On multivariable averages of divisor functions

Abstract: We deduce asymptotic formulas for the sums $\sum_{n_1,\ldots,n_r\le x} f(n_1\cdots n_r)$ and $\sum_{n_1,\ldots,n_r\le x} f([n_1\cdots n_r])$, where $r\ge 2$ is a fixed integer, $[n_1,\ldots,n_r]$ stands for the least common multiple of the integers $n_1,\ldots,n_r$ and $f$ is one of the divisor functions $\tau_{1,k}(n)$ ($k\ge 1$), $\tau^{(e)}(n)$ and $\tau^*(n)$. Our formulas refine and generalize a result of Lelechenko (2014). A new generalization of the Busche-Ramanujan identity is also pointed out.