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

Terms

a(0) =1a(1) =9a(2) =51a(3) =230a(4) =863a(5) =2864a(6) =8609a(7) =23883a(8) =61883a(9) =151214a(10) =350929a(11) =778113a(12) =1656265a(13) =3398229a(14) =6743791a(15) =12983181a(16) =24311044a(17) =44377016a(18) =79124476a(19) =138048542a(20) =236050912a(21) =396137492a(22) =653285736a(23) =1059923072a(24) =1693592112a(25) =2667563553a(26) =4145373780a(27) =6360553548a(28) =9643151582