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

Terms

a(0) =2a(1) =9a(2) =45a(3) =243a(4) =1377a(5) =8019a(6) =47385a(7) =282123a(8) =1686177a(9) =10097379a(10) =60525225a(11) =362974203a(12) =2177313777a(13) =13062288339a(14) =78368947065a(15) =470199333483