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.