Number of necklaces with n black and n white beads that avoid the pattern BBBB.

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =21a(5) =59a(6) =162a(7) =483a(8) =1453a(9) =4492a(10) =14048a(11) =44616a(12) =142936a(13) =462380a(14) =1506534a(15) =4941955a(16) =16303783a(17) =54066560a(18) =180111432a(19) =602481589a(20) =2022797529a(21) =6814336663a(22) =23026359634a(23) =78027238290a(24) =265085903996a(25) =902733374480