a(n) = 2*3^(2*n)-3^n.

Terms

a(0) =1a(1) =15a(2) =153a(3) =1431a(4) =13041a(5) =117855a(6) =1062153a(7) =9563751a(8) =86086881a(9) =774821295a(10) =6973509753a(11) =62761942071a(12) =564858541521