Powers of 3 which are not powers of 3^3.

Terms

a(0) =3a(1) =9a(2) =81a(3) =243a(4) =2187a(5) =6561a(6) =59049a(7) =177147a(8) =1594323a(9) =4782969a(10) =43046721a(11) =129140163a(12) =1162261467a(13) =3486784401a(14) =31381059609a(15) =94143178827a(16) =847288609443