Well, double, spherically symmetric

The simplest example in which the singular solutions of (4) form a continuous spectrum, but can be easily overlooked, concerns one of the simplest problems of transport theory the diffusion of monoenergetic neutrons in plane geometry. In this case, t, as well as y, z, cease to be variables because the flux is assumed to be independent of them. Furthermore, since the flux is zero except for a single value of E, this is not a significant variable either. Finally, the double variable 2 can be replaced by the direction cosine /jl = 0.x of the velocity with respect to the x axis because the flux does not depend on the azimuthal angle either. If the further simplification is made that the scattering is spherically symmetric and if the total cross section is measured in appropriate units, the transport equation assumes the form... 

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