a(n) is the number of free polyominoes of width 2 and size n.

Terms

a(0) =1a(1) =1a(2) =4a(3) =5a(4) =12a(5) =18a(6) =37a(7) =60a(8) =117a(9) =200a(10) =379a(11) =669a(12) =1250a(13) =2247a(14) =4168a(15) =7570a(16) =13987a(17) =25549a(18) =47108a(19) =86319a(20) =158978a(21) =291806a(22) =537105a(23) =986786a(24) =1815699a(25) =3337560a(26) =6140047a(27) =11289571a(28) =20767180a(29) =38189927