Number of perfect matchings (or domino tilings) in C_4 X P_n.

Terms

a(0) =1a(1) =2a(2) =9a(3) =32a(4) =121a(5) =450a(6) =1681a(7) =6272a(8) =23409a(9) =87362a(10) =326041a(11) =1216800a(12) =4541161a(13) =16947842a(14) =63250209a(15) =236052992a(16) =880961761a(17) =3287794050a(18) =12270214441a(19) =45793063712a(20) =170902040409a(21) =637815097922