a(n) = 4*8^n - 1.

Terms

a(0) =3a(1) =31a(2) =255a(3) =2047a(4) =16383a(5) =131071a(6) =1048575a(7) =8388607a(8) =67108863a(9) =536870911a(10) =4294967295a(11) =34359738367a(12) =274877906943