Number of disconnected regular simple graphs on n vertices with girth exactly 4.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =0a(8) =1a(9) =1a(10) =1a(11) =1a(12) =3a(13) =2a(14) =5a(15) =3a(16) =15a(17) =5a(18) =44a(19) =10a(20) =203a(21) =47a(22) =1415a(23) =1710a(24) =27771a(25) =197951a(26) =2613710a(27) =33117920a(28) =463707092a(29) =6709514282