a(n) = 2^phi(n) - 1 = A066781(n) - 1.

Terms

a(0) =1a(1) =1a(2) =3a(3) =3a(4) =15a(5) =3a(6) =63a(7) =15a(8) =63a(9) =15a(10) =1023a(11) =15a(12) =4095a(13) =63a(14) =255a(15) =255a(16) =65535a(17) =63a(18) =262143a(19) =255a(20) =4095a(21) =1023a(22) =4194303a(23) =255a(24) =1048575a(25) =4095a(26) =262143a(27) =4095a(28) =268435455a(29) =255