Number of mappings from n points to themselves with in-degree <= 2.

Terms

a(0) =1a(1) =1a(2) =3a(3) =6a(4) =15a(5) =31a(6) =75a(7) =164a(8) =388a(9) =887a(10) =2092a(11) =4884a(12) =11599a(13) =27443a(14) =65509a(15) =156427a(16) =375263a(17) =901353a(18) =2171313a(19) =5237581a(20) =12658815a(21) =30633725a(22) =74238228a(23) =180106656a(24) =437437445a(25) =1063425655a(26) =2587564434a(27) =6301175326a(28) =15356071604a(29) =37448674536