Assume that $G$ is a graph with cover ideal $J(G)$. For every integer $k\geq 1$, we denote the $k$-th symbolic power of $J(G)$ by $J(G)^{ (k)}$. We provide a sharp upper bound for the regularity of $J(G)^{ (k)}$ in terms of the star packing number of $G$. Also, for any integer $k\geq 2$, we study the difference between ${ \rm reg}(J(G)^{ (k)})$ and ${ \rm reg}(J(G)^{ (k-2)})$. As a consequence, we compute the regularity of $J(G)^{ (k)}$ when $G$ is a doubly Cohen-Macaulay graph. Furthermore, we determine ${ \rm reg}(J(G)^{ (k)})$ if $G$ is either a Cameron-Walker graph or a claw-free graph which has no cycle of length other that $3$ and $5$.
Categories: math.AC math.CO