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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/nth_prime".
+
+include "nat/primes.ma".
+include "nat/lt_arith.ma".
+
+(* upper bound by Bertrand's conjecture. *)
+(* Too difficult to prove.        
+let rec nth_prime n \def
+match n with
+  [ O \Rightarrow (S(S O))
+  | (S p) \Rightarrow
+    let previous_prime \def S (nth_prime p) in
+    min_aux previous_prime ((S(S O))*previous_prime) primeb].
+
+theorem example8 : nth_prime (S(S O)) = (S(S(S(S(S O))))).
+normalize.reflexivity.
+qed.
+
+theorem example9 : nth_prime (S(S(S O))) = (S(S(S(S(S(S(S O))))))).
+normalize.reflexivity.
+qed.
+
+theorem example10 : nth_prime (S(S(S(S O)))) = (S(S(S(S(S(S(S(S(S(S(S O))))))))))).
+normalize.reflexivity.
+qed. *)
+
+theorem smallest_factor_fact: \forall n:nat.
+n < smallest_factor (S n!).
+intros.
+apply not_le_to_lt.
+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.
+unfold lt.apply le_S_S.apply le_SO_fact.
+assumption.
+apply divides_smallest_factor_n.
+unfold lt.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 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 (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 (S n1)!).
+apply le_S.apply le_SSO_fact.
+unfold lt.apply le_S_S.assumption.
+qed.
+
+let rec nth_prime n \def
+match n with
+  [ O \Rightarrow (S(S O))
+  | (S p) \Rightarrow
+    let previous_prime \def (nth_prime p) 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 -
+it must compute factorial of 7 ...
+
+theorem example11 : nth_prime (S(S O)) = (S(S(S(S(S O))))).
+normalize.reflexivity.
+qed.
+
+theorem example12: nth_prime (S(S(S O))) = (S(S(S(S(S(S(S O))))))).
+normalize.reflexivity.
+qed.
+
+theorem example13 : nth_prime (S(S(S(S O)))) = (S(S(S(S(S(S(S(S(S(S(S O))))))))))).
+normalize.reflexivity.
+*) 
+
+theorem prime_nth_prime : \forall n:nat.prime (nth_prime n).
+intro.
+apply (nat_case n).
+change with (prime (S(S O))).
+apply (primeb_to_Prop (S(S O))).
+intro.
+change with
+(let previous_prime \def (nth_prime m) 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 (nth_prime m)!))).
+split.split.
+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.
+apply plus_minus_m_m.
+apply le_S_S.
+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 (nth_prime m)!).
+apply le_S_S.apply le_SO_fact.
+qed.
+
+(* properties of nth_prime *)
+theorem increasing_nth_prime: increasing nth_prime.
+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 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)).
+rewrite < Hcut in \vdash (? % ?).
+apply le_min_aux.
+apply plus_to_minus.
+apply plus_minus_m_m.
+apply le_S_S.
+apply le_n_fact_n.
+qed.
+
+variant lt_nth_prime_n_nth_prime_Sn :\forall n:nat. 
+(nth_prime n) < (nth_prime (S n)) \def increasing_nth_prime.
+
+theorem injective_nth_prime: injective nat nat nth_prime.
+apply increasing_to_injective.
+apply increasing_nth_prime.
+qed.
+
+theorem lt_SO_nth_prime_n : \forall n:nat. (S O) \lt nth_prime n.
+intros. elim n.unfold lt.apply le_n.
+apply (trans_lt ? (nth_prime n1)).
+assumption.apply lt_nth_prime_n_nth_prime_Sn.
+qed.
+
+theorem lt_O_nth_prime_n : \forall n:nat. O \lt nth_prime n.
+intros.apply (trans_lt O (S O)).
+unfold lt. apply le_n.apply lt_SO_nth_prime_n.
+qed.
+
+theorem ex_m_le_n_nth_prime_m: 
+\forall n: nat. nth_prime O \le n \to 
+\exists m. nth_prime m \le n \land n < nth_prime (S m).
+intros.
+apply increasing_to_le2.
+exact lt_nth_prime_n_nth_prime_Sn.assumption.
+qed.
+
+theorem lt_nth_prime_to_not_prime: \forall n,m. nth_prime n < m \to m < nth_prime (S n) 
+\to \lnot (prime m).
+intros.
+apply primeb_false_to_not_prime.
+letin previous_prime \def (nth_prime n).
+letin upper_bound \def (S previous_prime!).
+apply (lt_min_aux_to_false primeb upper_bound (upper_bound - (S previous_prime)) m).
+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 plus_minus_m_m.
+apply le_S_S.
+apply le_n_fact_n.
+assumption.
+qed.
+
+(* nth_prime enumerates all primes *)
+theorem prime_to_nth_prime : \forall p:nat. prime p \to
+\exists i. nth_prime i = p.
+intros.
+cut (\exists m. nth_prime m \le p \land p < nth_prime (S m)).
+elim Hcut.elim H1.
+cut (nth_prime a < p \lor nth_prime a = p).
+elim Hcut1.
+absurd (prime p).
+assumption.
+apply (lt_nth_prime_to_not_prime a).assumption.assumption.
+apply (ex_intro nat ? a).assumption.
+apply le_to_or_lt_eq.assumption.
+apply ex_m_le_n_nth_prime_m.
+simplify.unfold prime in H.elim H.assumption.
+qed.
+