Number of n-bead necklaces labeled with numbers -1..1 allowing reversal, with sum zero.

Terms

a(0) =1a(1) =2a(2) =2a(3) =5a(4) =7a(5) =18a(6) =32a(7) =84a(8) =185a(9) =486a(10) =1192a(11) =3179a(12) =8261a(13) =22290a(14) =59788a(15) =163181a(16) =445699a(17) =1228686a(18) =3396224a(19) =9440759a(20) =26322371a(21) =73680058a(22) =206822134a(23) =582308320a(24) =1643573625a(25) =4650401810a(26) =13186590962a(27) =37469003353