Number of n-bead necklace structures using exactly three different colored beads.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =5a(5) =18a(6) =43a(7) =126a(8) =339a(9) =946a(10) =2591a(11) =7254a(12) =20125a(13) =56450a(14) =158355a(15) =446618a(16) =1262225a(17) =3580686a(18) =10181479a(19) =29032254a(20) =82968843a(21) =237645250a(22) =682014587a(23) =1960981598a(24) =5647919645a(25) =16292761730a(26) =47069104613a(27) =136166703562a(28) =394418199725