--- /dev/null
+(**************************************************************************)
+(* __ *)
+(* ||M|| *)
+(* ||A|| A project by Andrea Asperti *)
+(* ||T|| *)
+(* ||I|| Developers: *)
+(* ||T|| A.Asperti, C.Sacerdoti Coen, *)
+(* ||A|| E.Tassi, S.Zacchiroli *)
+(* \ / *)
+(* \ / This file is distributed under the terms of the *)
+(* v GNU Lesser General Public License Version 2.1 *)
+(* *)
+(**************************************************************************)
+
+set "baseuri" "cic:/matita/nat/totient".
+
+include "nat/count.ma".
+include "nat/chinese_reminder.ma".
+
+definition totient : nat \to nat \def
+\lambda n. count n (\lambda m. eqb (gcd m n) (S O)).
+
+theorem totient3: totient (S(S(S O))) = (S(S O)).
+reflexivity.
+qed.
+
+theorem totient6: totient (S(S(S(S(S(S O)))))) = (S(S O)).
+reflexivity.
+qed.
+
+theorem totient_times: \forall n,m:nat. (gcd m n) = (S O) \to
+totient (n*m) = (totient n)*(totient m).
+intro.
+apply (nat_case n).
+intro.simplify.intro.reflexivity.
+intros 2.apply (nat_case m1).
+rewrite < sym_times.
+rewrite < (sym_times (totient O)).
+simplify.intro.reflexivity.
+intros.
+unfold totient.
+apply (count_times m m2 ? ? ?
+(\lambda b,a. cr_pair (S m) (S m2) a b) (\lambda x. x \mod (S m)) (\lambda x. x \mod (S m2))).
+intros.unfold cr_pair.
+apply (le_to_lt_to_lt ? (pred ((S m)*(S m2)))).
+unfold min.
+apply le_min_aux_r.
+change with ((S (pred ((S m)*(S m2)))) \le ((S m)*(S m2))).
+apply (nat_case ((S m)*(S m2))).apply le_n.
+intro.apply le_n.
+intros.
+generalize in match (mod_cr_pair (S m) (S m2) a b H1 H2 H).
+intro.elim H3.
+apply H4.
+intros.
+generalize in match (mod_cr_pair (S m) (S m2) a b H1 H2 H).
+intro.elim H3.
+apply H5.
+intros.
+generalize in match (mod_cr_pair (S m) (S m2) a b H1 H2 H).
+intro.elim H3.
+apply eqb_elim.
+intro.
+rewrite > eq_to_eqb_true.
+rewrite > eq_to_eqb_true.
+reflexivity.
+rewrite < H4.
+rewrite > sym_gcd.
+rewrite > gcd_mod.
+apply (gcd_times_SO_to_gcd_SO ? ? (S m2)).
+unfold lt.apply le_S_S.apply le_O_n.
+unfold lt.apply le_S_S.apply le_O_n.
+assumption.
+unfold lt.apply le_S_S.apply le_O_n.
+rewrite < H5.
+rewrite > sym_gcd.
+rewrite > gcd_mod.
+apply (gcd_times_SO_to_gcd_SO ? ? (S m)).
+unfold lt.apply le_S_S.apply le_O_n.
+unfold lt.apply le_S_S.apply le_O_n.
+rewrite > sym_times.
+assumption.
+unfold lt.apply le_S_S.apply le_O_n.
+intro.
+apply eqb_elim.
+intro.apply eqb_elim.
+intro.apply False_ind.
+apply H6.
+apply eq_gcd_times_SO.
+unfold lt.apply le_S_S.apply le_O_n.
+unfold lt.apply le_S_S.apply le_O_n.
+rewrite < gcd_mod.
+rewrite > H4.
+rewrite > sym_gcd.assumption.
+unfold lt.apply le_S_S.apply le_O_n.
+rewrite < gcd_mod.
+rewrite > H5.
+rewrite > sym_gcd.assumption.
+unfold lt.apply le_S_S.apply le_O_n.
+intro.reflexivity.
+intro.reflexivity.
+qed.
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