function fd2d(L)
  I = speye(L,L);
  subdiag = sparse(2:L,1:L-1,1,L,L);
  D = 2*I-subdiag-subdiag';
  A = kron(D,I)+kron(I,D);
  N=L*L;
  h = 1./(L+1);
  b = h*h*ones([N,1]);
  %t1=time();%octave 
  tic%matlab
  x=A\b;
  norm(x,inf)
  %t2=time();%octave
  %cputime=t2-t1
  cputime=toc%matlab
end
