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 A214813 Maximal contact number of a subset of n balls from the face-centered cubic lattice. 1
 0, 1, 3, 6, 9, 12, 15, 18, 21 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS If S is an arrangement of non-overlapping balls of radius 1, the contact number of S is the number of pairs of balls that just touch each other. a(13) >= 36 (take one ball and its 12 neighbors), so this is different from A008486. If b(n) denotes the maximal contact number of any arrangement of n balls then it is conjectured that a(n) = b(n) for n <= 9. It is also known that b(10)>=25, b(11)>=29, b(12)>=33 and of course b(13) >= a(13) >= 36. [Bezdek 2012] Note that Figure 1e of Bezdek's arxiv:1601.00145 shows at n=5 a sphere packing with 9 contacts on the hexagonal close package (!), not on the cubic close package (which equals the f.c.c.). [In Figure 1e there is one sphere that touches from above a set of 3 spheres in a middle layer right above the bottom sphere; so this needs the ABABA... layer structures of the h.c.p, and cannot be done with the ABCABC... layer structure of the f.c.c.] So Figure 1e is not demonstrating a(5)=9. The correct value for the f.c.c is apparently a(5)=8 (where two structures with 8 contacts exist.) - R. J. Mathar, Mar 13 2018 LINKS Bezdek, Karoly, Contact Numbers for Congruent Sphere Packings in Euclidean 3-Space, Discrete Comput. Geom. 48 (2012), no. 2, 298--309. MR2946449 K. Bezdek, M. A. Khan, Contact number for sphere packings, arXiv:1601.00145 [math.MG], 2016. K. Bezdek, S. Reid, Contact graphs of unit sphere packings revisited, J. Geom. 104 (1) (2013) 57-83. J. P. K. Doye, D. J. Wales, Magic numbers and growth sequences of small face-centered-cubic and decahedral clusters, Chem. Phys. Lett. 247 (1995) 339, Table 1 column n(fcc). G. Nebe and N. J. A. Sloane, Home page for this lattice CROSSREFS Cf. A004015, A005901, A038173. Sequence in context: A198263 A276192 A282143 * A119888 A305495 A003252 Adjacent sequences:  A214810 A214811 A214812 * A214814 A214815 A214816 KEYWORD nonn,more AUTHOR N. J. A. Sloane, Jul 31 2012 STATUS approved

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Last modified January 23 08:58 EST 2019. Contains 319376 sequences. (Running on oeis4.)