“…There is a considerable body of literature devoted to the analysis of topological and geometrical structures that appear in fluid flows, which has led to significant results e.g. on particle trajectories and vortex lines [2,13,21,11,25], on the relationship between the Euler equation and the group of volume-preserving diffeomorphisms [2,10,19,6], and on the connection of the helicity with the energy functional and the asymptotic linking number [3,16,17,29]. However, Kelvin's conjecture remains wide open, and indeed has been included as a major open problem in topological fluid mechanics in the surveys [27,23].…”