Unconditional uniqueness for the modified Korteweg–de Vries equation on the line

Abstract

We prove that the modified Korteweg–de Vries (mKdV) equation is unconditionally well-posed in $H^s(\mathbb R)$ for $s > 1/3$. Our method of proof combines the improvement of the energy method introduced recently by the first and third authors with the construction of a modified energy. Our approach also yields a priori estimates for the solutions of mKdV in $H^s(\mathbb R)$, for $s > 0$, and enables us to construct weak solutions at this level of regularity.