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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
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
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.)