a(n) = 1^n + 3^n + 9^n.

Terms

a(0) =3a(1) =13a(2) =91a(3) =757a(4) =6643a(5) =59293a(6) =532171a(7) =4785157a(8) =43053283a(9) =387440173a(10) =3486843451a(11) =31381236757a(12) =282430067923