Number of bracelet structures using a maximum of four different colored beads.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =7a(5) =11a(6) =33a(7) =73a(8) =237a(9) =703a(10) =2433a(11) =8309a(12) =30108a(13) =108991a(14) =403262a(15) =1497070a(16) =5607437a(17) =21076571a(18) =79595990a(19) =301492045a(20) =1145560579a(21) =4363503684a(22) =16660204452a(23) =63741248201a(24) =244339646708a(25) =938255682551