“…It has not been until fairly recently, with the works of Lindblad [61] for σ = 0, Coutand and Shkoller [25] for σ ≥ 0, and Shatah and Zeng [72,73], also for σ ≥ 0, and more recently by the first author and Ebin [34] for σ > 0, that existence and uniqueness for (1.4) have been addressed in full generality. Since the early 2000's, research on (1.4) has blossomed, as is illustrated by the sample list [1,3,4,5,6,7,10,8,2,9,12,11,15,16,17,18,19,20,21,22,23,24,26,27,29,30,31,32,33,40,41,42,44,45,46,47,49,50,51,52,54,55,56,…”