Number of 4-covers of an unlabeled n-set.

Terms

a(0) =1a(1) =4a(2) =17a(3) =65a(4) =230a(5) =736a(6) =2197a(7) =6093a(8) =15864a(9) =38960a(10) =90837a(11) =202005a(12) =430577a(13) =883057a(14) =1748909a(15) =3355213a(16) =6252575a(17) =11345602a(18) =20089514a(19) =34778306a(20) =58964020a(21) =98053576a(22) =160151566a(23) =257229974a(24) =406739271a(25) =633795181a(26) =974126408a(27) =1477999320a(28) =2215409037a(29) =3282874359