a(n) = 1^n + 5^n + 7^n.

Terms

a(0) =3a(1) =13a(2) =75a(3) =469a(4) =3027a(5) =19933a(6) =133275a(7) =901669a(8) =6155427a(9) =42306733a(10) =292240875a(11) =2026154869a(12) =14085427827a(13) =98109713533a(14) =684326588475