Number of nonisomorphic spanning trees of the triangular snake nC_3.

Terms

a(0) =1a(1) =3a(2) =7a(3) =21a(4) =57a(5) =171a(6) =495a(7) =1485a(8) =4401a(9) =13203a(10) =39447a(11) =118341a(12) =354537a(13) =1063611a(14) =3189375a(15) =9568125a(16) =28700001a(17) =86100003a(18) =258286887a(19) =774860661a(20) =2324542617a(21) =6973627851a(22) =20920765455a(23) =62762296365a(24) =188286534801a(25) =564859604403