On the volume of the intersection of two $L_p^n$ balls

Abstract

This note deals with the following problem, the case $p=1$, $q=2$ of which was introduced to us by Vitali Milman: What is the volume left in the $L_p^n$ ball after removing a t-multiple of the $L_q^n$ ball? Recall that the $L_r^n$ ball is the set $\{(t_1,t_2,\dots,t_n);\ t_i\in{\bf R},\ n^{-1}\sum_{i=1}^n|t_i|^r\le 1\}$ and note that for $0<p<q<\infty$ the $L_q^n$ ball is contained in the $L_p^n$ ball. In Corollary 4 we show that, after normalizing Lebesgue measure so that the volume of the $L_p^n$ ball is one, the answer to the problem above is of order $e^{-ct^pn^{p/q}}$ for $T<t<{1\over 2}n^ {{1\over p}-{1\over q}}$, where $c$ and $T$ depend on $p$ and $q$ but not on $n$. The main theorem, Theorem 3, deals with the corresponding question for the surface measure of the $L_p^n$ sphere.