a(n) = 3^n + 6^n + 9^n + 12^n.

Terms

a(0) =4a(1) =30a(2) =270a(3) =2700a(4) =28674a(5) =315900a(6) =3564810a(7) =40896900a(8) =474714594a(9) =5557298220a(10) =65464673850a(11) =774752404500