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diff --git a/helm/matita/library/nat/nth_prime.ma b/helm/matita/library/nat/nth_prime.ma
index 200ccba5a..f165705ac 100644
--- a/helm/matita/library/nat/nth_prime.ma
+++ b/helm/matita/library/nat/nth_prime.ma
@@ -39,11 +39,11 @@ normalize.reflexivity.
 qed. *)
 
 theorem smallest_factor_fact: \forall n:nat.
-n < smallest_factor (S (fact n)).
+n < smallest_factor (S (n !)).
 intros.
 apply not_le_to_lt.
-change with smallest_factor (S (fact n)) \le n \to False.intro.
-apply not_divides_S_fact n (smallest_factor(S (fact n))).
+change with smallest_factor (S (n !)) \le n \to False.intro.
+apply not_divides_S_fact n (smallest_factor(S (n !))).
 apply lt_SO_smallest_factor.
 simplify.apply le_S_S.apply le_SO_fact.
 assumption.
@@ -52,19 +52,19 @@ simplify.apply le_S_S.apply le_O_n.
 qed.
 
 theorem ex_prime: \forall n. (S O) \le n \to \exists m.
-n < m \land m \le (S (fact n)) \land (prime m).
+n < m \land m \le (S (n !)) \land (prime m).
 intros.
 elim H.
 apply ex_intro nat ? (S(S O)).
 split.split.apply le_n (S(S O)).
 apply le_n (S(S O)).apply primeb_to_Prop (S(S O)).
-apply ex_intro nat ? (smallest_factor (S (fact (S n1)))).
+apply ex_intro nat ? (smallest_factor (S ((S n1) !))).
 split.split.
 apply smallest_factor_fact.
 apply le_smallest_factor_n.
 (* Andrea: ancora hint non lo trova *)
 apply prime_smallest_factor_n.
-change with (S(S O)) \le S (fact (S n1)).
+change with (S(S O)) \le S ((S n1) !).
 apply le_S.apply le_SSO_fact.
 simplify.apply le_S_S.assumption.
 qed.
@@ -74,7 +74,7 @@ match n with
   [ O \Rightarrow (S(S O))
   | (S p) \Rightarrow
     let previous_prime \def (nth_prime p) in
-    let upper_bound \def S (fact previous_prime) in
+    let upper_bound \def S (previous_prime !) in
     min_aux (upper_bound - (S previous_prime)) upper_bound primeb].
     
 (* it works, but nth_prime 4 takes already a few minutes -
@@ -100,13 +100,13 @@ apply primeb_to_Prop (S(S O)).
 intro.
 change with
 let previous_prime \def (nth_prime m) in
-let upper_bound \def S (fact previous_prime) in
+let upper_bound \def S (previous_prime !) in
 prime (min_aux (upper_bound - (S previous_prime)) upper_bound primeb).
 apply primeb_true_to_prime.
 apply f_min_aux_true.
-apply ex_intro nat ? (smallest_factor (S (fact (nth_prime m)))).
+apply ex_intro nat ? (smallest_factor (S ((nth_prime m) !))).
 split.split.
-cut S (fact (nth_prime m))-(S (fact (nth_prime m)) - (S (nth_prime m))) = (S (nth_prime m)).
+cut S ((nth_prime m) !)-(S ((nth_prime m) !) - (S (nth_prime m))) = (S (nth_prime m)).
 rewrite > Hcut.exact smallest_factor_fact (nth_prime m).
 (* maybe we could factorize this proof *)
 apply plus_to_minus.
@@ -117,7 +117,7 @@ apply le_n_fact_n.
 apply le_smallest_factor_n.
 apply prime_to_primeb_true.
 apply prime_smallest_factor_n.
-change with (S(S O)) \le S (fact (nth_prime m)).
+change with (S(S O)) \le S ((nth_prime m) !).
 apply le_S_S.apply le_SO_fact.
 qed.
 
@@ -127,7 +127,7 @@ change with \forall n:nat. (nth_prime n) < (nth_prime (S n)).
 intros.
 change with
 let previous_prime \def (nth_prime n) in
-let upper_bound \def S (fact previous_prime) in
+let upper_bound \def S (previous_prime !) in
 (S previous_prime) \le min_aux (upper_bound - (S previous_prime)) upper_bound primeb.
 intros.
 cut upper_bound - (upper_bound -(S previous_prime)) = (S previous_prime).
@@ -172,9 +172,9 @@ theorem lt_nth_prime_to_not_prime: \forall n,m. nth_prime n < m \to m < nth_prim
 intros.
 apply primeb_false_to_not_prime.
 letin previous_prime \def nth_prime n.
-letin upper_bound \def S (fact previous_prime).
+letin upper_bound \def S (previous_prime !).
 apply lt_min_aux_to_false primeb upper_bound (upper_bound - (S previous_prime)) m.
-cut S (fact (nth_prime n))-(S (fact (nth_prime n)) - (S (nth_prime n))) = (S (nth_prime n)).
+cut S ((nth_prime n) !)-(S ((nth_prime n) !) - (S (nth_prime n))) = (S (nth_prime n)).
 rewrite > Hcut.assumption.
 apply plus_to_minus.
 apply le_minus_m.