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