Catalan numbers - 1.

Terms

a(0) =1a(1) =4a(2) =13a(3) =41a(4) =131a(5) =428a(6) =1429a(7) =4861a(8) =16795a(9) =58785a(10) =208011a(11) =742899a(12) =2674439a(13) =9694844a(14) =35357669a(15) =129644789a(16) =477638699a(17) =1767263189a(18) =6564120419a(19) =24466267019a(20) =91482563639a(21) =343059613649