Convergence over fractals for the periodic Schrödinger equation

Abstract

We consider a fractal refinement of Carleson's problem for pointwise convergence of solutions to the periodic Schr\"odinger equation to their initial datum. For $\alpha \in (0,d]$ and \[ s < \frac{d}{2(d+1)} (d + 1 - \alpha), \] we find a function in $H^s(\mathbb{T}^d)$ whose corresponding solution diverges in the limit $t \to 0$ on a set with strictly positive $\alpha$-Hausdorff measure. We conjecture this regularity threshold to be optimal. We also prove that \[ s > \frac{d}{2(d+2)}\left( d+2-\alpha \right) \] is sufficient for the solution corresponding to every datum in $H^s(\mathbb T^d)$ to converge to such datum $\alpha$-almost everywhere.