Laws Laplacian diffusion equation

The right-hand side of eqn. (9), which is the diffusion equation or Fick s second law, involves two spherically symmetric derivatives of p(r, t). In the general case of three-dimensional space, lacking any symmetry, it can be shown that the Laplacian operator... 

This is known as Pick s second law it states that the time derivative of the concentration at a given point in space is equal to the diffusion coefficient times the Laplacian of the concentration. In deriving this equation it has been assumed that the diffusion coefficient is independent of concentration. 

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