Number of 4-element ordered antichain covers of an unlabeled n-element set.

Terms

a(0) =25a(1) =429a(2) =3364a(3) =17602a(4) =71385a(5) =242347a(6) =720792a(7) =1934076a(8) =4777337a(9) =11021713a(10) =24008532a(11) =49790614a(12) =98954626a(13) =189457350a(14) =350941064a(15) =631167840a(16) =1105440045a(17) =1890167329a(18) =3162113836a(19) =5185330818a(20) =8348369731a(21) =13215102985a(22) =20593381200a(23) =31626858540a(24) =47916657405a(25) =71681161365