Number of unlabeled forests on n nodes that have exactly two nonempty components.

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =2a(5) =3a(6) =6a(7) =11a(8) =23a(9) =46a(10) =99a(11) =216a(12) =488a(13) =1121a(14) =2644a(15) =6334a(16) =15437a(17) =38132a(18) =95368a(19) =241029a(20) =614968a(21) =1582030a(22) =4100157a(23) =10697038a(24) =28075303a(25) =74086468a(26) =196470902a(27) =523383136a(28) =1400051585a(29) =3759508536