Before revision: They have since found to describe systems in non-linear optics, fluid mechanics and the theory of non-linear dynamical systems, as they provide a pair of invariants or conserved quantities for the three-wave equation.
After revision: beenThey have since found to describe systems in non-linear optics, fluid mechanics and the theory of non-linear dynamical systems, as they provide a pair of invariants or conserved quantities for the three-wave equation.