Number of 6-bead necklaces labeled with numbers -n..n allowing reversal, with sum zero.

Terms

a(0) =18a(1) =167a(2) =828a(3) =2821a(4) =7582a(5) =17339a(6) =35288a(7) =65769a(8) =114442a(9) =188463a(10) =296660a(11) =449709a(12) =660310a(13) =943363a(14) =1316144a(15) =1798481a(16) =2412930a(17) =3184951a(18) =4143084a(19) =5319125a(20) =6748302a(21) =8469451a(22) =10525192a(23) =12962105a(24) =15830906a(25) =19186623a(26) =23088772a(27) =27601533a(28) =32793926