[math] (\frac {\partial^2{}}{\partial{x^2}} +\frac {\partial^2{}}{\partial{x^2}})[/math]n + [math] D_L \frac {\partial^2{}}{\partial{z^2}}[/math] - [math] W \frac {\partial{}}{\partial{z}}[/math] n = 0
for the previous equation let consider the asymptotic solution has the form:
[math] n(x', y', z') = e^{\lambda_L z'} V(x,y,z) [/math]
so
[math] \nabla'^2 V = \lambda_L^2 V [/math]
where
[math] \nabla'^2 V = \frac {\partial^2{}}{\partial{x'^2}} + \frac {\partial^2{}}{\partial{y'^2}} + \frac {\partial^2{}}{\partial{z^2}}[/math]
and
[math] x' = \frac {D_L}{D} x [/math] [math] y' = \frac {D_L}{D} y [/math]
In spherical coordinates:
[math] \frac {1}{r^'^2} \frac{\partial{}}{\partial{r'}}r'^2 \frac{\partial{V}}{\partial{r'} + \frac {1}{r^'sin\theta'} \frac{\partial{}}{\partial{\theta'} sin\theta'\frac{\partial{V}}{\partial{\theta'} = \lambda_L^2 V [/math]