When solving Partial Differential Equations(PDEs), the Douglas-Gunn (D-G) method is sometimes used to transform the equations to tri-diagnal systems, when it comes to the boundary, how to treat the intermediate variables?
there are several Douglas-Gunn method to solve. Especially, you can apply numerical methods for solving partial differential equation. And another methods of numerical solution to solve your question. Now, I will look new sources for help you.