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A348404
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Number of achiral polyomino rings of length 2n with twofold rotational symmetry.
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3
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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
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OFFSET
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1,6
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LINKS
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FORMULA
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EXAMPLE
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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
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CROSSREFS
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Cf. A348402 (all unoriented), A348403 (chiral), A324407 (unoriented with fourfold rotational symmetry), A324408 (chiral with fourfold rotational symmetry), A324409 (achiral with fourfold rotational symmetry).
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KEYWORD
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nonn,more
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AUTHOR
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STATUS
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approved
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