for i=1:41
   x(i)=-pi/2+ pi/40*(i-1);
end
for i=1:41
   y(i)=-pi/2+ pi/40*(i-1);
end
 for i=1:41
    for j=1:41
      f(i,j)=3.0-cos(x(i))-cos(y(j));
    end
   end
 u(:)=3.0-cos(x(:));