Number of bipartite graphs with 4 edges on nodes {1..n}.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =3a(5) =140a(6) =1125a(7) =5355a(8) =19075a(9) =56133a(10) =143955a(11) =332475a(12) =706860a(13) =1404975a(14) =2640638a(15) =4733820a(16) =8149050a(17) =13543390a(18) =21825450a(19) =34227018a(20) =52388985a(21) =78463350a(22) =115233195a(23) =166252625a(24) =236008773a(25) =330108075a(26) =455489125a(27) =620664525a(28) =835994250