Number of undirected Knight's tours on a 3 X n board.

Terms

a(0) =0a(1) =0a(2) =0a(3) =8a(4) =0a(5) =0a(6) =52a(7) =396a(8) =560a(9) =3048a(10) =10672a(11) =57248a(12) =128864a(13) =646272a(14) =1838784a(15) =8636880a(16) =23400992a(17) =105865688a(18) =305753680a(19) =1322849752a(20) =3862974304a(21) =16225820000a(22) =48744080192a(23) =198673312880a(24) =607041217056