Decide whether the following sets are open, closed or neither. If a set is not open, find a point in the set for which there is no ε-neighborhood contained in the set. If a set is not closed, find a limit point that is not contained in the set.

a. Q (the set of rationals)

b. N (the set of naturals)

c. {xЄR : x>0}

d. (0,1] = {xЄR : 0<x<=1}

e. {1 + (1/4) + (1/9) + ... + (1/(n^2)) : nЄN}