a(n) = 5^n - n.

Terms

a(0) =1a(1) =4a(2) =23a(3) =122a(4) =621a(5) =3120a(6) =15619a(7) =78118a(8) =390617a(9) =1953116a(10) =9765615a(11) =48828114a(12) =244140613a(13) =1220703112a(14) =6103515611a(15) =30517578110a(16) =152587890609a(17) =762939453108