a(n) = 3^n*(2*n + 1)!/n!.A254381a(n) = 3^n*(2*n + 1)!/n!.Terms a(0) =1a(1) =18a(2) =540a(3) =22680a(4) =1224720a(5) =80831520a(6) =6304858560a(7) =567437270400External referencesoeis: A254381