Number of necklaces with n beads of 4 colors, no 2 adjacent beads the same color.

Terms

a(0) =4a(1) =6a(2) =8a(3) =24a(4) =48a(5) =130a(6) =312a(7) =834a(8) =2192a(9) =5934a(10) =16104a(11) =44368a(12) =122640a(13) =341802a(14) =956632a(15) =2690844a(16) =7596480a(17) =21524542a(18) =61171656a(19) =174342216a(20) =498112272a(21) =1426419858a(22) =4093181688a(23) =11767920118a(24) =33891544416a(25) =97764131646a(26) =282429537944