Number of closed knight's tours in the first 2n cells of a 3 X ceiling(2n/3) board.

Terms

a(0) =4a(1) =0a(2) =4a(3) =24a(4) =16a(5) =56a(6) =306a(7) =176a(8) =456a(9) =2632a(10) =1536a(11) =4828a(12) =26788a(13) =15424a(14) =44952a(15) =254288a(16) =147728a(17) =448032a(18) =2502568a(19) =1448416a(20) =4310048a(21) =24228704a(22) =14060048a(23) =42195584a(24) =236335248a(25) =136947616a(26) =409403328a(27) =2297294496a(28) =1332257856a(29) =3989883552