a(n) = T(2n,n), T given by A026725.

Terms

a(0) =1a(1) =2a(2) =7a(3) =27a(4) =108a(5) =440a(6) =1812a(7) =7514a(8) =31307a(9) =130883a(10) =548547a(11) =2303413a(12) =9686617a(13) =40783083a(14) =171868037a(15) =724837891a(16) =3058850316a(17) =12915186640a(18) =54554594416a(19) =230526280814a(20) =974414815782