AI中文摘要
Lutwak的仿射quermass积分理论是现代仿射Brunn--Minkowski理论的基础组成部分。该理论自20世纪80年代发展起来,提供了经典quermass积分的仿射类比,并导致了一系列尖锐的仿射等周不等式。这一计划中的核心问题,追溯到Lutwak1988年的工作,是关于归一化$L^{-n}$-moment quermass积分$I_{k,-n}$的Alexandrov--Fenchel型单调性原理。一种形式的该原理预测:$$ I_{m,-n}(K)^{1/m}\ge I_{k,-n}(K)^{1/k}, \quad 1\le m<k\le n. $$ 该问题被记录在Gardner2006年的《几何体 tomography》一书中作为问题列表的一部分,而与最高维度$k=n$的比较由Milman和Yehudayoff在2023年JAMS论文中建立。我们证明,所提出的单调性在完整范围内并不持续。更具体地说,对于每一个满足$1\le m<k\le n-1$且$n>(m+2)(k+2)-2$的整数三元组,存在一个原点对称的$C^2_+$凸体$K\subset\mathbb R^n$,使得$$ I_{m,-n}(K)^{1/m} < I_{k,-n}(K)^{1/k}. $$ 该例子是从欧几里得球体通过任意小的四次球面谐波扰动获得的。在正向方面,我们证明在三维空间中,端点链成立:对于每一个凸体$K\subset\mathbb R^3$,$$ I_{1,-3}(K)\ge I_{2,-3}(K)^{1/2}\ge I_{3,-3}(K)^{1/3}=1. $$ 两个不等式的等号情况恰好是椭球体,除了平移和非奇异仿射变换外。
英文摘要
Lutwak's affine quermassintegral theory is a foundational component of modern affine Brunn--Minkowski theory. Developed in the 1980s, it provides affine analogues of the classical quermassintegrals and has led to a rich family of sharp affine isoperimetric inequalities. A central question in this program, going back to Lutwak's 1988 work, is an Alexandrov--Fenchel-type monotonicity principle for the normalized $L^{-n}$-moment quermassintegrals $I_{k,-n}$. In one form, this principle predicts that \[
I_{m,-n}(K)^{1/m}\ge I_{k,-n}(K)^{1/k},
\qquad 1\le m<k\le n . \] The question was recorded in Gardner's 2006 book Geometric Tomography as part of its problem list, and the comparison with the top dimension, $k=n$, was established by Milman and Yehudayoff in their 2023 JAMS paper.
We show that the proposed monotonicity does not persist in the full range. More precisely, for every triple of integers $m,k,n$ satisfying $1\le m<k\le n-1$ and $n>(m+2)(k+2)-2$, there exists an origin-symmetric $C^2_+$ convex body $K\subset\mathbb R^n$ such that \[
I_{m,-n}(K)^{1/m} < I_{k,-n}(K)^{1/k}. \] The example is obtained from the Euclidean ball by an arbitrarily small degree-four spherical harmonic perturbation.
On the positive side, we prove that the endpoint chain is true in dimension three: for every convex body $K\subset\mathbb R^3$, \[
I_{1,-3}(K)\ge I_{2,-3}(K)^{1/2}\ge I_{3,-3}(K)^{1/3}=1. \] The equality cases in both non-trivial inequalities are exactly ellipsoids, up to translation and nonsingular affine transformations.