Number of bracelet structures using exactly four different colored beads.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =2a(5) =11a(6) =33a(7) =137a(8) =478a(9) =1851a(10) =6845a(11) =26148a(12) =98406a(13) =374010a(14) =1416251a(15) =5380907a(16) =20440250a(17) =77795428a(18) =296384565a(19) =1131011633a(20) =4321964768a(21) =16541275068a(22) =63400061153a(23) =243358803904a(24) =935431121462