Number of tilings of a 9 X n rectangle using 3n trominoes of shape I.

Terms

a(0) =1a(1) =1a(2) =1a(3) =19a(4) =57a(5) =121a(6) =783a(7) =2861a(8) =8133a(9) =37160a(10) =143419a(11) =468816a(12) =1876855a(13) =7263468a(14) =25496863a(15) =97187247a(16) =372086645a(17) =1352780401a(18) =5071962134a(19) =19220628318a(20) =71025008365a(21) =265095817718a(22) =997839772024