--- /dev/null
+(**************************************************************************)
+(* ___ *)
+(* ||M|| *)
+(* ||A|| A project by Andrea Asperti *)
+(* ||T|| *)
+(* ||I|| Developers: *)
+(* ||T|| A.Asperti, C.Sacerdoti Coen, *)
+(* ||A|| E.Tassi, S.Zacchiroli *)
+(* \ / *)
+(* \ / Matita is distributed under the terms of the *)
+(* v GNU Lesser General Public License Version 2.1 *)
+(* *)
+(**************************************************************************)
+
+set "baseuri" "cic:/matita/nat/factorial".
+
+include "nat/lt_arith.ma".
+
+let rec fact n \def
+ match n with
+ [ O \Rightarrow (S O)
+ | (S m) \Rightarrow (S m)*(fact m)].
+
+theorem le_SO_fact : \forall n. (S O) \le (fact n).
+intro.elim n.simplify.apply le_n.
+change with (S O) \le (S n1)*(fact n1).
+apply trans_le ? ((S n1)*(S O)).simplify.
+apply le_S_S.apply le_O_n.
+apply le_times_r.assumption.
+qed.
+
+theorem le_SSO_fact : \forall n. (S O) < n \to (S(S O)) \le (fact n).
+intro.apply nat_case n.intro.apply False_ind.apply not_le_Sn_O (S O) H.
+intros.change with (S (S O)) \le (S m)*(fact m).
+apply trans_le ? ((S(S O))*(S O)).apply le_n.
+apply le_times.exact H.apply le_SO_fact.
+qed.
+
+theorem le_n_fact_n: \forall n. n \le (fact n).
+intro. elim n.apply le_O_n.
+change with S n1 \le (S n1)*(fact n1).
+apply trans_le ? ((S n1)*(S O)).
+rewrite < times_n_SO.apply le_n.
+apply le_times.apply le_n.
+apply le_SO_fact.
+qed.
+
+theorem lt_n_fact_n: \forall n. (S(S O)) < n \to n < (fact n).
+intro.apply nat_case n.intro.apply False_ind.apply not_le_Sn_O (S(S O)) H.
+intros.change with (S m) < (S m)*(fact m).
+apply lt_to_le_to_lt ? ((S m)*(S (S O))).
+rewrite < sym_times.
+simplify.
+apply le_S_S.rewrite < plus_n_O.
+apply le_plus_n.
+apply le_times_r.apply le_SSO_fact.
+simplify.apply le_S_S_to_le.exact H.
+qed.
+