Number of symmetry sites in all planted 3-trees with n nodes.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =1a(5) =3a(6) =5a(7) =14a(8) =27a(9) =65a(10) =142a(11) =338a(12) =773a(13) =1832a(14) =4296a(15) =10231a(16) =24296a(17) =58128a(18) =139132a(19) =334350a(20) =804441a(21) =1940239a(22) =4685806a(23) =11335797a(24) =27455949a(25) =66585170a(26) =161646826