The number of single edges on the boundary of ordered trees with n edges.

Terms

a(0) =0a(1) =1a(2) =2a(3) =6a(4) =19a(5) =61a(6) =199a(7) =661a(8) =2234a(9) =7668a(10) =26674a(11) =93858a(12) =333524a(13) =1195288a(14) =4315468a(15) =15681838a(16) =57312643a(17) =210529213a(18) =776872243a(19) =2878482523a(20) =10704933793a(21) =39945106573a(22) =149511432793a(23) =561182969173