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

Terms

a(0) =0a(1) =0a(2) =0a(3) =6a(4) =48a(5) =260a(6) =1200a(7) =5106a(8) =20720a(9) =81876a(10) =318000a(11) =1223136a(12) =4675440a(13) =17815020a(14) =67769552a(15) =257700906a(16) =980240880a(17) =3731753180a(18) =14222737200a(19) =54278580036a(20) =207438938000a(21) =793940475900