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 A320495 Coordination sequence of thinnest 5-neighbor packing of the plane with congruent hexagons with respect to a point of type A. 7
 1, 4, 6, 12, 14, 19, 22, 28, 32, 34, 39, 44, 46, 52, 54, 58, 62, 69, 69, 75, 77, 82, 87, 93, 92, 98, 100, 107, 111, 117, 114, 122, 123, 132, 134, 140, 137, 146, 148, 156, 157, 163, 160, 171, 172, 180, 179, 187, 183, 196, 195, 203, 202, 211, 208, 220, 218, 226 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS "5-neighbor" means that each hexagon has a point in common with exactly five other hexagons. This packing is actually the thinnest 5-neighbor packing in the plane using any centrally symmetric congruent polygons. More formally, this sequence is the coordination sequence of the vertex-edge graph of the packing with respect to a vertex of type A. (The automorphism group of the tiling has four orbits on vertices, indicated by the letters A, B, C, D in the figure.) REFERENCES William Moser and Janos Pach, Research Problems in Discrete Geometry: Packing and Covering, DIMACS Technical Report 93-32, May 1993. See Fig. 19.1b, page 32. There is an error in the figure: the hexagon at the right of the bottom row should not be shaded. The figure shown here is correct. LINKS Rémy Sigrist, Table of n, a(n) for n = 0..1000 Rémy Sigrist, Illustration of first terms Rémy Sigrist, PARI program for A320495 N. J. A. Sloane, The packing and its graph. (The hexagons are shaded, the base point is marked A, and the green dots indicate the centers of large empty hexagrams.) FORMULA Based on the b-file, this appears to have g.f. = f/g, where f = -2*x^24+x^23-x^22-x^21+5*x^20+6*x^19+4*x^18+5*x^17+ 13*x^16+16*x^15+9*x^14+8*x^13+13*x^12+16*x^11+11*x^10 +9*x^9+14*x^8+13*x^7+12*x^6+11*x^5+9*x^4+8*x^3+5*x^2+4*x+1 and g = (1-x^2)*(1-x^6)*(1+x^4)*(1+x^8). - N. J. A. Sloane, Oct 25 2018 PROG (PARI) See Links section. CROSSREFS Cf. A320492, A320493, A320494, A320496, A320497, A320498. Sequence in context: A318712 A140599 A282280 * A047406 A136415 A310596 Adjacent sequences:  A320492 A320493 A320494 * A320496 A320497 A320498 KEYWORD nonn AUTHOR N. J. A. Sloane, Oct 22 2018 EXTENSIONS More terms from Rémy Sigrist, Oct 24 2018 STATUS approved

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Last modified April 19 17:22 EDT 2021. Contains 343117 sequences. (Running on oeis4.)