a(n) = T(4*n,n), where T = Catalan triangle (A008315).

A026005

a(n) = T(4*n,n), where T = Catalan triangle (A008315).

Terms

    a(0) =1a(1) =4a(2) =27a(3) =208a(4) =1700a(5) =14364a(6) =123970a(7) =1085760a(8) =9612108a(9) =85795600a(10) =770755843a(11) =6960408624a(12) =63127818572a(13) =574609830760

External references