Number of double-closed subsets of {1..n}.

Terms

a(0) =1a(1) =2a(2) =3a(3) =6a(4) =8a(5) =16a(6) =24a(7) =48a(8) =60a(9) =120a(10) =180a(11) =360a(12) =480a(13) =960a(14) =1440a(15) =2880a(16) =3456a(17) =6912a(18) =10368a(19) =20736a(20) =27648a(21) =55296a(22) =82944a(23) =165888a(24) =207360a(25) =414720a(26) =622080a(27) =1244160a(28) =1658880a(29) =3317760