On the largest empty axis-parallel box amidst $n$ points

Abstract: We give the first nontrivial upper and lower bounds on the maximum volume of an empty axis-parallel box inside an axis-parallel unit hypercube in $\RR<sup>d$ containing $n$ points. For a fixed $d$, we show that the maximum volume is of the order $\Theta(\frac{1}{n})$. We then use the fact that the maximum volume is $\Omega(\frac{1}{n})$ in our design of the first efficient $(1-\eps)$-approximation algorithm for the following problem: Given an axis-parallel $d$-dimensional box $R$ in $\RR<sup>d$ containing $n$ points, compute a maximum-volume empty axis-parallel $d$-dimensional box contained in $R$. The running time of our algorithm is nearly linear in $n$, for small $d$, and increases only by an $O(\log{n})$ factor when one goes up one dimension. No previous efficient exact or approximation algorithms were known for this problem for $d \geq 4$. As the problem has been recently shown to be NP-hard in arbitrary high dimensions (i.e., when $d$ is part of the input), the existence of efficient exact algorithms is unlikely. We also obtain tight estimates on the maximum volume of an empty axis-parallel hypercube inside an axis-parallel unit hypercube in $\RR<sup>d$ containing $n$ points. For a fixed $d$, this maximum volume is of the same order order $\Theta(\frac{1}{n})$. A faster $(1-\eps)$-approximation algorithm, with a milder dependence on $d$ in the running time, is obtained in this case.