(************************************************************************)
(*         *      The Rocq Prover / The Rocq Development Team           *)
(*  v      *         Copyright INRIA, CNRS and contributors             *)
(* <O___,, * (see version control and CREDITS file for authors & dates) *)
(*   \VV/  **************************************************************)
(*    //   *    This file is distributed under the terms of the         *)
(*         *     GNU Lesser General Public License Version 2.1          *)
(*         *     (see LICENSE file for the text of the license)         *)
(************************************************************************)

From Stdlib Require Import Rbase.
From Stdlib Require Import Rfunctions.
From Stdlib Require Import SeqSeries.
From Stdlib Require Import Rtrigo1.
From Stdlib Require Import Ranalysis1.
From Stdlib Require Import Ranalysis3.
From Stdlib Require Import Exp_prop.
From Stdlib Require Import MVT.
From Stdlib Require Import Lra Lia.
#[local] Open Scope R_scope.

(**********)
Lemma derivable_pt_inv :
  forall (f:R -> R) (x:R),
    f x <> 0 -> derivable_pt f x -> derivable_pt (/ f) x.
Proof.
  intros f x H X; cut (derivable_pt (fct_cte 1 / f) x -> derivable_pt (/ f) x).
  - intro X0; apply X0.
    apply derivable_pt_div.
    + apply derivable_pt_const.
    + assumption.
    + assumption.
  - unfold div_fct, inv_fct, fct_cte; intros (x0,p);
      unfold derivable_pt; exists x0;
      unfold derivable_pt_abs; unfold derivable_pt_lim;
      unfold derivable_pt_abs in p; unfold derivable_pt_lim in p;
      intros; elim (p eps H0); intros; exists x1; intros;
      unfold Rdiv in H1; unfold Rdiv; rewrite <- (Rmult_1_l (/ f x));
      rewrite <- (Rmult_1_l (/ f (x + h))).
    apply H1; assumption.
Qed.

(**********)
Lemma pr_nu_var :
  forall (f g:R -> R) (x:R) (pr1:derivable_pt f x) (pr2:derivable_pt g x),
    f = g -> derive_pt f x pr1 = derive_pt g x pr2.
Proof.
  unfold derivable_pt, derive_pt; intros f g x (x0,p0) (x1,p1) ->.
  apply uniqueness_limite with g x; assumption.
Qed.

(**********)
Lemma pr_nu_var2 :
  forall (f g:R -> R) (x:R) (pr1:derivable_pt f x) (pr2:derivable_pt g x),
    (forall h:R, f h = g h) -> derive_pt f x pr1 = derive_pt g x pr2.
Proof.
  unfold derivable_pt, derive_pt; intros f g x (x0,p0) (x1,p1) H.
  assert (H0 := uniqueness_step2 _ _ _ p0).
  assert (H1 := uniqueness_step2 _ _ _ p1).
  cut (limit1_in (fun h:R => (f (x + h) - f x) / h) (fun h:R => h <> 0) x1 0).
  - intro H2; assert (H3 := uniqueness_step1 _ _ _ _ H0 H2).
    assumption.
  - unfold limit1_in; unfold limit_in; unfold dist;
      simpl; unfold Rdist; unfold limit1_in in H1;
      unfold limit_in in H1; unfold dist in H1; simpl in H1;
      unfold Rdist in H1.
    intros; elim (H1 eps H2); intros.
    elim H3; intros.
    exists x2.
    split.
    + assumption.
    + intros; do 2 rewrite H; apply H5; assumption.
Qed.

(**********)
Lemma derivable_inv :
  forall f:R -> R, (forall x:R, f x <> 0) -> derivable f -> derivable (/ f).
Proof.
  intros f H X.
  unfold derivable; intro x.
  apply derivable_pt_inv.
  - apply (H x).
  - apply (X x).
Qed.

Lemma derive_pt_inv :
  forall (f:R -> R) (x:R) (pr:derivable_pt f x) (na:f x <> 0),
    derive_pt (/ f) x (derivable_pt_inv f x na pr) =
    - derive_pt f x pr / Rsqr (f x).
Proof.
  intros;
    replace (derive_pt (/ f) x (derivable_pt_inv f x na pr)) with
    (derive_pt (fct_cte 1 / f) x
      (derivable_pt_div (fct_cte 1) f x (derivable_pt_const 1 x) pr na)).
  - rewrite derive_pt_div; rewrite derive_pt_const; unfold fct_cte;
      rewrite Rmult_0_l; rewrite Rmult_1_r; unfold Rminus;
      rewrite Rplus_0_l; reflexivity.
  - apply pr_nu_var2.
    intro; unfold div_fct, fct_cte, inv_fct.
    unfold Rdiv; ring.
Qed.

(** Rabsolu *)
Lemma Rabs_derive_1 : forall x:R, 0 < x -> derivable_pt_lim Rabs x 1.
Proof.
  intros.
  unfold derivable_pt_lim; intros.
  exists (mkposreal x H); intros.
  rewrite (Rabs_right x).
  - rewrite (Rabs_right (x + h)).
    + rewrite Rplus_comm.
      unfold Rminus; rewrite Rplus_assoc; rewrite Rplus_opp_r.
      rewrite Rplus_0_r; unfold Rdiv; rewrite Rinv_r.
      * rewrite Rplus_opp_r; rewrite Rabs_R0; apply H0.
      * apply H1.
    + apply Rle_ge.
      destruct (Rcase_abs h) as [Hlt|Hgt].
      * rewrite (Rabs_left h Hlt) in H2.
        left; rewrite Rplus_comm; apply Rplus_lt_reg_l with (- h); rewrite Rplus_0_r;
          rewrite <- Rplus_assoc; rewrite Rplus_opp_l; rewrite Rplus_0_l;
          apply H2.
      * apply Rplus_le_le_0_compat.
        -- left; apply H.
        -- apply Rge_le; apply Hgt.
  - left; apply H.
Qed.

Lemma Rabs_derive_2 : forall x:R, x < 0 -> derivable_pt_lim Rabs x (-1).
Proof.
  intros.
  unfold derivable_pt_lim; intros.
  cut (0 < - x).
  - intro; exists (mkposreal (- x) H1); intros.
    rewrite (Rabs_left x).
    + rewrite (Rabs_left (x + h)).
      * replace ((-(x + h) - - x) / h - -1) with 0 by now field.
        rewrite Rabs_R0; apply H0.
      * destruct (Rcase_abs h) as [Hlt|Hgt].
        -- apply Ropp_lt_cancel.
           rewrite Ropp_0; rewrite Ropp_plus_distr; apply Rplus_lt_0_compat.
           ++ apply H1.
           ++ apply Ropp_0_gt_lt_contravar; apply Hlt.
        -- rewrite (Rabs_right h Hgt) in H3.
           apply Rplus_lt_reg_l with (- x); rewrite Rplus_0_r; rewrite <- Rplus_assoc;
             rewrite Rplus_opp_l; rewrite Rplus_0_l; apply H3.
    + apply H.
  - apply Ropp_0_gt_lt_contravar; apply H.
Qed.

(** Rabsolu is derivable for all x <> 0 *)
Lemma Rderivable_pt_abs : forall x:R, x <> 0 -> derivable_pt Rabs x.
Proof.
  intros.
  destruct (total_order_T x 0) as [[Hlt|Heq]|Hgt].
  - unfold derivable_pt; exists (-1).
    apply (Rabs_derive_2 x Hlt).
  - elim H; exact Heq.
  - unfold derivable_pt; exists 1.
    apply (Rabs_derive_1 x Hgt).
Qed.

(** Rabsolu is continuous for all x *)
Lemma Rcontinuity_abs : continuity Rabs.
Proof.
  unfold continuity; intro.
  case (Req_dec x 0); intro.
  - unfold continuity_pt; unfold continue_in;
      unfold limit1_in; unfold limit_in;
      simpl; unfold Rdist; intros; exists eps;
      split.
    + apply H0.
    + intros; rewrite H; rewrite Rabs_R0; unfold Rminus; rewrite Ropp_0;
        rewrite Rplus_0_r; rewrite Rabs_Rabsolu; elim H1;
        intros; rewrite H in H3; unfold Rminus in H3; rewrite Ropp_0 in H3;
        rewrite Rplus_0_r in H3; apply H3.
  - apply derivable_continuous_pt; apply (Rderivable_pt_abs x H).
Qed.

(** Finite sums : Sum a_k x^k *)
Lemma continuity_finite_sum :
  forall (An:nat -> R) (N:nat),
    continuity (fun y:R => sum_f_R0 (fun k:nat => An k * y ^ k) N).
Proof.
  intros; unfold continuity; intro.
  induction  N as [| N HrecN].
  - simpl.
    apply continuity_pt_const.
    unfold constant; intros; reflexivity.
  - replace (fun y:R => sum_f_R0 (fun k:nat => An k * y ^ k) (S N)) with
      ((fun y:R => sum_f_R0 (fun k:nat => (An k * y ^ k)%R) N) +
         (fun y:R => (An (S N) * y ^ S N)%R))%F.
    + apply continuity_pt_plus.
      * apply HrecN.
      * replace (fun y:R => An (S N) * y ^ S N) with
          (mult_real_fct (An (S N)) (fun y:R => y ^ S N)).
        -- apply continuity_pt_scal.
           apply derivable_continuous_pt.
           apply derivable_pt_pow.
        -- reflexivity.
    + reflexivity.
Qed.

Lemma derivable_pt_lim_fs :
  forall (An:nat -> R) (x:R) (N:nat),
    (0 < N)%nat ->
    derivable_pt_lim (fun y:R => sum_f_R0 (fun k:nat => An k * y ^ k) N) x
    (sum_f_R0 (fun k:nat => INR (S k) * An (S k) * x ^ k) (pred N)).
Proof.
  intros; induction  N as [| N HrecN].
  { elim (Nat.lt_irrefl _ H). }
  assert (N = 0%nat \/ (0 < N)%nat) by nia.
  elim H0; intro.
  { rewrite H1.
    simpl.
    change (fun y:R => An 0%nat * 1 + An 1%nat * (y * 1)) with
      (fct_cte (An 0%nat * 1) + mult_real_fct (An 1%nat) (id * fct_cte 1))%F.
    replace (1 * An 1%nat * 1) with (0 + An 1%nat * (1 * fct_cte 1 x + id x * 0))
      by (unfold fct_cte, id; ring).
    apply derivable_pt_lim_plus.
    - apply derivable_pt_lim_const.
    - apply derivable_pt_lim_scal.
      apply derivable_pt_lim_mult.
      + apply derivable_pt_lim_id.
      + apply derivable_pt_lim_const. }
  change (fun y:R => sum_f_R0 (fun k:nat => An k * y ^ k) (S N)) with
    ((fun y:R => sum_f_R0 (fun k:nat => (An k * y ^ k)%R) N) +
       (fun y:R => (An (S N) * y ^ S N)%R))%F.
  replace (sum_f_R0 (fun k:nat => INR (S k) * An (S k) * x ^ k) (pred (S N)))
    with
      (sum_f_R0 (fun k:nat => INR (S k) * An (S k) * x ^ k) (pred N) +
        An (S N) * (INR (S (pred (S N))) * x ^ pred (S N))).
  2:{ assert (pred (S N) = S (pred N)) by lia.
      rewrite H2.
      rewrite tech5.
      apply Rplus_eq_compat_l.
      rewrite <- H2.
      change (pred (S N)) with N.
      ring. }
  apply derivable_pt_lim_plus.
  { apply HrecN. assumption. }
  change (fun y:R => An (S N) * y ^ S N) with
  (mult_real_fct (An (S N)) (fun y:R => y ^ S N)).
  apply derivable_pt_lim_scal.
  apply derivable_pt_lim_pow.
Qed.

Lemma derivable_pt_lim_finite_sum :
  forall (An:nat -> R) (x:R) (N:nat),
    derivable_pt_lim (fun y:R => sum_f_R0 (fun k:nat => An k * y ^ k) N) x
    match N with
      | O => 0
      | _ => sum_f_R0 (fun k:nat => INR (S k) * An (S k) * x ^ k) (pred N)
    end.
Proof.
  intros.
  induction  N as [| N HrecN].
  - simpl.
    rewrite Rmult_1_r.
    replace (fun _:R => An 0%nat) with (fct_cte (An 0%nat));
      [ apply derivable_pt_lim_const | reflexivity ].
  - apply derivable_pt_lim_fs; apply Nat.lt_0_succ.
Qed.

Lemma derivable_pt_finite_sum :
  forall (An:nat -> R) (N:nat) (x:R),
    derivable_pt (fun y:R => sum_f_R0 (fun k:nat => An k * y ^ k) N) x.
Proof.
  intros.
  unfold derivable_pt.
  assert (H := derivable_pt_lim_finite_sum An x N).
  induction  N as [| N HrecN].
  - exists 0; apply H.
  - exists
      (sum_f_R0 (fun k:nat => INR (S k) * An (S k) * x ^ k) (pred (S N)));
      apply H.
Qed.

Lemma derivable_finite_sum :
  forall (An:nat -> R) (N:nat),
    derivable (fun y:R => sum_f_R0 (fun k:nat => An k * y ^ k) N).
Proof.
  intros; unfold derivable; intro; apply derivable_pt_finite_sum.
Qed.

(** Regularity of hyperbolic functions *)
Lemma derivable_pt_lim_cosh : forall x:R, derivable_pt_lim cosh x (sinh x).
Proof.
  intro.
  unfold cosh, sinh; unfold Rdiv.
  replace (fun x0:R => (exp x0 + exp (- x0)) * / 2) with
  ((exp + comp exp (- id)) * fct_cte (/ 2))%F; [ idtac | reflexivity ].
  replace ((exp x - exp (- x)) * / 2) with
  ((exp x + exp (- x) * -1) * fct_cte (/ 2) x +
    (exp + comp exp (- id))%F x * 0).
  - apply derivable_pt_lim_mult.
    + apply derivable_pt_lim_plus.
      * apply derivable_pt_lim_exp.
      * apply derivable_pt_lim_comp.
        -- apply derivable_pt_lim_opp.
           apply derivable_pt_lim_id.
        -- apply derivable_pt_lim_exp.
    + apply derivable_pt_lim_const.
  - unfold plus_fct, mult_real_fct, comp, opp_fct, id, fct_cte; ring.
Qed.

Lemma derivable_pt_lim_sinh : forall x:R, derivable_pt_lim sinh x (cosh x).
Proof.
  intro.
  unfold cosh, sinh; unfold Rdiv.
  replace (fun x0:R => (exp x0 - exp (- x0)) * / 2) with
  ((exp - comp exp (- id)) * fct_cte (/ 2))%F; [ idtac | reflexivity ].
  replace ((exp x + exp (- x)) * / 2) with
  ((exp x - exp (- x) * -1) * fct_cte (/ 2) x +
    (exp - comp exp (- id))%F x * 0).
  - apply derivable_pt_lim_mult.
    + apply derivable_pt_lim_minus.
      * apply derivable_pt_lim_exp.
      * apply derivable_pt_lim_comp.
        -- apply derivable_pt_lim_opp.
           apply derivable_pt_lim_id.
        -- apply derivable_pt_lim_exp.
    + apply derivable_pt_lim_const.
  - unfold plus_fct, mult_real_fct, comp, opp_fct, id, fct_cte; ring.
Qed.

Lemma derivable_pt_exp : forall x:R, derivable_pt exp x.
Proof.
  intro.
  unfold derivable_pt.
  exists (exp x).
  apply derivable_pt_lim_exp.
Qed.

Lemma derivable_pt_cosh : forall x:R, derivable_pt cosh x.
Proof.
  intro.
  unfold derivable_pt.
  exists (sinh x).
  apply derivable_pt_lim_cosh.
Qed.

Lemma derivable_pt_sinh : forall x:R, derivable_pt sinh x.
Proof.
  intro.
  unfold derivable_pt.
  exists (cosh x).
  apply derivable_pt_lim_sinh.
Qed.

Lemma derivable_exp : derivable exp.
Proof.
  unfold derivable; apply derivable_pt_exp.
Qed.

Lemma derivable_cosh : derivable cosh.
Proof.
  unfold derivable; apply derivable_pt_cosh.
Qed.

Lemma derivable_sinh : derivable sinh.
Proof.
  unfold derivable; apply derivable_pt_sinh.
Qed.

Lemma derive_pt_exp :
  forall x:R, derive_pt exp x (derivable_pt_exp x) = exp x.
Proof.
  intro; apply derive_pt_eq_0.
  apply derivable_pt_lim_exp.
Qed.

Lemma derive_pt_cosh :
  forall x:R, derive_pt cosh x (derivable_pt_cosh x) = sinh x.
Proof.
  intro; apply derive_pt_eq_0.
  apply derivable_pt_lim_cosh.
Qed.

Lemma derive_pt_sinh :
  forall x:R, derive_pt sinh x (derivable_pt_sinh x) = cosh x.
Proof.
  intro; apply derive_pt_eq_0.
  apply derivable_pt_lim_sinh.
Qed.

Lemma sinh_lt : forall x y, x < y -> sinh x < sinh y.
intros x y xy; destruct (MVT_cor2 sinh cosh x y xy) as [c [Pc _]].
- intros; apply derivable_pt_lim_sinh.
- apply Rplus_lt_reg_l with (Ropp (sinh x)); rewrite Rplus_opp_l, Rplus_comm.
  unfold Rminus at 1 in Pc; rewrite Pc; apply Rmult_lt_0_compat;[ | ].
  + unfold cosh; apply Rmult_lt_0_compat;[|apply Rinv_0_lt_compat, Rlt_0_2].
    now apply Rplus_lt_0_compat; apply exp_pos.
  + now apply Rlt_0_minus; assumption.
Qed.
