Darcy (with simple Dirichlet and Neumann)

方程: - \triangledown \cdot(a\triangledown u(x) ) = 0 ,x\in [0,1]^{2} u(x)= 1,\ forall x_ {1} = 0, u(x)= 0,\ forall x_ {1} = 1, -\frac{\partial u}{\partial n}=1, \forall x_{2}= 0 , -\frac{\partial u}{\partial n}=0, \forall x_{2}=1。