Casson–Gordon signature characterization of ribbon 2-bridge knots

Let $K_{m^2,q}$ be a 2-bridge knot, and let $\mathcal{R}$ denote the family of ribbon knots. The Casson–Gordon signature invariants associated to the double branched cover are the invariants $\sigma_{CG}(K_{m^2,q};2,k,r)$, with $k$ dividing $m$ and $0\leq r\leq k-1$. Casson–Gordon characterization. The knot $K_{m^2,q}$ is ribbon if and only if all Casson–Gordon signature invariants associated to its double branched cover vanish if and only if $K_{m^2,q}\in\mathcal{R}$. This characterizes the ribbon members of the relevant family of 2-bridge knots in terms of Casson–Gordon signatures; the supplied source does not establish whether the statement is resolved.