Spherically averaged maximal function and scattering for the 2D cubic derivative Schr\"odinger equation

We prove scattering for the 2D cubic derivative Schr\"odinger equation with small data in the critical Besov space with one degree angular regularity. The main new ingredient is that we prove a spherically averaged maximal function estimate for the 2D Schr\"odinger equation. We also prove a global well-posedness result for the 2D Schr\"odinger map in the critical Besov space with one degree angular regularity. The key ingredients for the latter results are the spherically averaged maximal function estimate, null form structure observed in \cite{Bej}, as well as the generalised spherically averaged Strichartz estimates obtained in \cite{Guo2} in order to exploit the null form structure.