a(n) = T(2n,n-1), T given by A026747.

Terms

a(0) =1a(1) =6a(2) =30a(3) =143a(4) =671a(5) =3132a(6) =14601a(7) =68101a(8) =318035a(9) =1487661a(10) =6971222a(11) =32727472a(12) =153926409a(13) =725264305a(14) =3423262180a(15) =16185240446a(16) =76648901377a(17) =363557014067