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diff --git a/helm/matita/library/nat/factorial.ma b/helm/matita/library/nat/factorial.ma
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+(**************************************************************************)
+(*       ___	                                                            *)
+(*      ||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.
+