a(n) = 1^n + 5^n + 9^n.

Terms

a(0) =3a(1) =15a(2) =107a(3) =855a(4) =7187a(5) =62175a(6) =547067a(7) =4861095a(8) =43437347a(9) =389373615a(10) =3496550027a(11) =31429887735a(12) =282673677107