Vertex diagrams of order 2n.

Terms

a(0) =1a(1) =1a(2) =6a(3) =50a(4) =518a(5) =6354a(6) =89782a(7) =1435330a(8) =25625910a(9) =505785122a(10) =10944711398a(11) =257834384850