∇ ∇ ⋅ ⋅ ∇ ∇ D P ( r , t | r 0 , t 0 ) = ∇ ∇ ⋅ ⋅ D ∇ ∇ P ( r , t | r 0 , t 0 ) + ∇ ∇ ⋅ ⋅ P ( r , t | r 0 , t 0 ) ∇ ∇ D = ∇ ∇ ⋅ ⋅ D ∇ ∇ P ( r , t | r 0 , t 0 ) + ∇ ∇ ⋅ ⋅ P ( r , t | r 0 , t 0 ) F ( r ) γ γ − − ∇ ∇ ⋅ ⋅ P ( r , t | r 0 , t 0 ) D β β F ( r )