\frac{\partial p(\theta,z,t)}{\partial z}=-A\theta^2p(\theta,z,t)-\frac{n}{2c}\theta^2\frac{\partial p(\theta,z,t)}{\partial t}+\frac{D}{\theta}\frac{\partial p(\theta,z,t)}{\partial\theta}+D\frac{\partial^2p(\theta,z,t)}{\partial\theta^2}