p_{i,j+1}=(\frac{\Delta z D}{\Delta\theta^2}-\frac{\Delta z D}{2\Delta\theta\theta_{i,j}})p_{i-1,j} + (1-\frac{2\Delta z D}{\Delta\theta^2}-\Delta zA\theta_{i,j}^2)p_{i,j} + (\frac{\Delta z D}{2\Delta\theta\theta_{i,j}}+\frac{\Delta z D}{\Delta\theta^2})p_{i+1,j}