On the Mean Values of the Function $\tau_k(n)$ in Sequences of Natural   Numbers
  We obtain an asymptotic formula for the mean value of the function $\tau_k(n)$, which is the number of solutions of the equation $x_1...\, x_k=n$ in natural numbers $x_1,..., x_k$ in some special sequences of natural numbers. 
K.M. Eminyan
math.NT