Number of n-bead necklace structures using exactly two different colored beads.

Terms

a(0) =0a(1) =1a(2) =1a(3) =3a(4) =3a(5) =7a(6) =9a(7) =19a(8) =29a(9) =55a(10) =93a(11) =179a(12) =315a(13) =595a(14) =1095a(15) =2067a(16) =3855a(17) =7315a(18) =13797a(19) =26271a(20) =49939a(21) =95419a(22) =182361a(23) =349715a(24) =671091a(25) =1290871a(26) =2485533a(27) =4794087a(28) =9256395a(29) =17896831