= Solution
Put $\mu=\cos\theta$ and $S=j/\kappa$. Integrating the <radiative transfer equation> over solid angle gives
$$
\frac d{d\tau}\int_{4\pi}\mu I\,d\Omega
=\int_{4\pi}I\,d\Omega-4\pi S.
$$
The first integral is the <radiative flux> $F$. A stationary atmosphere with no local energy source has $dF/d\tau=0$, so
$$
\boxed{4\pi\frac j\kappa
=\int_{4\pi}I\,d\Omega}.
$$
By definition, the <mean intensity> is $J=(4\pi)^{-1}\int I\,d\Omega$. Therefore
$$
\boxed{\frac j\kappa=J},
\qquad
\boxed{4\pi\frac j\kappa=4\pi J},
$$
which is radiative equilibrium: the angle-integrated emission and absorption rates are equal.
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