a(n) = T(2n, n), T given by A026758.

Terms

a(0) =1a(1) =2a(2) =7a(3) =27a(4) =109a(5) =453a(6) =1922a(7) =8284a(8) =36155a(9) =159435a(10) =709246a(11) =3178992a(12) =14343567a(13) =65099245a(14) =297015765a(15) =1361584755a(16) =6268757195a(17) =28975155915a(18) =134410918700a(19) =625578384150