Number of 2n-bead black-white reversible necklaces with n black beads.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =8a(5) =16a(6) =50a(7) =133a(8) =440a(9) =1387a(10) =4752a(11) =16159a(12) =56822a(13) =200474a(14) =718146a(15) =2587018a(16) =9398520a(17) =34324174a(18) =126068558a(19) =465093571a(20) =1723176308a(21) =6407924300a(22) =23910576230a(23) =89494164973a(24) =335913918902