Number of bracelets (turnover necklaces) of n beads of 4 colors.

Terms

a(0) =4a(1) =10a(2) =20a(3) =55a(4) =136a(5) =430a(6) =1300a(7) =4435a(8) =15084a(9) =53764a(10) =192700a(11) =704370a(12) =2589304a(13) =9608050a(14) =35824240a(15) =134301715a(16) =505421344a(17) =1909209550a(18) =7234153420a(19) =27489127708a(20) =104717491064a(21) =399827748310