= Positivity limitation of a linear Eddington intensity
{title2=$I(0,-1)=-F/(4\pi)$}
For the affine angular ansatz $I=A+C\cos\theta$, the <radiative flux> is $F=4\pi C/3$. Setting the inward hemispheric flux to zero gives $A(0)=2C/3$, but then the formal intensity at $\cos\theta=-1$ is negative when $F>0$. Exact no-incoming radiation and nonnegative intensity cannot both be represented by a nonzero affine polynomial on all directions. The <Eddington surface boundary condition> therefore specifies approximate moments, rather than an exact emergent angular radiation field.
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