Normal covering numbers for $S_n$ and $A_n$ and additive combinatorics

The normal covering number $\gamma (G)$ of a finite group $G$ is the minimum number of proper subgroups whose conjugates cover the group. We give various estimates for $\gamma (S_n)$ and $\gamma (A_n)$ depending on the arithmetic structure of $n$. In particular we determine the limsups over $\gamma (S_n)\mathrm{/}n$ and $\gamma (A_n)\mathrm{/}n$ over the sequences of even and odd integers, as well as the liminf of $\gamma (S_n)\mathrm{/}n$ over even integers. In general we explain how the values of $\gamma (S_n)\mathrm{/}n$ and $\gamma (A_n)\mathrm{/}n$ are related to problems in additive combinatorics. These results answer most of the questions posed by Bubboloni, Praeger, and Spiga as Problem 20.17 of the Kourovka Notebook.