a(n) = T(2*n, n), where T is given by A026552.

Terms

a(0) =1a(1) =2a(2) =7a(3) =24a(4) =93a(5) =362a(6) =1452a(7) =5880a(8) =24089a(9) =99386a(10) =412637a(11) =1721500a(12) =7211536a(13) =30312960a(14) =127790379a(15) =540082784a(16) =2287577537a(17) =9707988994a(18) =41269156159a(19) =175705272784a(20) =749099069183