a(n) = T(2*n, n), where T is given by A026519.

Terms

a(0) =1a(1) =1a(2) =5a(3) =16a(4) =65a(5) =251a(6) =1016a(7) =4117a(8) =16913a(9) =69865a(10) =290455a(11) =1212905a(12) =5085224a(13) =21389824a(14) =90226449a(15) =381519416a(16) =1616684241a(17) =6863544233a(18) =29187402749a(19) =124305180842a(20) =530108333515