The number of free polyominoes of width 2 and height n.

Terms

a(0) =2a(1) =6a(2) =12a(3) =30a(4) =65a(5) =158a(6) =362a(7) =875a(8) =2064a(9) =4984a(10) =11914a(11) =28764a(12) =69155a(13) =166956a(14) =402372a(15) =971413a(16) =2343518a(17) =5657754a(18) =13654968a(19) =32966010a(20) =79577189a(21) =192116330a(22) =463786190a(23) =1119678911a(24) =2703086892a(25) =6525829036a(26) =15754607062a(27) =38034986040a(28) =91824246215a(29) =221683340568