a(n) = n^9 - 1.

Terms

a(0) =0a(1) =511a(2) =19682a(3) =262143a(4) =1953124a(5) =10077695a(6) =40353606a(7) =134217727a(8) =387420488a(9) =999999999a(10) =2357947690a(11) =5159780351a(12) =10604499372a(13) =20661046783a(14) =38443359374a(15) =68719476735a(16) =118587876496a(17) =198359290367a(18) =322687697778a(19) =511999999999a(20) =794280046580