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+(* taken from https://coq.inria.fr/distrib/8.2/contribs/QArithSternBrocot.sqrt2.html *)
+(* Note: this file contains no "Proof" command (invariant to preserve)
+ in order to exercise 070_coq-test-regression-wholefile-no-proof. *)
+
+Require Export ArithRing.
+Require Export Compare_dec.
+Require Export Wf_nat.
+Require Export Arith.
+Require Export Omega.
+
+Theorem minus_minus : forall a b c : nat, a - b - c = a - (b + c).
+intros a; elim a; auto.
+intros n' Hrec b; case b; auto.
+Qed.
+
+Remark expand_mult2 : forall x : nat, 2 * x = x + x.
+intros x; ring.
+Qed.
+
+Theorem lt_neq : forall x y : nat, x < y -> x <> y.
+unfold not in |- *; intros x y H H1; elim (lt_irrefl x);
+ pattern x at 2 in |- *; rewrite H1; auto.
+Qed.
+Hint Resolve lt_neq.
+
+Theorem monotonic_inverse :
+ forall f : nat -> nat,
+ (forall x y : nat, x < y -> f x < f y) ->
+ forall x y : nat, f x < f y -> x < y.
+intros f Hmon x y Hlt; case (le_gt_dec y x); auto.
+intros Hle; elim (le_lt_or_eq _ _ Hle).
+intros Hlt'; elim (lt_asym _ _ Hlt); apply Hmon; auto.
+intros Heq; elim (lt_neq _ _ Hlt); rewrite Heq; auto.
+Qed.
+
+Theorem mult_lt : forall a b c : nat, c <> 0 -> a < b -> a * c < b * c.
+intros a b c; elim c.
+intros H; elim H; auto.
+intros c'; case c'.
+intros; omega.
+intros c'' Hrec Hneq Hlt;
+ repeat rewrite <- (fun x : nat => mult_n_Sm x (S c'')).
+auto with *.
+Qed.
+
+Remark add_sub_square_identity :
+ forall a b : nat,
+ (b + a - b) * (b + a - b) = (b + a) * (b + a) + b * b - 2 * ((b + a) * b).
+intros a b; rewrite minus_plus.
+repeat rewrite mult_plus_distr_r || rewrite <- (mult_comm (b + a)).
+replace (b * b + a * b + (b * a + a * a) + b * b) with
+ (b * b + a * b + (b * b + a * b + a * a)); try (ring; fail).
+rewrite expand_mult2; repeat rewrite minus_plus; auto with *.
+Qed.
+
+Theorem sub_square_identity :
+ forall a b : nat, b <= a -> (a - b) * (a - b) = a * a + b * b - 2 * (a * b).
+intros a b H; rewrite (le_plus_minus b a H); apply add_sub_square_identity.
+Qed.
+
+Theorem square_monotonic : forall x y : nat, x < y -> x * x < y * y.
+intros x; case x.
+intros y; case y; simpl in |- *; auto with *.
+intros x' y Hlt; apply lt_trans with (S x' * y).
+rewrite (mult_comm (S x') y); apply mult_lt; auto.
+apply mult_lt; omega.
+Qed.
+
+Theorem root_monotonic : forall x y : nat, x * x < y * y -> x < y.
+exact (monotonic_inverse (fun x : nat => x * x) square_monotonic).
+Qed.
+
+Remark square_recompose : forall x y : nat, x * y * (x * y) = x * x * (y * y).
+intros; ring.
+Qed.
+
+Remark mult2_recompose : forall x y : nat, x * (2 * y) = x * 2 * y.
+intros; ring.
+Qed.
+Section sqrt2_decrease.
+Variables (p q : nat) (pos_q : 0 < q) (hyp_sqrt : p * p = 2 * (q * q)).
+
+Theorem sqrt_q_non_zero : 0 <> q * q.
+generalize pos_q; case q.
+intros H; elim (lt_n_O 0); auto.
+intros n H.
+simpl in |- *; discriminate.
+Qed.
+Hint Resolve sqrt_q_non_zero.
+
+Ltac solve_comparison :=
+ apply root_monotonic; repeat rewrite square_recompose; rewrite hyp_sqrt;
+ rewrite mult2_recompose; apply mult_lt; auto with arith.
+
+Theorem comparison1 : q < p.
+replace q with (1 * q); try ring.
+replace p with (1 * p); try ring.
+solve_comparison.
+Qed.
+
+Theorem comparison2 : 2 * p < 3 * q.
+solve_comparison.
+Qed.
+
+Theorem comparison3 : 4 * q < 3 * p.
+solve_comparison.
+Qed.
+Hint Resolve comparison1 comparison2 comparison3: arith.
+
+Theorem comparison4 : 3 * q - 2 * p < q.
+apply plus_lt_reg_l with (2 * p).
+rewrite <- le_plus_minus; try (simple apply lt_le_weak; auto with arith).
+replace (3 * q) with (2 * q + q); try ring.
+apply plus_lt_le_compat; auto.
+repeat rewrite (mult_comm 2); apply mult_lt; auto with arith.
+Qed.
+
+Remark mult_minus_distr_l : forall a b c : nat, a * (b - c) = a * b - a * c.
+intros a b c; repeat rewrite (mult_comm a); apply mult_minus_distr_r.
+Qed.
+
+Remark minus_eq_decompose :
+ forall a b c d : nat, a = b -> c = d -> a - c = b - d.
+intros a b c d H H0; rewrite H; rewrite H0; auto.
+Qed.
+
+Theorem new_equality :
+ (3 * p - 4 * q) * (3 * p - 4 * q) = 2 * ((3 * q - 2 * p) * (3 * q - 2 * p)).
+repeat rewrite sub_square_identity; auto with arith.
+repeat rewrite square_recompose; rewrite mult_minus_distr_l.
+apply minus_eq_decompose; try rewrite hyp_sqrt; ring.
+Qed.
+End sqrt2_decrease.
+Hint Resolve lt_le_weak comparison2: sqrt.
+
+Theorem sqrt2_not_rational :
+ forall p q : nat, q <> 0 -> p * p = 2 * (q * q) -> False.
+intros p q; generalize p; clear p; elim q using (well_founded_ind lt_wf).
+clear q; intros q Hrec p Hneq; generalize (neq_O_lt _ (sym_not_equal Hneq));
+ intros Hlt_O_q Heq.
+apply (Hrec (3 * q - 2 * p) (comparison4 _ _ Hlt_O_q Heq) (3 * p - 4 * q)).
+apply sym_not_equal; apply lt_neq; apply plus_lt_reg_l with (2 * p);
+ rewrite <- plus_n_O; rewrite <- le_plus_minus; auto with *.
+apply new_equality; auto.
+Qed.