Solution of the propeller conjecture in $\mathbb{R}^3$

TL;DR

This paper proves the Propeller Conjecture in three dimensions, establishing a sharp inequality for measurable partitions of imensional space, with implications for computational complexity and the Unique Games conjecture.

Contribution

It provides the first proof of the 3D Propeller Conjecture, using computer-assisted verification of finite numerical inequalities.

Findings

Sharp bound  3 for partitions in imensional space

Equality characterized by specific sector-based partitions

Implication for the hardness threshold in Kernel Clustering

Abstract

It is shown that every measurable partition ${A_1,..., A_k}$ of $\mathbb{R}^3$ satisfies $$\sum_{i=1}^k||\int_{A_i} xe^{-\frac12||x||_2^2}dx||_2^2\le 9\pi^2.\qquad(*)$$ Let ${P_1,P_2,P_3}$ be the partition of $\mathbb{R}^2$ into $120^\circ$ sectors centered at the origin. The bound is sharp, with equality holding if $A_i=P_i\times \mathbb{R}$ for $i\in {1,2,3}$ and $A_i=\emptyset$ for $i\in \{4,...,k\}$ (up to measure zero corrections, orthogonal transformations and renumbering of the sets $\{A_1,...,A_k\}$). This settles positively the 3-dimensional Propeller Conjecture of Khot and Naor (FOCS 2008). The proof of reduces the problem to a finite set of numerical inequalities which are then verified with full rigor in a computer-assisted fashion. The main consequence (and motivation) of $(*)$ is complexity-theoretic: the Unique Games hardness threshold of the Kernel Clustering problem with $4 \times 4$ centered and spherical hypothesis matrix equals $\frac{2\pi}{3}$.