Packings of regular pentagons in the plane

Abstract:

We show that every packing of congruent regular pentagons in the Euclidean plane has density at most $(5-\sqrt5)/3\approx 0.92$. More specifically, this article proves the pentagonal ice-ray conjecture of Henley $(1986)$, and Kuperberg and Kuperberg $(1990)$, which asserts that an optimal packing of congruent regular pentagons in the plane is a double lattice, formed by aligned vertical columns of upward pointing pentagons alternating with aligned vertical columns of downward pointing pentagons. The strategy is based on estimates of the areas of Delaunay triangles. Our strategy reduces the pentagonal ice-ray conjecture to area minimization problems that involve at most four Delaunay triangles. These minimization problems are solved by computer. The computer-assisted portions of the proof use techniques such as interval arithmetic, automatic differentiation, and a meet-in-the-middle algorithm.

Comments and Corrigenda:

Abstract of Version 1: We show that every packing of regular pentagons in the Euclidean plane has density less than 0.9611. Our proof is computer-assisted. We also give a detailed strategy for proving the Kuperberg-Kuperberg conjecture, which asserts that the optimal packing of regular pentagons in the plane is a double lattice, formed by aligned vertical columns of upward pointing pentagons alternating with aligned vertical columns of downward pointing pentagons. The strategy is based on estimates of the areas of Delaunay triangles. Our strategy reduces the Kuperberg conjecture to area minimization problems that involve at most four acute Delaunay triangles.