Published online by Cambridge University Press: 23 May 2016
We improve recent results of Bourgain and Shparlinski to show that, for almost all primes $p$, there is a multiple
$mp$ that can be written in binary as
$$\begin{eqnarray}mp=1+2^{m_{1}}+\cdots +2^{m_{k}},\quad 1\leq m_{1}<\cdots <m_{k},\end{eqnarray}$$
$k=6$ (corresponding to Hamming weight seven). We also prove that there are infinitely many primes
$p$ with a multiplicative subgroup
$A=\langle g\rangle \subset \mathbb{F}_{p}^{\ast }$, for some
$g\in \{2,3,5\}$, of size
$|A|\gg p/(\log p)^{3}$, where the sum–product set
$A\cdot A+A\cdot A$ does not cover
$\mathbb{F}_{p}$ completely.