A348404 Number of achiral polyomino rings of length 2n with twofold rotational symmetry.

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

Offset: 1

Author

John Mason, Oct 18 2021

Keywords

• nonn
• more

Examples

			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
		

Links

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

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).

Formula

a(n) = A348402(n) - A348403(n).