N = 100;
format long
[A,b] = twodlaplace(N);
disp('Konditionszahl von A');
condest(A),
x = A \ b;


D = diag(diag(A)); f = D \ b;
LU = A-D;
disp('Spektralradius von B');
normest(inv(sqrt(D)) * LU * inv(sqrt(D))),

xk = zeros((N-1)^2,1); err = []; res = [];
for k = 1:1000,
   xk = -D \ ( LU*xk ) + f;
   err = [err;norm(x-xk)];
   res = [res;norm(A*xk - b)];
end
semilogy(err,'r-'),
hold on
semilogy(res,'b-'),
hold off
makelarge
err(end) / norm(x),