I have the next drift-diffusion equation:

\frac{\partial \phi(x,y,z)}{\partial t} + u_x(z) \frac{\partial \phi(x,y,z)}{\partial x} = \nabla D(z) \nabla \phi (x,y,z)

where D(z) is a matrice.
To solve this equation i perform the next change of variable: X = x + u(z) t , Y= y , Z=z, T=t and then I apply the Fourier transformation and when I solve this integrals with maple, at the first integral maple said that it is undefined.

Maybe I can solve the problem applying other change of variable or taking into account in the change of variable \nabla D(z) like for example Z=z - \nabla D(z) or maybe I should replace also in \nabla D(z) the change of variable (I replace only in \phi). I am not sure if I can doing this options. Some one can help me?