Number of directed trees with n nodes.

Terms

a(0) =1a(1) =2a(2) =6a(3) =25a(4) =114a(5) =591a(6) =3298a(7) =19532a(8) =120687a(9) =771373a(10) =5061741a(11) =33943662a(12) =231751331a(13) =1606587482a(14) =11283944502a(15) =80157645245a(16) =575105238243