ASYMPTOTIC AND VARIATIONAL DERIVATIONS OF THE SIMPLIFIED P(N) EQUATIONS

It is demonstrated that the simplified P(N) equations are a leading Or der asymptotic limit of the transport equation. This limit is one of l ocally nearly planar transport involving scattering which is highly pe aked in the forward direction. This asymptotic equivalence is valid in full generality, in that it allows time-dependent transport in a hete rogeneous medium. It is also shown that the simplified P(N) equations can be derived variationally, making use of certain trial functions in a variational characterization of the even-parity transport equation. In this case, however, there appears to be an inherent restriction to time-independent transport in a homogeneous medium.