The number of unlabeled trees with n nodes rooted at 3 indistinguishable roots.

Terms

a(0) =0a(1) =0a(2) =1a(3) =4a(4) =15a(5) =51a(6) =175a(7) =573a(8) =1866a(9) =5978a(10) =19000a(11) =59859a(12) =187503a(13) =584012a(14) =1811212a(15) =5595239a(16) =17228943a(17) =52898764a(18) =162013452a(19) =495100454a(20) =1510029296a(21) =4597430832a(22) =13975327501a(23) =42422033217a(24) =128606150706a(25) =389423872694