A Density version of Waring's problem

In this paper, we study a density version of Waring's problem. We prove that a positive density subset of $k$th-powers forms an asymptotic additive basis of order $O(k^2)$ provided that the relative lower density of the set is greater than $(1 - \mathcal{Z}_k^{-1}/2)^{1/k}$, where $\mathcal{Z}_k$ is certain constant depending on $k$ for which it holds that $\mathcal{Z}_k > 1$ for every $k$ and $\lim_{k \rightarrow \infty} \mathcal{Z}_k = 1$.