Finiteness and strict growth conjecture for compact packings with n radii

For each nNn\in\mathbb{N}, let Πn\Pi_n denote the set of compact packings of the plane with nn distinct circle radii, represented by radius ratios (rn1,rn2,,r1)(r_{n-1},r_{n-2},\ldots,r_1). The construction obtained by filling interstitial gaps of a compact (n1)(n-1)-packing corresponds to the truncation (rn2,,r1)Πn1(r_{n-2},\ldots,r_1)\in\Pi_{n-1}. Finiteness and strict growth conjecture. For every nNn\in\mathbb{N}, the set Πn\Pi_n is finite and the sequence (Πn)nN(|\Pi_n|)_{n\in\mathbb{N}} is strictly increasing. Furthermore, for every nNn\in\mathbb{N}, there exists an element (rn1,rn2,,r1)Πn(r_{n-1},r_{n-2},\ldots,r_1)\in\Pi_n with (rn2,,r1)Πn1(r_{n-2},\ldots,r_1)\notin\Pi_{n-1}; equivalently, not every compact nn-packing arises from filling interstitial gaps of a compact (n1)(n-1)-packing. The conjecture extends the observed finiteness of Π2\Pi_2 and Π3\Pi_3 and predicts genuinely new compact packings at every number of radii, rather than only those obtained by recursively filling gaps.

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Primary source

Miek Messerschmidt, “On compact packings of the plane with circles of three radii”, arXiv:1709.03487 (2019).

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