A348403 Number of chiral pairs of polyomino rings of length 2n with twofold rotational symmetry.

0, 0, 0, 0, 0, 0, 1, 3, 9, 21, 51, 117, 262, 598, 1307, 2987, 6456, 14752, 31859, 72839, 157611, 360472, 782802, 1791140, 3904323, 8936996, 19553272

Offset: 1

Author

John Mason, Oct 18 2021

Keywords

• nonn
• more

Examples

			a(7)=1 because of:
  OOOO
  O..OO
  OO..O
  .OOOO
		

Links

• D. H. Redelmeier, Counting polyominoes: yet another attack, Discrete Math., 36 (1981), 191-203.

Crossrefs

Cf. A348402 (all unoriented), A348404 (achiral), A324407 (unoriented with fourfold rotational symmetry), A324408 (chiral with fourfold rotational symmetry), A324409 (achiral with fourfold rotational symmetry).

Formula

a(n) = A348402(n) - A348404(n).