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Projections of the uniform distribution on the cube -- a large deviation perspective

Probability 2021-09-21 v2 Functional Analysis

Abstract

Let Θ(n)\Theta^{(n)} be a random vector uniformly distributed on the unit sphere Sn1\mathbb S^{n-1} in Rn\mathbb R^n. Consider the projection of the uniform distribution on the cube [1,1]n[-1,1]^n to the line spanned by Θ(n)\Theta^{(n)}. The projected distribution is the random probability measure μΘ(n)\mu_{\Theta^{(n)}} on R\mathbb R given by μΘ(n)(A):=12n[1,1]n1{u,Θ(n)A}du, \mu_{\Theta^{(n)}}(A) := \frac 1 {2^n} \int_{[-1,1]^n} \mathbb 1\{\langle u, \Theta^{(n)} \rangle \in A\} du, for Borel subets AA of R\mathbb{R}. It is well known that, with probability 11, the sequence of random probability measures μΘ(n)\mu_{\Theta^{(n)}} converges weakly to the centered Gaussian distribution with variance 1/31/3. We prove a large deviation principle for the sequence μΘ(n)\mu_{\Theta^{(n)}} on the space of probability measures on R\mathbb R with speed nn. The (good) rate function is explicitly given by I(ν(α)):=12log(1α22)I(\nu(\alpha)) := - \frac{1}{2} \log ( 1 - \|\alpha\|_2^2) whenever ν(α)\nu(\alpha) is the law of a random variable of the form \begin{align*} \sqrt{1 - \|\alpha\|_2^2 } \frac{Z}{\sqrt 3} + \sum_{ k = 1}^\infty \alpha_k U_k, \end{align*} where ZZ is standard Gaussian independent of U1,U2,U_1,U_2,\ldots which are i.i.d. Unif[1,1]\text{Unif}[-1,1], and α1α2\alpha_1 \geq \alpha_2 \geq \ldots is a non-increasing sequence of non-negative reals with α2<1\|\alpha\|_2<1. We obtain a similar result for random projections of the uniform distribution on the discrete cube {1,+1}n\{-1,+1\}^n.

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Cite

@article{arxiv.2103.16430,
  title  = {Projections of the uniform distribution on the cube -- a large deviation perspective},
  author = {Samuel G. G. Johnston and Zakhar Kabluchko and Joscha Prochno},
  journal= {arXiv preprint arXiv:2103.16430},
  year   = {2021}
}

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12 pages