\Gamma _l=\Gamma _r; \qquad \delta x_{LP}=\delta x_{PR} = \delta x .
we get D_l=D_r=D The equation further reduces to F(\phi_r-\phi_l)=D(\phi_R-2\phi_P+\phi_L) This can be written as P(\phi_r-\phi_l)=(\phi_R-2\phi_P+\phi_L) , where P is the Péclet number.