Find an example of a group G and two subgroups H and K such that the set in exercise 47 is not a subgroup of G.
a) Group G = Integers, H = Even Integers, K = Odd Integers
b) Group G = Real Numbers, H = Positive Real Numbers, K = Negative Real Numbers
c) Group G = Rational Numbers, H = Integers, K = Fractions
d) Group G = Complex Numbers, H = Real Numbers, K = Imaginary Numbers