An improved necessary condition for the Schr\"odinger maximal estimate

We improve the necessary condition for Carleson’s problem regarding convergence for the Schr"odinger equation in dimensions $n\ge 3$. We prove that if the solution converges almost everywhere to its initial datum as time tends to zero, for all data in $H^s({\mathbb{R}}^n)$, then $s\ge \frac{n}{2(n+2)}$.