Convex order under majorization for projections of ell-p balls

Abstract

Let $n\ge1$ and $1\le p\le2$, and let $X$ be uniformly distributed on the unit ball $B_p^n$ of $\ell_p^n$. We prove that if $(a_1^2,\ldots,a_n^2)$ majorizes $(b_1^2,\ldots,b_n^2)$, then $\langle b,X\rangle^2$ is dominated by $\langle a,X\rangle^2$ in convex order. For $p<2$ this gives a nontrivial comparison of the complete squared-projection laws, whereas at $p=2$ it reduces to equality by rotational invariance. The result strengthens known majorization inequalities for individual power moments and provides a single distributional source for several familiar projection inequalities. Its proof combines a two-dimensional hinge-function comparison with a preservation theorem for Beta--Rademacher perturbations, which lifts elementary majorization transfers through the Dirichlet representation of the uniform measure on $B_p^n$. The comparison is stable under a common independent radial scaling and therefore extends to $\ell_p$-radial measures with nonincreasing radial density. Consequences include simultaneous inequalities for moments, hinge functions, and Laplace transforms, together with Schur-convexity of Gaussian-regularized central-section densities. The range is sharp in the stated direction: for $p>2$, a support obstruction already occurs between a coordinate direction and a two-coordinate direction.