a(n) = 5^n + 4.

Terms

a(0) =5a(1) =9a(2) =29a(3) =129a(4) =629a(5) =3129a(6) =15629a(7) =78129a(8) =390629a(9) =1953129a(10) =9765629a(11) =48828129a(12) =244140629a(13) =1220703129a(14) =6103515629a(15) =30517578129a(16) =152587890629a(17) =762939453129