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 A348404 Number of achiral polyomino rings of length 2n with twofold rotational symmetry. 3
 0, 1, 0, 1, 1, 3, 2, 6, 4, 14, 8, 30, 18, 71, 40, 155, 89, 358, 198, 786, 445, 1808, 998, 3994, 2250, 9158, 5068 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,6 LINKS Table of n, a(n) for n=1..27. D. H. Redelmeier, Counting polyominoes: yet another attack, Discrete Math., 36 (1981), 191-203. FORMULA a(n) = A348402(n) - A348403(n). EXAMPLE a(2)=1 because of: OO OO a(4)=1 because of: OOO O.O OOO a(5)=1 because of: OOOO O..O OOOO CROSSREFS Cf. A348402 (all unoriented), A348403 (chiral), A324407 (unoriented with fourfold rotational symmetry), A324408 (chiral with fourfold rotational symmetry), A324409 (achiral with fourfold rotational symmetry). Sequence in context: A162255 A164073 A286595 * A218615 A177828 A248479 Adjacent sequences: A348401 A348402 A348403 * A348405 A348406 A348407 KEYWORD nonn,more AUTHOR John Mason, Oct 18 2021 STATUS approved

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Last modified July 24 15:22 EDT 2024. Contains 374584 sequences. (Running on oeis4.)