A new criterion for knots with free periods

Let $p\ge 2$ and $q\mathrm{\ne }0$ an integer. A knot $K$ in the three-sphere is said to be a $(p,q)$-lens knot if and only if it covers a link in the lens space $L(p,q)$. In this paper, we use the second coefficient of the HOMFLY polynomial to provide a necessary condition for a knot to be a $(p,q)$-lens knot. As an application, it is shown that this criterion rules out the possibility of being $(5,1)$-lens for 80 among the 84 knots with less than 9 crossings.