A362709 a(n) is the number of almost almost unitary polyominoes of size n. An almost almost unitary polyomino is one in which all but 2 of its perimeter walls have length 1.

0, 1, 2, 2, 4, 12, 15, 29, 58, 116, 199, 403, 790, 1509, 2857, 5700, 10941, 21032

Offset: 1

Author

John Mason, Apr 30 2023

Keywords

• nonn
• hard
• more

Examples

			a(4)=2 because of:
  O  O
  O  OO
  O   O
  O
		

Links

• George Sicherman, Catalogue of Almost Almost Unitary Polyominoes

Crossrefs

Cf. A245660, A362708.