Relating Eddington factors to flux limiters.

Variable Eddington factors and flux-limiters have been introduced in the P-1 and diffusion equations, respectively, to handle situations when the underlying intensity is too anisotropic for the unmodified theories to remain valied. The author presents a derivation of a relation between the two for which a new approach to the diffusion approximation is used. Algebraic expressions for Eddington factors satisfying the moment conditions are not satisfactory for closing the P-1 equations but, by using the derived relation, yield acceptable flux-limited diffusion theories.