a(n) = 4^n - n^3.

Terms

a(0) =1a(1) =3a(2) =8a(3) =37a(4) =192a(5) =899a(6) =3880a(7) =16041a(8) =65024a(9) =261415a(10) =1047576a(11) =4192973a(12) =16775488a(13) =67106667a(14) =268432712a(15) =1073738449a(16) =4294963200a(17) =17179864271a(18) =68719470904a(19) =274877900085