On the covering number of symmetric groups of even degree

If a group $G$ is the union of proper subgroups $H_1,\dots ,H_k$, we say that the collection $\{H_1,\dots H_k\}$ is a cover of $G$, and the size of a minimal cover (supposing one exists) is the covering number of $G$, denoted $\sigma (G)$. Mar'oti showed that $\sigma (S_n)=2^{n-1}$ for $n$ odd and sufficiently large, and he also gave asymptotic bounds for $n$ even. In this paper, we determine the exact value of $\sigma (S_n)$ when $n$ is divisible by $6$.