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

Terms

a(0) =1a(1) =2a(2) =1a(3) =0a(4) =17a(5) =118a(6) =513a(7) =1844a(8) =6049a(9) =18954a(10) =58049a(11) =175816a(12) =529713a(13) =1592126a(14) =4780225a(15) =14345532a(16) =43042625a(17) =129135250a(18) =387414657a(19) =1162254608a(20) =3486776401a(21) =10460343942a(22) =31381048961a(23) =94143166660a(24) =282429522657a(25) =847288593818