a(n) = 4^n - n^6.

Terms

a(0) =1a(1) =3a(2) =-48a(3) =-665a(4) =-3840a(5) =-14601a(6) =-42560a(7) =-101265a(8) =-196608a(9) =-269297a(10) =48576a(11) =2422743a(12) =13791232a(13) =62282055a(14) =260905920a(15) =1062351199a(16) =4278190080a(17) =17155731615a(18) =68685464512a(19) =274830861063