Radon transforms with small derivatives and distance inequalities for convex bodies

Julián Haddad, Alexander Koldobsky

Published 2023-12-28Version 1

Generalizing the slicing inequality for functions on convex bodies from [11], it was proved in [4] that there exists an absolute constant $c$ so that for any $n\in \mathbb N$, any $q\in [0,n-1)$ which is not an odd integer, any origin-symmetric convex body $K$ of volume one in $\mathbb R^n$ and any infinitely smooth probability density $f$ on $K$ we have $$\max_{\xi \in S^{n-1}} {\frac 1{\cos(\pi q/2)}\mathcal R f(\xi, \cdot)_t^{(q)}(0)} \ge \left( \frac {c(q+1)}{n}\right)^{\frac{q+1}2}.$$ Here $\mathcal R f(\xi,t)$ is the Radon transform of $f$, and the fractional derivative of the order $q$ is taken with respect to the variable $t\in \mathbb R$ with fixed $\xi\in S^{n-1}.$ In this note we show that there exist an origin-symmetric convex body $K$ of volume 1 in $\mathbb R^n$ and a continuous probability density $g$ on $K$ so that $$\max_{\xi\in S^{n-1}} {\frac 1{\cos(\pi q/2)}\mathcal R g(\xi, \cdot)_t^{(q)}(0)} \leq \frac 1{\sqrt n} (c(q+1))^{\frac{q+1}2}.$$ In the case $q=0$ this was proved in [5,6], and it was used there to obtain a lower estimate for the maximal outer volume ratio distance from an arbitrary origin-symmetric convex body $K$ to the class of intersection bodies. We extend the latter result to the class $L_{-1-q}^n$ of bodies in $\mathbb R^n$ that embed in $L_{-1-q}.$ Namely, for every $q\in [0,n)$ there exists an origin-symmetric convex body $K$ in $\mathbb R^n$ so that ${d_{\operatorname{ovr}}}(K, L_{-1-q}^n) \ge c n^{\frac 1{2(q+1)}}.$