Number of unlabeled 3-trees on n vertices.

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =1a(5) =2a(6) =5a(7) =15a(8) =58a(9) =275a(10) =1505a(11) =9003a(12) =56931a(13) =372973a(14) =2506312a(15) =17165954a(16) =119398333a(17) =841244274a(18) =5993093551a(19) =43109340222a(20) =312747109787