a(n) = 2^n - (3-(-1)^n)/2.

Terms

a(0) =0a(1) =0a(2) =3a(3) =6a(4) =15a(5) =30a(6) =63a(7) =126a(8) =255a(9) =510a(10) =1023a(11) =2046a(12) =4095a(13) =8190a(14) =16383a(15) =32766a(16) =65535a(17) =131070a(18) =262143a(19) =524286a(20) =1048575a(21) =2097150a(22) =4194303a(23) =8388606a(24) =16777215a(25) =33554430a(26) =67108863a(27) =134217726a(28) =268435455a(29) =536870910