Representing and counting the subgroups of the group Z_m x Z_n
  We deduce a simple representation and the invariant factor decompositions of the subgroups of the group $\Bbb{Z}_m \times \Bbb{Z}_n$, where $m$ and $n$ are arbitrary positive integers. We obtain formulas for the total number of subgroups and the number of subgroups of a given order. 
Mario Hampejs, Nicki Holighaus, L\'aszl\'o T\'oth, and Christoph   Wiesmeyr
math.GR math.CO math.NT