Functional L\"owner Ellipsoids

Abstract

We extend the notion of the smallest volume ellipsoid containing a convex body in~$\mathbb{R}^{d}$ to the setting of logarithmically concave functions. We consider a vast class of logarithmically concave functions whose superlevel sets are concentric ellipsoids. For a fixed function from this class, we consider the set of all its "affine" positions. For any log-concave function $f$ on $\mathbb{R}^{d},$ we consider functions belonging to this set of "affine" positions, and find the one with the smallest integral under the condition that it is pointwise greater than or equal to $f.$ We study the properties of existence and uniqueness of the solution to this problem. For any $s \in [0,\infty),$ we consider the construction dual to the recently defined John $s$-function \cite{ivanov2020functional}. We prove that such a construction determines a unique function and call it the \emph{L\"owner $s$-function} of $f.$ We study the L\"owner $s$-functions as $s$ tends to zero and to infinity. Finally, extending the notion of the outer volume ratio, we define the outer integral ratio of a log-concave function and give an asymptotically tight bound on it. \end{abstract}