from http://abstract.ups.edu/aata/exercises-cyclic.html (problem 13):
"For n≤20, which groups U(n) are cyclic? Make a conjecture as to what is true in general. Can you prove your conjecture?"
where U(n) is the group of natural numbers < n and co-prime to n, under the operation of multiplication (mod n)
I find that U(n) is acyclic for n = 8,12,15,16,20, but unable to form a conjecture.
