a(n) = T(2n,n-1), T given by A026769.

Terms

a(0) =1a(1) =6a(2) =31a(3) =156a(4) =784a(5) =3962a(6) =20173a(7) =103522a(8) =535294a(9) =2787700a(10) =14613710a(11) =77072816a(12) =408737760a(13) =2178631156a(14) =11666175215a(15) =62734622764a(16) =338660977020