The generalized Busemann intersection inequality

Let LL be a measurable star set in Rn\mathbb R^n, let 1j<k<n1\leq j<k<n, and let Gn,rG_{n,r} denote the Grassmannian of rr-dimensional linear subspaces with invariant probability measure dτ0d_*\tau_0 or dζ0d_*\zeta_0. Write VkV_k for kk-dimensional volume and V~m\widetilde V_m for the corresponding generalized section functional. Then

Gn,k[Vk(Lτ0)]n/jdτ0(bkbj)n/j(Gn,j[V~k(Lζ0)]n/kdζ0)k/j.\int_{G_{n,k}}[V_k(L\cap\tau_0)]^{n/j}\,d_*\tau_0 \leq \left(\frac{b_k}{b_j}\right)^{n/j}\left(\int_{G_{n,j}}[\widetilde V_k(L\cap\zeta_0)]^{n/k}\,d_*\zeta_0\right)^{k/j}.

Generalized Busemann intersection inequality. More generally, for an arbitrary real number mm such that the integral on the right-hand side exists in the Lebesgue sense,

Gn,k[V~m(Lτ0)]n/jdτ0(bkbj)n/j(Gn,j[V~m(Lζ0)]n/kdζ0)k/j.\int_{G_{n,k}}[\widetilde V_m(L\cap\tau_0)]^{n/j}\,d_*\tau_0 \leq \left(\frac{b_k}{b_j}\right)^{n/j}\left(\int_{G_{n,j}}[\widetilde V_m(L\cap\zeta_0)]^{n/k}\,d_*\zeta_0\right)^{k/j}.

This would generalize the Busemann intersection inequality and the known kk-plane-transform estimate from convex bodies to arbitrary measurable star sets and from the kk-section functional to arbitrary admissible real powers mm. The source presents it as a conjecture and gives no resolution.

Sources & referencesView supporting material

Primary source

Boris Rubin, “Norm Estimates for k-Plane Transforms and Geometric Inequalities”, arXiv:1801.00186 (2017).

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