a(n) = 2^n + (-1)^(n+1).

Terms

a(0) =0a(1) =3a(2) =3a(3) =9a(4) =15a(5) =33a(6) =63a(7) =129a(8) =255a(9) =513a(10) =1023a(11) =2049a(12) =4095a(13) =8193a(14) =16383a(15) =32769a(16) =65535a(17) =131073a(18) =262143a(19) =524289a(20) =1048575a(21) =2097153a(22) =4194303a(23) =8388609a(24) =16777215a(25) =33554433a(26) =67108863a(27) =134217729a(28) =268435455a(29) =536870913