∂ ∂ p ( x , t ) ∂ ∂ t = − − ∑ ∑ i = 1 N ∂ ∂ ∂ ∂ x i [ μ μ i ( x , t ) p ( x , t ) ] + 1 2 ∑ ∑ k = 1 M ∑ ∑ i = 1 N ∂ ∂ ∂ ∂ x i { σ σ i k ( x , t ) ∑ ∑ j = 1 N ∂ ∂ ∂ ∂ x j [ σ σ j k ( x , t ) p ( x , t ) ] }