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-
-(*************************************************************)
-(* This file is distributed under the terms of the *)
-(* GNU Lesser General Public License Version 2.1 *)
-(*************************************************************)
-(* Benjamin.Gregoire@inria.fr Laurent.Thery@inria.fr *)
-(*************************************************************)
-
-(***********************************************************************
- Root.v
-
- Proof that a polynomial has at most n roots
-************************************************************************)
-Require Import ZArith.
-Require Import List.
-Require Import UList.
-Require Import Tactic.
-Require Import Permutation.
-
-Open Scope Z_scope.
-
-Section Root.
-
-Variable A: Set.
-Variable P: A -> Prop.
-Variable plus mult: A -> A -> A.
-Variable op: A -> A.
-Variable zero one: A.
-
-
-Let pol := list A.
-
-Definition toA z :=
-match z with
- Z0 => zero
-| Zpos p => iter_pos _ (plus one) zero p
-| Zneg p => op (iter_pos _ (plus one) zero p)
-end.
-
-Fixpoint eval (p: pol) (x: A) {struct p} : A :=
-match p with
- nil => zero
-| a::p1 => plus a (mult x (eval p1 x))
-end.
-
-Fixpoint div (p: pol) (x: A) {struct p} : pol * A :=
-match p with
- nil => (nil, zero)
-| a::nil => (nil, a)
-| a::p1 =>
- (snd (div p1 x)::fst (div p1 x),
- (plus a (mult x (snd (div p1 x)))))
-end.
-
-Hypothesis Pzero: P zero.
-Hypothesis Pplus: forall x y, P x -> P y -> P (plus x y).
-Hypothesis Pmult: forall x y, P x -> P y -> P (mult x y).
-Hypothesis Pop: forall x, P x -> P (op x).
-Hypothesis plus_zero: forall a, P a -> plus zero a = a.
-Hypothesis plus_comm: forall a b, P a -> P b -> plus a b = plus b a.
-Hypothesis plus_assoc: forall a b c, P a -> P b -> P c -> plus a (plus b c) = plus (plus a b) c.
-Hypothesis mult_zero: forall a, P a -> mult zero a = zero.
-Hypothesis mult_comm: forall a b, P a -> P b -> mult a b = mult b a.
-Hypothesis mult_assoc: forall a b c, P a -> P b -> P c -> mult a (mult b c) = mult (mult a b) c.
-Hypothesis mult_plus_distr: forall a b c, P a -> P b -> P c -> mult a (plus b c) = plus (mult a b) (mult a c).
-Hypothesis plus_op_zero: forall a, P a -> plus a (op a) = zero.
-Hypothesis mult_integral: forall a b, P a -> P b -> mult a b = zero -> a = zero \/ b = zero.
-(* Not necessary in Set just handy *)
-Hypothesis A_dec: forall a b: A, {a = b} + {a <> b}.
-
-Theorem eval_P: forall p a, P a -> (forall i, In i p -> P i) -> P (eval p a).
-intros p a Pa; elim p; simpl; auto with datatypes.
-intros a1 l1 Rec H; apply Pplus; auto.
-Qed.
-
-Hint Resolve eval_P.
-
-Theorem div_P: forall p a, P a -> (forall i, In i p -> P i) -> (forall i, In i (fst (div p a)) -> P i) /\ P (snd (div p a)).
-intros p a Pa; elim p; auto with datatypes.
-intros a1 l1; case l1.
-simpl; intuition.
-intros a2 p2 Rec Hi; split.
-case Rec; auto with datatypes.
-intros H H1 i.
-replace (In i (fst (div (a1 :: a2 :: p2) a))) with
- (snd (div (a2::p2) a) = i \/ In i (fst (div (a2::p2) a))); auto.
-intros [Hi1 | Hi1]; auto.
-rewrite <- Hi1; auto.
-change ( P (plus a1 (mult a (snd (div (a2::p2) a))))); auto with datatypes.
-apply Pplus; auto with datatypes.
-apply Pmult; auto with datatypes.
-case Rec; auto with datatypes.
-Qed.
-
-
-Theorem div_correct:
- forall p x y, P x -> P y -> (forall i, In i p -> P i) -> eval p y = plus (mult (eval (fst (div p x)) y) (plus y (op x))) (snd (div p x)).
-intros p x y; elim p; simpl.
-intros; rewrite mult_zero; try rewrite plus_zero; auto.
-intros a l; case l; simpl; auto.
-intros _ px py pa; rewrite (fun x => mult_comm x zero); repeat rewrite mult_zero; try apply plus_comm; auto.
-intros a1 l1.
-generalize (div_P (a1::l1) x); simpl.
-match goal with |- context[fst ?A] => case A end; simpl.
-intros q r Hd Rec px py pi.
-assert (pr: P r).
-case Hd; auto.
-assert (pa1: P a1).
-case Hd; auto.
-assert (pey: P (eval q y)).
-apply eval_P; auto.
-case Hd; auto.
-rewrite Rec; auto with datatypes.
-rewrite (fun x y => plus_comm x (plus a y)); try rewrite <- plus_assoc; auto.
-apply f_equal2 with (f := plus); auto.
-repeat rewrite mult_plus_distr; auto.
-repeat (rewrite (fun x y => (mult_comm (plus x y))) || rewrite mult_plus_distr); auto.
-rewrite (fun x => (plus_comm x (mult y r))); auto.
-repeat rewrite plus_assoc; try apply f_equal2 with (f := plus); auto.
-2: repeat rewrite mult_assoc; try rewrite (fun y => mult_comm y (op x));
- repeat rewrite mult_assoc; auto.
-rewrite (fun z => (plus_comm z (mult (op x) r))); auto.
-repeat rewrite plus_assoc; try apply f_equal2 with (f := plus); auto.
-2: apply f_equal2 with (f := mult); auto.
-repeat rewrite (fun x => mult_comm x r); try rewrite <- mult_plus_distr; auto.
-rewrite (plus_comm (op x)); try rewrite plus_op_zero; auto.
-rewrite (fun x => mult_comm x zero); try rewrite mult_zero; try rewrite plus_zero; auto.
-Qed.
-
-Theorem div_correct_factor:
- forall p a, (forall i, In i p -> P i) -> P a ->
- eval p a = zero -> forall x, P x -> eval p x = (mult (eval (fst (div p a)) x) (plus x (op a))).
-intros p a Hp Ha H x px.
-case (div_P p a); auto; intros Hd1 Hd2.
-rewrite (div_correct p a x); auto.
-generalize (div_correct p a a).
-rewrite plus_op_zero; try rewrite (fun x => mult_comm x zero); try rewrite mult_zero; try rewrite plus_zero; try rewrite H; auto.
-intros H1; rewrite <- H1; auto.
-rewrite (fun x => plus_comm x zero); auto.
-Qed.
-
-Theorem length_decrease: forall p x, p <> nil -> (length (fst (div p x)) < length p)%nat.
-intros p x; elim p; simpl; auto.
-intros H1; case H1; auto.
-intros a l; case l; simpl; auto.
-intros a1 l1.
-match goal with |- context[fst ?A] => case A end; simpl; auto with zarith.
-intros p1 _ H H1.
-apply lt_n_S; apply H; intros; discriminate.
-Qed.
-
-Theorem root_max:
-forall p l, ulist l -> (forall i, In i p -> P i) -> (forall i, In i l -> P i) ->
- (forall x, In x l -> eval p x = zero) -> (length p <= length l)%nat -> forall x, P x -> eval p x = zero.
-intros p l; generalize p; elim l; clear l p; simpl; auto.
-intros p; case p; simpl; auto.
-intros a p1 _ _ _ _ H; contradict H; auto with arith.
-intros a p1 Rec p; case p.
-simpl; auto.
-intros a1 p2 H H1 H2 H3 H4 x px.
-assert (Hu: eval (a1 :: p2) a = zero); auto with datatypes.
-rewrite (div_correct_factor (a1 :: p2) a); auto with datatypes.
-match goal with |- mult ?X _ = _ => replace X with zero end; try apply mult_zero; auto.
-apply sym_equal; apply Rec; auto with datatypes.
-apply ulist_inv with (1 := H).
-intros i Hi; case (div_P (a1 :: p2) a); auto.
-intros x1 H5; case (mult_integral (eval (fst (div (a1 :: p2) a)) x1) (plus x1 (op a))); auto.
-apply eval_P; auto.
-intros i Hi; case (div_P (a1 :: p2) a); auto.
-rewrite <- div_correct_factor; auto.
-intros H6; case (ulist_app_inv _ (a::nil) p1 x1); simpl; auto.
-left.
-apply trans_equal with (plus zero x1); auto.
-rewrite <- (plus_op_zero a); try rewrite <- plus_assoc; auto.
-rewrite (fun x => plus_comm (op x)); try rewrite H6; try rewrite plus_comm; auto.
-apply sym_equal; apply plus_zero; auto.
-apply lt_n_Sm_le;apply lt_le_trans with (length (a1 :: p2)); auto with zarith.
-apply length_decrease; auto with datatypes.
-Qed.
-
-Theorem root_max_is_zero:
-forall p l, ulist l -> (forall i, In i p -> P i) -> (forall i, In i l -> P i) ->
- (forall x, In x l -> eval p x = zero) -> (length p <= length l)%nat -> forall x, (In x p) -> x = zero.
-intros p l; generalize p; elim l; clear l p; simpl; auto.
-intros p; case p; simpl; auto.
-intros _ _ _ _ _ x H; case H.
-intros a p1 _ _ _ _ H; contradict H; auto with arith.
-intros a p1 Rec p; case p.
-simpl; auto.
-intros _ _ _ _ _ x H; case H.
-simpl; intros a1 p2 H H1 H2 H3 H4 x H5.
-assert (Ha1: a1 = zero).
-assert (Hu: (eval (a1::p2) zero = zero)).
-apply root_max with (l := a :: p1); auto.
-rewrite <- Hu; simpl; rewrite mult_zero; try rewrite plus_comm; sauto.
-case H5; clear H5; intros H5; subst; auto.
-apply Rec with p2; auto with arith.
-apply ulist_inv with (1 := H).
-intros x1 Hx1.
-case (In_dec A_dec zero p1); intros Hz.
-case (in_permutation_ex _ zero p1); auto; intros p3 Hp3.
-apply root_max with (l := a::p3); auto.
-apply ulist_inv with zero.
-apply ulist_perm with (a::p1); auto.
-apply permutation_trans with (a:: (zero:: p3)); auto.
-apply permutation_skip; auto.
-apply permutation_sym; auto.
-simpl; intros x2 [Hx2 | Hx2]; subst; auto.
-apply H2; right; apply permutation_in with (1 := Hp3); auto with datatypes.
-simpl; intros x2 [Hx2 | Hx2]; subst.
-case (mult_integral x2 (eval p2 x2)); auto.
-rewrite <- H3 with x2; sauto.
-rewrite plus_zero; auto.
-intros H6; case (ulist_app_inv _ (x2::nil) p1 x2) ; auto with datatypes.
-rewrite H6; apply permutation_in with (1 := Hp3); auto with datatypes.
-case (mult_integral x2 (eval p2 x2)); auto.
-apply H2; right; apply permutation_in with (1 := Hp3); auto with datatypes.
-apply eval_P; auto.
-apply H2; right; apply permutation_in with (1 := Hp3); auto with datatypes.
-rewrite <- H3 with x2; sauto; try right.
-apply sym_equal; apply plus_zero; auto.
-apply Pmult; auto.
-apply H2; right; apply permutation_in with (1 := Hp3); auto with datatypes.
-apply eval_P; auto.
-apply H2; right; apply permutation_in with (1 := Hp3); auto with datatypes.
-apply permutation_in with (1 := Hp3); auto with datatypes.
-intros H6; case (ulist_app_inv _ (zero::nil) p3 x2) ; auto with datatypes.
-simpl; apply ulist_perm with (1:= (permutation_sym _ _ _ Hp3)).
-apply ulist_inv with (1 := H).
-rewrite H6; auto with datatypes.
-replace (length (a :: p3)) with (length (zero::p3)); auto.
-rewrite permutation_length with (1 := Hp3); auto with arith.
-case (mult_integral x1 (eval p2 x1)); auto.
-rewrite <- H3 with x1; sauto; try right.
-apply sym_equal; apply plus_zero; auto.
-intros HH; case Hz; rewrite <- HH; auto.
-Qed.
-
-End Root.