a(n) = 8*5^n+1.

Terms

a(0) =9a(1) =41a(2) =201a(3) =1001a(4) =5001a(5) =25001a(6) =125001a(7) =625001a(8) =3125001a(9) =15625001a(10) =78125001a(11) =390625001a(12) =1953125001a(13) =9765625001a(14) =48828125001a(15) =244140625001