Number of n-bead necklaces with exactly three different colored beads.

Terms

a(0) =0a(1) =0a(2) =2a(3) =9a(4) =30a(5) =91a(6) =258a(7) =729a(8) =2018a(9) =5613a(10) =15546a(11) =43315a(12) =120750a(13) =338259a(14) =950062a(15) =2678499a(16) =7573350a(17) =21480739a(18) =61088874a(19) =174184755a(20) =497812638a(21) =1425847623a(22) =4092087522a(23) =11765822365a(24) =33887517870a(25) =97756387365a(26) =282414624746a(27) =816999710223