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

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =7a(5) =15a(6) =44a(7) =128a(8) =415a(9) =1367a(10) =4654a(11) =16080a(12) =56450a(13) =200170a(14) =716728a(15) =2585850a(16) =9393119a(17) =34319667a(18) =126047906a(19) =465076160a(20) =1723097066a(21) =6407856892a(22) =23910271224a(23) =89493903438a(24) =335912741682