Paper detail
Recursive Scheme for Angles of Random Simplices, and Applications to Random Polytopes
Consider a random simplex $[X_1,\ldots,X_n]$ defined as the convex hull of independent identically distributed random points $X_1,\ldots,X_n$ in $\mathbb{R}^{n-1}$ with the following beta density: $$ f_{n-1,β} (x) \propto (1-\|x\|^2)^β 1_{\{\|x\| < 1\}}, \qquad x\in\mathbb{R}^{n-1}, \quad β>-1. $$ Let $J_{n,k}(β)$ be the expected internal angle of the simplex $[X_1,\ldots,X_n]$ at its face $[X_1,\ldots,X_k]$. Define $\tilde J_{n,k}(β)$ analogously for i.i.d. random points distributed according to the beta' density $$ \tilde f_{n-1,β} (x) \propto (1+\|x\|^2)^{-β}, \qquad x\in\mathbb{R}^{n-1}, \quad β> \frac{n-1}{2}. $$ We derive formulae for $J_{n,k}(β)$ and $\tilde J_{n,k}(β)$ which make it possible to compute these quantities symbolically, in finitely many steps, for any integer or half-integer value of $β$. For $J_{n,1}(\pm 1/2)$ we even provide explicit formulae in terms of products of Gamma functions. We give applications of these results to two seemingly unrelated problems of stochastic geometry. (i) We compute the expected $f$-vectors of the typical Poisson-Voronoi cells in dimensions up to $10$. (ii) Consider the random polytope $K_{n,d} := [U_1,\ldots,U_n]$ where $U_1,\ldots,U_n$ are i.i.d. random points sampled uniformly inside some $d$-dimensional convex body $K$ with smooth boundary and unit volume. M. Reitzner proved the existence of the limit of the normalized expected $f$-vector of $K_{n,d}$: $$ \lim_{n\to\infty} n^{-{\frac{d-1}{d+1}}}\mathbb E \mathbf f(K_{n,d}) = \mathbf c_d \cdot Ω(K), $$ where $Ω(K)$ is the affine surface area of $K$, and $\mathbf c_d$ is an unknown vector not depending on $K$. We compute $\mathbf c_d$ explicitly in dimensions up to $d=10$ and also solve the analogous problem for random polytopes with vertices distributed uniformly on the sphere.
