a(n) = 3^n*n!*(n+2)!/2!.A064633a(n) = 3^n*n!*(n+2)!/2!.Terms a(0) =1a(1) =9a(2) =216a(3) =9720a(4) =699840a(5) =73483200a(6) =10581580800External referencesoeis: A064633