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 A324407 Number of unoriented polyomino rings of length 4n with fourfold rotational symmetry. 3
 1, 1, 1, 2, 3, 6, 10, 21, 38, 80, 157, 336, 691, 1493, 3164, 6899, 14880, 32628, 71212, 156856, 345216, 762870, 1689978, 3743888, 8338405, 18507829, 41410352, 92053712, 206790477, 460256854, 1037575558, 2311730792, 5227759707, 11657738806, 26436550400 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS Redelmeier uses these rings to enumerate polyominoes of the regular tiling {4,4} with fourfold rotational symmetry (A144553) and an even number of cells. Each cell of a polyomino ring is adjacent to (shares an edge with) exactly two other cells. For unoriented rings, a chiral ring and its congruent reflection are counted as one. For n odd, the center of the ring is a vertex of the tiling; for n even, the center is the center of a tile. LINKS D. H. Redelmeier, Counting polyominoes: yet another attack, Discrete Math., 36 (1981), 191-203. FORMULA a(n) = A324406(n) - A324408(n) = (A324406(n) + A324409(n)) / 2 = A324408(n) + A324409(n). EXAMPLE For a(1)=1, the four cells form a square. For a(2)=1, the eight cells form a 3 X 3 square with the center cell omitted. For a(3)=1, the twelve cells form a 4 X 4 square with the four inner cells omitted. For a(4)=2, the sixteen cells of one ring form a 5 X 5 square with the nine inner cells omitted; the other ring is similar, but with each corner cell omitted and replaced with the cell diagonally toward the center from that corner cell. CROSSREFS Cf. A324406 (oriented), A324408 (chiral), A324409 (achiral). Cf. A144553. Sequence in context: A215067 A008928 A124343 * A032291 A063687 A002988 Adjacent sequences:  A324404 A324405 A324406 * A324408 A324409 A324410 KEYWORD nonn,hard AUTHOR Robert A. Russell, Feb 26 2019 STATUS approved

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Last modified November 29 08:47 EST 2020. Contains 338762 sequences. (Running on oeis4.)