a(n) = 6^(n-1) * n! * Catalan(n-1).A221955a(n) = 6^(n-1) * n! * Catalan(n-1).Terms a(0) =1a(1) =12a(2) =432a(3) =25920a(4) =2177280a(5) =235146240a(6) =31039303680External referencesoeis: A221955