a(n) = 5^n - n^3.

Terms

a(0) =1a(1) =4a(2) =17a(3) =98a(4) =561a(5) =3000a(6) =15409a(7) =77782a(8) =390113a(9) =1952396a(10) =9764625a(11) =48826794a(12) =244138897a(13) =1220700928a(14) =6103512881a(15) =30517574750a(16) =152587886529a(17) =762939448212