Ill-posedness of the cubic nonlinear half-wave equation and other fractional NLS on the real line

Abstract: In this paper, we study ill-posedness of cubic fractional nonlinear Schr\"odinger equations. First, we consider the cubic nonlinear half-wave equation (NHW) on $\mathbb R$. In particular, we prove the following ill-posedness results: (i) failure of local uniform continuity of the solution map in $H^s(\mathbb R)$ for $s\in (0,\frac 12)$, and also for $s=0$ in the focusing case; (ii) failure of $C^3$-smoothness of the solution map in $L^2(\mathbb R)$; (iii) norm inflation and, in particular, failure of continuity of the solution map in $H^s(\mathbb R)$, $s<0$. By a similar argument, we also prove norm inflation in negative Sobolev spaces for the cubic fractional NLS. Surprisingly, we obtain norm inflation above the scaling critical regularity in the case of dispersion $|D|^\beta$ with $\beta>2$.