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URL: https://oeis.org/A390655

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A390655
a(n) is the number of 2n-celled fixed polyominoes with at least 180 degree rotational symmetry about a vertex of a square in the lattice.
3
0, 1, 4, 14, 50, 186, 704, 2685, 10312, 39841, 154642, 602468, 2354592, 9227412, 36246300, 142671941, 562600898, 2222092230, 8789217470, 34809890792, 138027690188, 547891969992, 2176951996156, 8657510269463, 34458522549812, 137256376227738, 547112224619120, 2182260930530668, 8709713766597954
OFFSET
1,3
COMMENTS
Column R180V of the Shirakawa link.
LINKS
Toshihiro Shirakawa, Enumeration of Polyominoes up to Size N=59, arXiv:2510.22446 [math.CO], 2025. See p. 5-6.
FORMULA
a(n) = 4*A351616(n) + 2*A351191(n/2) + 2*A351160(n) + 2*A234007(n/2) + A346800(n/2).
KEYWORD
nonn
AUTHOR
John Mason, Nov 14 2025
STATUS
approved