A390654 a(n) is the number of 2n-celled fixed polyominoes with at least 180 degree rotational symmetry about the midpoint of a horizontal edge of a square in the lattice.

1, 3, 10, 35, 127, 472, 1778, 6760, 25896, 99783, 386320, 1501661, 5856748, 22908230, 89828072, 353006402, 1389925354, 5482090062, 21655518914, 85662816994, 339281765621, 1345314845818, 5339982398682, 21216353719303, 84369161695005, 335775967151692, 1337341813545314, 5330144686746579, 21257810880609714

Offset

1,2

Comments

Column R180M of the Shirakawa link.

Links

Table of n, a(n) for n=1..29.

Toshihiro Shirakawa, Enumeration of Polyominoes up to Size N=59, arXiv:2510.22446 [math.CO], 2025. See p. 5-6.

Formula

a(n) = 2*A234008(n) + A346799(n).

Crossrefs

Cf. A001168, A234008, A346799, A390653, A390655.

Sequence in context: A149036 A316596 A047127 * A114196 A346739 A078789

Adjacent sequences: A390651 A390652 A390653 * A390655 A390656 A390657

Keyword

nonn

Author

John Mason, Nov 14 2025

Status

approved