Number of 2-trees rooted at any symmetric edge.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =3a(5) =6a(6) =10a(7) =21a(8) =38a(9) =84a(10) =156a(11) =352a(12) =677a(13) =1562a(14) =3054a(15) =7148a(16) =14196a(17) =33603a(18) =67474a(19) =161121a(20) =326519a(21) =785305a(22) =1603111a(23) =3878194a(24) =7965845a(25) =19365811a(26) =39983300a(27) =97610247a(28) =202424674a(29) =495955345