Knotted vortices such as those produced in water by Kleckner and Irvine tend to transform by reconnection to collections of unknotted and unlinked circles. The reconnection number $R(K)$ of an oriented knot of link $K$ is the least number of reconnections (oriented re-smoothings) needed to unknot/unlink $K$. Putting this problem into the context of knot cobordism, we show, using Rasmussen's Invariant that the reconnection number of a positive knot is equal to twice the genus of its Seifert spanning surface. In particular an $(a,b)$ torus knot has $R = (a-1)(b-1).$ For an arbitrary unsplittable positive knot or link $K$, $R(K) = c(K) - s(K) + 1$ where $c(K)$ is the number of crossings of $K$ and $s(K)$ is the number of Seifert circles of $K.$ Examples of vortex dynamics are illustrated in the paper.
Categories: math.GT