Number of n X 3 nonconsecutive chess tableaux.

Terms

a(0) =1a(1) =0a(2) =0a(3) =1a(4) =1a(5) =7a(6) =27a(7) =128a(8) =640a(9) =3351a(10) =18313a(11) =103404a(12) =600538a(13) =3571717a(14) =21683185a(15) =134005373a(16) =841259885a(17) =5355078350a(18) =34512405410a(19) =224908338137