a(n) = n*8^n - 1.

Terms

a(0) =7a(1) =127a(2) =1535a(3) =16383a(4) =163839a(5) =1572863a(6) =14680063a(7) =134217727a(8) =1207959551a(9) =10737418239a(10) =94489280511a(11) =824633720831