Number of bracelet structures using exactly two different colored beads.

Terms

a(0) =0a(1) =1a(2) =1a(3) =3a(4) =3a(5) =7a(6) =8a(7) =17a(8) =22a(9) =43a(10) =62a(11) =121a(12) =189a(13) =361a(14) =611a(15) =1161a(16) =2055a(17) =3913a(18) =7154a(19) =13647a(20) =25481a(21) =48733a(22) =92204a(23) =176905a(24) =337593a(25) =649531a(26) =1246862a(27) =2405235a(28) =4636389a(29) =8964799