Menon's identity and arithmetical sums representing functions of several   variables
  We generalize Menon's identity by considering sums representing arithmetical functions of several variables. As an application, we give a formula for the number of cyclic subgroups of the direct product of several cyclic groups of arbitrary orders. We also point out extensions of Menon's identity in the one variable case, which seems to not appear in the literature. 
L\'aszl\'o T\'oth
math.NT math.GR