Number of identity bracelets of n beads of 4 colors.

Terms

a(0) =4a(1) =6a(2) =4a(3) =15a(4) =72a(5) =266a(6) =1044a(7) =3780a(8) =14056a(9) =51132a(10) =188604a(11) =693845a(12) =2572920a(13) =9566046a(14) =35758628a(15) =134134080a(16) =505159200a(17) =1908539864a(18) =7233104844a(19) =27486455049a(20) =104713295712a(21) =399817073946