function A = Dmatrix(N)
%Diffusion Matrix
D = 1;
A = zeros(N,N);
A(1,1) = -2*D; A(1,2) = D; A(1,N) = D;
A(N,N) = -2*D; A(N,N-1) = D; A(N,1) = D;
for i=2:N-1
   A(i,i) = -2*D;
   A(i,i+1) = D;
   A(i,i-1) = D;
end
A = (N-1)*(N-1)*A;
A = sparse(A);