Since , you need to show that when is small enogh . If
is 0, it is clear that is never in . If , you have only to care of the limit direction in (+ or -) which makes
positive. The other direction gives the constant function 0. In this case you have to show
for small enogh. If is positive it is almost immediate you can do it (when ). If is negative and the inequality
is also fulfilled for small enough.