$$\frac{dy}{dx}=\cos(y)$$
Is critical point $y=\frac{3\pi}{2}$ semi-stable?
Because according to "Zill engineering math 6th", it says semi-stable means one side diverge to infinity and other side converge case. However, $y=\frac{3\pi}{2}$ case is one side converges to $\frac{5\pi}{2}$ and other side converges to $\frac{\pi}{2}$.
How can I define stability of that critical point?