On a question of Ruzsa about densities of sumsets

RESUMO

Let $\mathsf{d}$, $\mathsf{d}^{\ast}$, and $\mathsf{d}_{\ast}$ denote, respectively, the asymptotic density, the upper asymptotic density, and the lower asymptotic density on $\mathbb N$. We show that there are no constants $0<\nu<1$ and $c>0$ such that $$ \mathsf{d}_{\ast}(A+A)\ge c\bigl(\mathsf{d}^{\ast}(A+A)\bigr)^{1-\nu}\mathsf{d}(A)^{\nu} $$ for every set $A\subseteq \mathbb{N}$ which admits asymptotic density. This answers in the negative a question of I.Z. Ruzsa.