Number of 2-multiantichains of an n-set.

Terms

a(0) =1a(1) =2a(2) =5a(3) =17a(4) =71a(5) =317a(6) =1415a(7) =6197a(8) =26591a(9) =112157a(10) =466775a(11) =1923077a(12) =7863311a(13) =31972397a(14) =129459335a(15) =522571157a(16) =2104535231a(17) =8460991037a(18) =33972711095a(19) =136277478437a(20) =546270602351