a(n) = 3^n + 5^n + 7^n.

Terms

a(0) =3a(1) =15a(2) =83a(3) =495a(4) =3107a(5) =20175a(6) =134003a(7) =903855a(8) =6161987a(9) =42326415a(10) =292299923a(11) =2026332015a(12) =14085959267a(13) =98111307855a(14) =684331371443