Sharp Strichartz estimates on quadratic irrational tori

We prove sharp $L^4$ Strichartz estimates for two-dimensional periodic Schrödinger flows with integral diagonal symbols $a{k}_1^2+b{k}_2^2$ where $a,b\in \mathbb{Z}\setminus \{0\}$. The estimates exhibit a sharp arithmetic dichotomy according to whether $-ab$ is a square: for frequencies bounded by $N$, the optimal loss is $(\log N{)}^{1\mathrm{/}4}$ when $-ab$ is not a square and $N^{1\mathrm{/}4}$ when $-ab$ is a square. As a consequence, we show that arbitrarily small perturbations of the rectangular aspect ratio can lead to a transition between global well-posedness and norm inflation for cubic hyperbolic NLS.