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

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =7a(5) =13a(6) =35a(7) =85a(8) =257a(9) =765a(10) =2518a(11) =8359a(12) =28968a(13) =101340a(14) =361270a(15) =1297879a(16) =4707969a(17) =17179435a(18) =63068876a(19) =232615771a(20) =861725794a(21) =3204236779a(22) =11955836258a(23) =44748176653a(24) =167959144032a(25) =632058070310