Birooted trees: number of unlabeled trees with n nodes rooted at 2 indistinguishable roots.

Terms

a(0) =0a(1) =1a(2) =2a(3) =6a(4) =15a(5) =43a(6) =116a(7) =329a(8) =918a(9) =2609a(10) =7391a(11) =21099a(12) =60248a(13) =172683a(14) =495509a(15) =1424937a(16) =4102693a(17) =11830006a(18) =34148859a(19) =98686001a(20) =285459772a(21) =826473782a(22) =2394774727a(23) =6944343654a(24) =20151175658a(25) =58513084011a(26) =170007600051a(27) =494230862633