Random Polyhedral Cones: Distributional Results via Gale Duality

Abstract: Let $U_1,\ldots,U_n$ be independent random vectors uniformly distributed on the unit sphere $\mathbb S^{{d-1}\subseteq\mathbb} R<sup>d$, where $n\ge d$, and consider the random polyhedral cone [ \mathcal W_{n,d}:=\mathop{\mathrm{pos}} (U_1,\ldots,U_n) = {λ1 U_1+ \ldots + λ_n U_n: λ_1\geq 0, \ldots, λ_n \geq 0}. ] We establish several distributional results for $\mathcal W</em>{n,d}$ and the associated spherical polytope $\mathcal W_{n,d}\cap\mathbb S<sup>{d-1}$. Our main contributions include: (i) Let $α<em>d$ denote the solid angle of $\mathcal W</em>{d,d}$ and write $m(d,k):=\mathbb E[α<em>d<sup>k]$ for its $k$-th moment. We prove the symmetry $m(d,k)=m(k,d)$. As an application, we compute $$\mathop{\mathrm{Var}}[α_d]=2<sup>{-d}(d+1)<sup>{-1}-4<sup>{-d}$$ and derive a closed formula for the third moment. (ii) For $n=d+1,d+2,d+3$ we determine the probability that $\mathcal W</em>{n,d}\cap\mathbb S<sup>{d-1}$ is a spherical simplex, a spherical analogue of the classical Sylvester problem. In the case $n=d+2$ we also determine the distribution of the number of vertices of $\mathcal W_{d+2,d}\cap\mathbb S<sup>{d-1}$. (iii) Let $f_\ell(\mathcal W_{n,d})$ denote the number of $\ell$-dimensional faces of $\mathcal W_{n,d}$. We prove a distributional limit theorem for $f_\ell(\mathcal W_{n,d})$ in the regime $n=d+k$ and $\ell=d-q$, where $k,q\in\mathbb N$ are fixed and $d\to\infty$. The limit law is a weighted sum of independent chi squared variables, with weights given by explicit eigenvalues of a convolution operator on the sphere. A unifying ingredient is an explicit coupling producing i.i.d. uniform vectors $U_1,\ldots,U_n\in\mathbb S<sup>{d-1}$ together with i.i.d. uniform vectors $V_1,\ldots,V_n\in\mathbb S<sup>{n-d-1}$ whose associated oriented matroids are Gale dual.