Number of polyominoes with perimeter at most 2n.

Terms

a(0) =0a(1) =1a(2) =2a(3) =5a(4) =11a(5) =36a(6) =122a(7) =538a(8) =2526a(9) =13166a(10) =71153a(11) =400109a(12) =2300430a(13) =13504780a(14) =80547558a(15) =487327904