Global well-posedness of the derivative nonlinear Schrödinger equation with periodic boundary condition in $H^{\frac12}$

Abstract: We establish the global well-posedness of the derivative nonlinear Schr\"odinger equation with periodic boundary condition in the Sobolev space $H^{\frac12}$, provided that the mass of initial data is less than $4\pi$. This result matches the one by Miao, Wu, and Xu and its recent mass threshold improvement by Guo and Wu in the non-periodic setting. Below $H^{\frac12}$, we show that the uniform continuity of the solution map on bounded subsets of $H^s$ does not hold, for any gauge equivalent equation.