Number of bracelet structures using exactly three different colored beads.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =5a(5) =14a(6) =31a(7) =82a(8) =202a(9) =538a(10) =1401a(11) =3838a(12) =10395a(13) =28890a(14) =80207a(15) =225368a(16) =634265a(17) =1796648a(18) =5100325a(19) =14535298a(20) =41513434a(21) =118880650a(22) =341094843a(23) =980665898a(24) =2824223495a(25) =8146908210a(26) =23535345372a(27) =68084937912a(28) =197211483155a(29) =571915789978