Number of, not necessarily connected, regular simple graphs on n vertices with girth exactly 6.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =1a(7) =0a(8) =0a(9) =0a(10) =0a(11) =0a(12) =1a(13) =1a(14) =2a(15) =1a(16) =2a(17) =1a(18) =7a(19) =2a(20) =35a(21) =3a(22) =389a(23) =4a(24) =7579a(25) =6a(26) =181233a(27) =9a(28) =4624493a(29) =12