Number of n-step polygons on hexagonal lattice.

Terms

a(0) =1a(1) =0a(2) =0a(3) =12a(4) =24a(5) =60a(6) =180a(7) =588a(8) =1968a(9) =6840a(10) =24240a(11) =87252a(12) =318360a(13) =1173744a(14) =4366740a(15) =16370700a(16) =61780320a(17) =234505140a(18) =894692736a(19) =3429028116a(20) =13195862760a(21) =50968206912a(22) =197517813636a(23) =767766750564