(a) Find all subrings of Z12, Z18, and Z24.
(b) Construct the Hasse diagram for each of these collections of subrings, where the partial order arises from set inclusion. Compare these diagrams with those for the set of positive divisors of n (n = 12; 18; 24), where the partial order now comes from the divisibility relation.
c) Find the formula for the number of subrings in Zn,n > 1