a(n) = A026615(2*n-1, n-1).

Terms

a(0) =1a(1) =5a(2) =17a(3) =60a(4) =217a(5) =798a(6) =2970a(7) =11154a(8) =42185a(9) =160446a(10) =613054a(11) =2351440a(12) =9048522a(13) =34916300a(14) =135059220a(15) =523521630a(16) =2033066025a(17) =7908332190a(18) =30807696150a(19) =120173896920a(20) =469334610030