Number of n-bead necklaces using exactly three colors with no adjacent beads having the same color.

Terms

a(0) =0a(1) =0a(2) =2a(3) =3a(4) =6a(5) =11a(6) =18a(7) =33a(8) =58a(9) =105a(10) =186a(11) =349a(12) =630a(13) =1179a(14) =2190a(15) =4113a(16) =7710a(17) =14599a(18) =27594a(19) =52485a(20) =99878a(21) =190743a(22) =364722a(23) =699249a(24) =1342182a(25) =2581425a(26) =4971066a(27) =9587577a(28) =18512790a(29) =35792565