Strichartz estimates on two-dimensional irrational tori

Abstract

We prove an $L^4$ Strichartz estimate for the dispersion $n_1^2+\alpha n_2^2$ on $\mathbb T^2$, for every $\alpha>0$. The loss is controlled by the Shannon entropy of the normalized squared Fourier coefficients and the cardinality of their support $S$. In particular, we obtain a norm loss of $(\log\#S)^{1/2}$ on fixed time intervals, with constants locally uniform in $\alpha$. This provides an irrational-dispersion counterpart of the cardinality estimate of Herr and Kwak [Forum Math. Pi (2024)] and improves the $N^\varepsilon$ loss of Bourgain and Demeter [Ann. of Math. (2015)] to $(\log N)^{1/2}$ on frequency boxes. The proof combines weighted incidence estimates, smooth localization, and an entropy recursion.