Let the operator A m be defined such A m f := ∑ ∑ n = 1 m ( − − ∂ ∂ x ) n [ D n ( x , t ) f ( x , t ) ] . The probability density evolves by ∂ ∂ t ρ ρ ≈ ≈ A m ρ ρ . Different order of m gives different level of approximation.