G.f. is 1 + x*c(x), where c(x) is the g.f. of the Catalan numbers (A000108).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =5a(5) =14a(6) =42a(7) =132a(8) =429a(9) =1430a(10) =4862a(11) =16796a(12) =58786a(13) =208012a(14) =742900a(15) =2674440a(16) =9694845a(17) =35357670a(18) =129644790a(19) =477638700a(20) =1767263190a(21) =6564120420a(22) =24466267020a(23) =91482563640a(24) =343059613650