semi-stable points

Before I mentioned that in order to determine if a point is stable, unstable, or semistable you can take the derivative of f(y) and plug the critical points into f '(y). Well, if for some "y" you get that f '(y) = 0, this does not mean that the critical point is semistable, it simply means that you can't use this derivative test, and you have to check the intervals around that critical point. However, if f '(y) > 0, then the point is unstable and if f '(y) < 0, then the point is stable.