Number of ordered 2-multiantichains on an n-set.

Terms

a(0) =1a(1) =2a(2) =6a(3) =26a(4) =126a(5) =602a(6) =2766a(7) =12266a(8) =52926a(9) =223802a(10) =932526a(11) =3844106a(12) =15722526a(13) =63936602a(14) =258902286a(15) =1045109546a(16) =4209004926a(17) =16921851002a(18) =67945160046a(19) =272554432586