Number of trees on n vertices rooted at an oriented non-edge.

Terms

a(0) =0a(1) =0a(2) =1a(3) =4a(4) =14a(5) =45a(6) =140a(7) =424a(8) =1269a(9) =3760a(10) =11080a(11) =32517a(12) =95190a(13) =278154a(14) =811887a(15) =2367973a(16) =6903453a(17) =20120905a(18) =58639016a(19) =170894228a(20) =498084608a(21) =1451899005a(22) =4232957241a(23) =12343454790a(24) =36001675800a(25) =105028397290a(26) =306472665459a(27) =894497511566