a(n) = 3^n*tetranacci(n) or (2^n)*A001648(n).

Terms

a(0) =3a(1) =27a(2) =189a(3) =1215a(4) =6318a(5) =37179a(6) =216513a(7) =1253151a(8) =7223661a(9) =41806692a(10) =241805655a(11) =1398221271a(12) =8084811933a(13) =46753521975a(14) =270362105694