Some sharp inequalities of Mizohata--Takeuchi-type

Abstract: Let $\Sigma$ be a strictly convex, compact patch of a $C^2$ hypersurface in $\mathbb{R}^n$, with non-vanishing Gaussian curvature and surface measure $d\sigma$ induced by the Lebesgue measure in $\mathbb{R}^n$. The Mizohata--Takeuchi conjecture states that \begin{equation*} \int |\widehat{gd\sigma}|^2w \leq C |Xw|\infty \int |g|^2 \end{equation*} for all $g\in L^2(\Sigma)$ and all weights $w:\mathbb{R}^n\rightarrow [0,+\infty)$, where $X$ denotes the $X$-ray transform. As partial progress towards the conjecture, we show, as a straightforward consequence of recently-established decoupling inequalities, that for every $\epsilon>0$, there exists a positive constant $C\epsilon$, which depends only on $\Sigma$ and $\epsilon$, such that for all $R \geq 1$ and all weights $w:\mathbb{R}^n\rightarrow [0,+\infty)$ we have \begin{equation*} \int_{B_R} |\widehat{gd\sigma}|^2w \leq C_\epsilon R^\epsilon \sup_T \left(\int T w^{\frac{n+1}{2}}\right)^{\frac{2}{n+1}}\int |g|^2, \end{equation*} where $T$ ranges over the family of all tubes in $\mathbb{R}^n$ of dimensions $R^{1/2} \times \dots \times R^{1/2} \times R$. From this we deduce the Mizohata--Takeuchi conjecture with an $R^{\frac{n-1}{n+1}}$-loss; i.e., that \begin{equation*} \int{B_R} |\widehat{gd\sigma}|^2w \leq C_\epsilon R^{\frac{n-1}{n+1}+ \epsilon}|Xw|_\infty\int |g|^2 \end{equation*} for any ball $B_R$ of radius $R$ and any $\epsilon>0$. The power $(n-1)/(n+1)$ here cannot be replaced by anything smaller unless properties of $\widehat{gd\sigma}$ beyond 'decoupling axioms' are exploited. We also provide estimates which improve this inequality under various conditions on the weight, and discuss some new cases where the conjecture holds.