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# Optimal Tiling of the Euclidean Space Using Permutation-Symmetric Bodies

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Mark Braverman and Dor Minzer. Optimal Tiling of the Euclidean Space Using Permutation-Symmetric Bodies. In 36th Computational Complexity Conference (CCC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 200, pp. 5:1-5:48, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)
https://doi.org/10.4230/LIPIcs.CCC.2021.5

## Abstract

What is the least surface area of a permutation-symmetric body B whose ℤⁿ translations tile ℝⁿ? Since any such body must have volume 1, the isoperimetric inequality implies that its surface area must be at least Ω(√n). Remarkably, Kindler et al. showed that for general bodies B this is tight, i.e. that there is a tiling body of ℝⁿ whose surface area is O(√n). In theoretical computer science, the tiling problem is intimately related to the study of parallel repetition theorems (which are an important component in PCPs), and more specifically in the question of whether a "strong version" of the parallel repetition theorem holds. Raz showed, using the odd cycle game, that strong parallel repetition fails in general, and subsequently these ideas were used in order to construct non-trivial tilings of ℝⁿ. In this paper, motivated by the study of a symmetric parallel repetition, we consider the permutation-symmetric variant of the tiling problem in ℝⁿ. We show that any permutation-symmetric body that tiles ℝⁿ must have surface area at least Ω(n/√{log n}), and that this bound is tight, i.e. that there is a permutation-symmetric tiling body of ℝⁿ with surface area O(n/√{log n}). We also give matching bounds for the value of the symmetric parallel repetition of Raz’s odd cycle game. Our result suggests that while strong parallel repetition fails in general, there may be important special cases where it still applies.

## Subject Classification

##### ACM Subject Classification
• Theory of computation → Problems, reductions and completeness
##### Keywords
• PCP
• Parallel Repetition
• Tilings

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