A360421 a(n) = the number of X-frame polyominoes with n cells, reduced for symmetry.

0, 0, 0, 0, 1, 2, 7, 20, 49, 109, 216, 414, 723, 1228, 1958, 3065, 4580, 6734

Offset: 1

Author

John Mason, Feb 06 2023

Keywords

• nonn
• more

Comments

An X-frame polyomino has a perimeter that forms a self-avoiding polygon such that as you traverse the perimeter counterclockwise you encounter turns in the order LLRLLRLLRLLR.

Examples

			a(5) = 1 because of:
    O
   OOO
    O
The a(7) = 7 polyominoes are:
    O      O      O       O     O     O     O
   OOOOO  OOOOO  OOOOO  OOOOO  OOOO  OOOO  OOOO
    O       O       O     O     O      O    OO
                                O      O
		

Crossrefs

Cf. A028247, A270060, A360419, A360420.