Number of at most 4-way branching ordered (i.e., plane) trees.

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =70a(5) =246a(6) =875a(7) =3144a(8) =11385a(9) =41470a(10) =151778a(11) =557712a(12) =2056210a(13) =7602700a(14) =28180050a(15) =104677280a(16) =389571983a(17) =1452293766a(18) =5422187130a(19) =20271296100a(20) =75878518695a(21) =284339792110