(1/2)*(A204991).

Terms

a(0) =1a(1) =1a(2) =3a(3) =2a(4) =15a(5) =3a(6) =7a(7) =4a(8) =63a(9) =15a(10) =1023a(11) =6a(12) =4095a(13) =7a(14) =15a(15) =8a(16) =255a(17) =63a(18) =262143a(19) =30a(20) =63a(21) =1023a(22) =2047a(23) =12a(24) =1048575a(25) =4095a(26) =262143a(27) =14a(28) =268435455a(29) =15