Number of imprimitive (periodic) 2n-bead black-white reversible necklaces with n black beads.

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =2a(5) =1a(6) =4a(7) =1a(8) =8a(9) =3a(10) =17a(11) =1a(12) =56a(13) =1a(14) =134a(15) =18a(16) =440a(17) =1a(18) =1434a(19) =1a(20) =4758a(21) =135a(22) =16160a(23) =1a(24) =57254a(25) =16a(26) =200475a(27) =1387a(28) =718152a(29) =1