On the Covering Number of Small Symmetric Groups and Some Sporadic Simple Groups

A set of proper subgroups is a covering for a group if its union is the whole group. The minimal number of subgroups needed to cover $G$ is called the covering number of $G$, denoted by $\sigma (G)$. Determining $\sigma (G)$ is an open problem for many non-solvable groups. For symmetric groups $S_n$, Mar'oti determined $\sigma (S_n)$ for odd $n$ with the exception of $n=9$ and gave estimates for $n$ even. In this paper we determine $\sigma (S_n)$ for $n=8$, $9$, $10$ and $12$. In addition we find the covering number for the Mathieu group $M_{12}$ and improve an estimate given by Holmes for the Janko group $J_1$.