Exact $\ell^\infty$-separation radius of Sobol' sequences in dimension 2

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Auteur principal: Suzuki, Kosuke
Format: Preprint
Publié: 2025
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author Suzuki, Kosuke
author_facet Suzuki, Kosuke
contents Quasi-uniformity is a fundamental geometric property of point sets, crucial for applications such as kernel interpolation, Gaussian process regression, and space-filling experimental designs. While quasi-Monte Carlo methods are widely recognized for their low-discrepancy characteristics, understanding their quasi-uniformity remains important for practical applications. For the two-dimensional Sobol' sequence, Sobol' and Shukhman (2007) conjectured that the separation radius of the first $N$ points achieves the optimal rate $N^{-1/2}$, which would imply quasi-uniformity. This conjecture was disproved by Goda (2024), who computed exact values of the $\ell^2$-separation radius for a sparse subsequence $N = 2^{2^v-1}$. However, the general behavior of the Sobol' sequence for arbitrary $N$ remained unclear. In this paper, we derive exact expressions for the $\ell^\infty$-separation radius of the first $N = 2^m$ points of the two-dimensional Sobol' sequence for all $m \in \mathbb{N}$. As an immediate consequence, we show that the separation radius of Sobol' points is $O(N^{-3/4})$, which is strictly worse than the optimal rate $N^{-1/2}$, revealing that the two-dimensional Sobol' sequence has a suboptimal mesh ratio that grows at least as $N^{1/4}$.
format Preprint
id arxiv_https___arxiv_org_abs_2508_14803
institution arXiv
publishDate 2025
record_format arxiv
spellingShingle Exact $\ell^\infty$-separation radius of Sobol' sequences in dimension 2
Suzuki, Kosuke
Numerical Analysis
05B40, 11K36, 52C15
Quasi-uniformity is a fundamental geometric property of point sets, crucial for applications such as kernel interpolation, Gaussian process regression, and space-filling experimental designs. While quasi-Monte Carlo methods are widely recognized for their low-discrepancy characteristics, understanding their quasi-uniformity remains important for practical applications. For the two-dimensional Sobol' sequence, Sobol' and Shukhman (2007) conjectured that the separation radius of the first $N$ points achieves the optimal rate $N^{-1/2}$, which would imply quasi-uniformity. This conjecture was disproved by Goda (2024), who computed exact values of the $\ell^2$-separation radius for a sparse subsequence $N = 2^{2^v-1}$. However, the general behavior of the Sobol' sequence for arbitrary $N$ remained unclear. In this paper, we derive exact expressions for the $\ell^\infty$-separation radius of the first $N = 2^m$ points of the two-dimensional Sobol' sequence for all $m \in \mathbb{N}$. As an immediate consequence, we show that the separation radius of Sobol' points is $O(N^{-3/4})$, which is strictly worse than the optimal rate $N^{-1/2}$, revealing that the two-dimensional Sobol' sequence has a suboptimal mesh ratio that grows at least as $N^{1/4}$.
title Exact $\ell^\infty$-separation radius of Sobol' sequences in dimension 2
topic Numerical Analysis
05B40, 11K36, 52C15
url https://arxiv.org/abs/2508.14803