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%I #24 Apr 15 2024 06:58:57
%S 1,1,4,7,18,31,70,126,261,484,960,1800,3514,6643,12852,24457,47151,
%T 90157,173740,333498,643230,1238664,2392650,4620006,8939657,17302033,
%U 33538048,65042495,126289800,245361171,477153040,928510506,1808276211,3523813566,6871685532,13408154100,26178323928,51139027135
%N Number of Barlow packings with group P63mc that repeat after 2n layers.
%H D. Bundala, M. Codish, L. Cruz-Filipe et al., <a href="http://arxiv.org/abs/1412.5302">Optimal-Depth Sorting Networks</a>, arXiv preprint arXiv:1412.5302 [cs.DS], 2014. (See Table 4)
%H J. E. Iglesias, <a href="https://doi.org/10.1524/zkri.2006.221.4.237">Enumeration of closest-packings by the space group: a simple approach</a>, Z. Krist. 221 (2006) 237-245, Table 3.
%H T. J. McLarnan, <a href="http://dx.doi.org/10.1524/zkri.1981.155.3-4.269">The numbers of polytypes in close-packings and related structures</a>, Zeits. Krist. 155, 269-291 (1981).
%Y Appears to be column k=2 of A309784.
%Y Cf. A045668.
%K nonn
%O 6,3
%A _N. J. A. Sloane_
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