| 0 1 2 3 -+-------- 0| 0 1 2 3 1| 1 2 3 0 2| 2 3 0 1 3| 3 0 1 2
Z_{q_1} x Z_{q_2} x ... x Z_{q_n} x Z^r
where q_1 divides q_2 divides ... divides q_n and r>= 0.
r is called the rank of the abelian group, and q_i the
invariants. The rank and invariants are uniquely determined
by the group. For example, there are exactly six nonisomorphic
abelian groups of order 360 = 2^3 . 3^2 . 5, namely
Z_360 Z_2 x Z_180 Z_2 x Z_2 x Z_90 Z_2 x Z_6 x Z_30 Z_3 x Z_120 Z_6 x Z_60