a(n) = 3^(2*n+1).

Terms

a(0) =3a(1) =27a(2) =243a(3) =2187a(4) =19683a(5) =177147a(6) =1594323a(7) =14348907a(8) =129140163a(9) =1162261467a(10) =10460353203a(11) =94143178827a(12) =847288609443