Ordered trees with branches of length at most 3.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =13a(5) =39a(6) =121a(7) =388a(8) =1277a(9) =4288a(10) =14630a(11) =50575a(12) =176762a(13) =623563a(14) =2217379a(15) =7939821a(16) =28603591a(17) =103600632a(18) =377033451a(19) =1378023887a(20) =5056021292a(21) =18615654196a(22) =68758804039a(23) =254706453524a(24) =946038872000