Number of unlabeled 2-trees with n nodes.

Terms

a(0) =0a(1) =1a(2) =1a(3) =1a(4) =2a(5) =5a(6) =12a(7) =39a(8) =136a(9) =529a(10) =2171a(11) =9368a(12) =41534a(13) =188942a(14) =874906a(15) =4115060a(16) =19602156a(17) =94419351a(18) =459183768a(19) =2252217207a(20) =11130545494a(21) =55382155396a(22) =277255622646