“…Several publications in the applied mathematics, engineering and physics research literature (see [6,7,[10][11][12]15,18] and references therein) exploited a remarkable feature shared by some celebrated explicit solutions to the two-dimensional incompressible Euler equations, in Lagrangian variables (such as Kirchhoff's elliptical vortex [16] found in 1876, Gerstner's flow [9] found in 1809 and re-discovered in 1863 by Rankine [19], and the Ptolemaic vortices found in 1984 by Abrashkin and Yakubovich [1]), namely that in all of them the labelling map is harmonic at all times.…”