## Dirichlet problem for 2D Poisson equation - finite difference method

**A simple example**

- regular mesh on a square, homogenneous boundary condition, right hand side of the equation identically equal to 1 (or 1 on one half of the square and -1 on the other half, respectively):

n=20; % n - number of steps at directions x, y G=numgrid('N',n); % G - the mesh D=delsq(G); % D - the system matrix N=sum(G(:)>0); % N - the number of unknowns F=ones(N,1); % F - the right hand side % F(1:round(N/2))=-1; % an alternative - right hand side changed u=D\F; % u - the solution % graph of the solution: V=G; V(G>0)=full(u(G(G>0))); mesh(V)you should obtain this, or that figure, respectively.

Look at the system matrix (for small n):

n=5; % n - number of steps at directions x, y G=numgrid('N',n); % G - the mesh D=delsq(G); % D - the system matrix full(D) % shows the matrix D on the screen