i think it is closed, if we define a sequence :
i hope my definition of closed subset is right. Maybe my result is wrong coz i choose a special case.
another question ,what about
Is neither open nor closed (in the usual topology anyway). It is not open as for any point in the set there is no open ball contained in the set which contains the point. It is not closed as it does not contain all its limit points.another question ,what about