a(n) = 2*(n-1)*Catalan(n).

Terms

a(0) =0a(1) =4a(2) =20a(3) =84a(4) =336a(5) =1320a(6) =5148a(7) =20020a(8) =77792a(9) =302328a(10) =1175720a(11) =4576264a(12) =17829600a(13) =69535440a(14) =271455660a(15) =1060730100a(16) =4148633280a(17) =16239715800a(18) =63621474840a(19) =249436575960a(20) =978650680800