“…Existence and uniqueness of local solutions behind this regularity threshold, and in particular for initial data in the so-called energy space D(A 1/2 ) × H, is still a largely open problem (the only result we are aware of in this direction is contained in the recent paper [9]). Global-in-time solutions to problem (1.1)- (1.3) are known to exist in many different special cases, involving either special initial data (such as analytic data [5,2,6,7], quasianalytic data [18,11], lacunary data [16,13,10,11,14]), or special nonlinearities [19], or dispersive equations and small data [12,8,20,17], or spectral gap operators [11].…”