a(n) = T(2*n, n), where T is given by A026584.

Terms

a(0) =1a(1) =1a(2) =5a(3) =19a(4) =69a(5) =341a(6) =1203a(7) =6336a(8) =22593a(9) =121483a(10) =438533a(11) =2381512a(12) =8677763a(13) =47419503a(14) =173984792a(15) =954961034a(16) =3522101709a(17) =19397198595a(18) =71831252031a(19) =396646918211