Published online by Cambridge University Press: 20 September 2016
We prove two generalizations of Furstenberg’s Diophantine result regarding the density of an orbit of an irrational point in the $1$-torus under the action of multiplication by a non-lacunary multiplicative semigroup of
$\mathbb{N}$. We show that for any sequences
$\{a_{n}\},\{b_{n}\}\subset \mathbb{Z}$ for which the quotients of successive elements tend to
$1$ as
$n$ goes to infinity, and any infinite sequence
$\{c_{n}\}$, the set
$\{a_{n}b_{m}c_{k}x:n,m,k\in \mathbb{N}\}$ is dense modulo
$1$ for every irrational
$x$. Moreover, by ergodic-theoretical methods, we prove that if
$\{a_{n}\},\{b_{n}\}$ are a sequence having smooth
$p$-adic interpolation for some prime number
$p$, then for every irrational
$x$, the sequence
$\{p^{n}a_{m}b_{k}x:n,m,k\in \mathbb{N}\}$ is dense modulo 1.