a(n) = 5^n + 7^n + 9^n.

Terms

a(0) =3a(1) =21a(2) =155a(3) =1197a(4) =9587a(5) =78981a(6) =664715a(7) =5684637a(8) =49202147a(9) =429727221a(10) =3779025275a(11) =33407214477a(12) =296514964307