Ratio sets of random sets

Abstract

We study the typical behavior of the size of the ratio set $A/A$ for a random subset $A\subset \{1,\dots , n\}$. For example, we prove that $|A/A|\sim \frac{2\text{Li}_2(3/4)}{\pi^2}n^2 $ for almost all subsets $A \subset\{1,\dots ,n\}$. We also prove that the proportion of visible lattice points in the lattice $A_1\times\cdots \times A_d$, where $A_i$ is taken at random in $[1,n]$ with $\mathbb P(m\in A_i)=\alpha_i$ for any $m\in [1,n]$, is asymptotic to a constant $\mu(\alpha_1,\dots,\alpha_d)$ that involves the polylogarithm of order $d$.