How do I find all singularities of$ \ \frac{\cos z - \cos (2z)}{z^4} \ $
It seems like there is only one (z = 0)?
How do I decide if it is isolated or nonisolated?
And if it is isolated, how do I decide if it is removable or not removable?
If it is non isolated, how do I decide the orders of the singularities?
Thanks!!!