a(n) = 3^n + 6^n + 9^n.

Terms

a(0) =3a(1) =18a(2) =126a(3) =972a(4) =7938a(5) =67068a(6) =578826a(7) =5065092a(8) =44732898a(9) =397517868a(10) =3547309626a(11) =31744033812a(12) =284606850258