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

Terms

a(0) =2a(1) =15a(2) =117a(3) =945a(4) =7857a(5) =66825a(6) =578097a(7) =5062905a(8) =44726337a(9) =397498185a(10) =3547250577a(11) =31743856665a(12) =284606318817