Number of fault-free tilings of a 4 X 2n rectangle with L tetrominoes.

Terms

a(0) =2a(1) =6a(2) =10a(3) =18a(4) =38a(5) =84a(6) =186a(7) =410a(8) =904a(9) =1994a(10) =4398a(11) =9700a(12) =21394a(13) =47186a(14) =104072a(15) =229538a(16) =506262a(17) =1116596a(18) =2462730a(19) =5431722a(20) =11980040a(21) =26422810a(22) =58277342a(23) =128534724a(24) =283492258a(25) =625261858a(26) =1379058440a(27) =3041609138a(28) =6708480134a(29) =14796018708