The $m$th order Orlicz projection bodies

Abstract

Let $M_{n, m}(\mathbb{R})$ be the space of $n\times m$ real matrices. Define $\mathcal{K}_o^{n,m}$ as the set of convex compact subsets in $M_{n,m}(\mathbb{R})$ with nonempty interior containing the origin $o\in M_{n, m}(\mathbb{R})$, and $\mathcal{K}_{(o)}^{n,m}$ as the members of $\mathcal{K}_o^{n,m}$ containing $o$ in their interiors. Let $Φ: M_{1, m}(\mathbb{R}) \rightarrow [0, \infty)$ be a convex function such that $Φ(o)=0$ and $Φ(z)+Φ(-z)>0$ for $z\neq o.$ In this paper, we propose the $m$th order Orlicz projection operator $Π_Φ^m: \mathcal{K}_{(o)}^{n,1}\rightarrow \mathcal{K}_{(o)}^{n,m}$, and study its fundamental properties, including the continuity and affine invariance. We establish the related higher-order Orlicz-Petty projection inequality, which states that the volume of $Π_Φ^{m, *}(K)$, the polar body of $Π_Φ^{m}(K)$, is maximized at origin-symmetric ellipsoids among convex bodies with fixed volume. Furthermore, when $Φ$ is strictly convex, we prove that the maximum is uniquely attained at origin-symmetric ellipsoids. Our proof is based on the classical Steiner symmetrization and its higher-order analogue. We also investigate the special case for $Φ_{Q}=ϕ\circ h_Q$, where $h_Q$ denotes the support function of $Q\in \mathcal{K}^{1, m}_o$ and $ϕ: [0, \infty)\rightarrow [0, \infty)$ is a convex function such that $ϕ(0)=0$ and $ϕ$ is strictly increasing on $[0, \infty).$ We establish a higher-order Orlicz-Petty projection inequality related to $Π_{Φ_Q}^{m, *} (K)$. Although $Φ_Q$ may not be strictly convex, we characterize the equality under the additional assumption on $Q$ and $ϕ$, such as $Q\in \mathcal{K}_{(o)}^{1,m}$ and the strict convexity of $ϕ$.