Number of symmetry sites in all planted 1,3-trees with 2n nodes.

Terms

a(0) =0a(1) =1a(2) =1a(3) =4a(4) =6a(5) =15a(6) =30a(7) =74a(8) =160a(9) =379a(10) =867a(11) =2057a(12) =4817a(13) =11465a(14) =27214a(15) =65102a(16) =155753a(17) =374208a(18) =900073a(19) =2170500a(20) =5240723a(21) =12676162a(22) =30697119a(23) =74435204a(24) =180679171