If by "Rk" you mean , then is not open because it is not true that for every point in it there exists an open interval containing the point which is completely contained in the set, and it is not closed because it doesn't contain all its accumulation points (the point zero=0), or also: it is not closed because its complement is not open (for the point x = 0 in the complement there doesn't exist an interval containing it and which is contained in the complement).

Tonio