Difference sequence of A020991.

Terms

a(0) =3a(1) =3a(2) =9a(3) =11a(4) =1a(5) =3a(6) =33a(7) =43a(8) =1a(9) =3a(10) =1a(11) =11a(12) =1a(13) =3a(14) =129a(15) =171a(16) =1a(17) =3a(18) =1a(19) =11a(20) =1a(21) =3a(22) =1a(23) =43a(24) =1a(25) =3a(26) =1a(27) =11a(28) =1a(29) =3