a(n) = T(2n,n-1), T given by A026648.

Terms

a(0) =1a(1) =6a(2) =21a(3) =108a(4) =370a(5) =1908a(6) =6587a(7) =34248a(8) =119142a(9) =623700a(10) =2182818a(11) =11487528a(12) =40391988a(13) =213451560a(14) =753295635a(15) =3993910416a(16) =14136717574a(17) =75152083716a(18) =266650874678