Reduction of the Fokker-Planck Equation with an Absorbing or Reflecting Boundary to the Diffusion Equation and the Radiation Boundary Condition

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Abstract
It is shown, from microscopic considerations based on the Fokker-Planck equation, that the boundary condition (used in conjunction with the diffusion equation) in which the particle density is set to zero on a perfectly absorbing surface is untenable, and, for the first time, the boundary condition for any plane (partially or perfectly) absorbing surface is derived.