Trails of length n on hexagonal lattice.

Terms

a(0) =1a(1) =6a(2) =30a(3) =150a(4) =738a(5) =3570a(6) =17118a(7) =81498a(8) =385710a(9) =1817046a(10) =8528478a(11) =39903462a(12) =186198642a(13) =866861394a(14) =4027766490a(15) =18681900270a(16) =86518735722a(17) =400136695074