Library Stdlib.Reals.Rderiv

From Stdlib Require Import Rbase.
From Stdlib Require Import Rfunctions.
From Stdlib Require Import Rlimit.
From Stdlib Require Import Lra.
From Stdlib Require Import Lia.
Local Open Scope R_scope.

Definition D_x (D:R -> Prop) (y x:R) : Prop := D x /\ y <> x.

Definition continue_in (f:R -> R) (D:R -> Prop) (x0:R) : Prop :=
  limit1_in f (D_x D x0) (f x0) x0.

Definition D_in (f d:R -> R) (D:R -> Prop) (x0:R) : Prop :=
  limit1_in (fun x:R => (f x - f x0) / (x - x0)) (D_x D x0) (d x0) x0.

Lemma cont_deriv :
  forall (f d:R -> R) (D:R -> Prop) (x0:R),
    D_in f d D x0 -> continue_in f D x0.

Lemma Dconst :
  forall (D:R -> Prop) (y x0:R), D_in (fun x:R => y) (fun x:R => 0) D x0.

Lemma Dx :
  forall (D:R -> Prop) (x0:R), D_in (fun x:R => x) (fun x:R => 1) D x0.

Lemma Dadd :
  forall (D:R -> Prop) (df dg f g:R -> R) (x0:R),
    D_in f df D x0 ->
    D_in g dg D x0 ->
    D_in (fun x:R => f x + g x) (fun x:R => df x + dg x) D x0.

Lemma Dmult :
  forall (D:R -> Prop) (df dg f g:R -> R) (x0:R),
    D_in f df D x0 ->
    D_in g dg D x0 ->
    D_in (fun x:R => f x * g x) (fun x:R => df x * g x + f x * dg x) D x0.

Lemma Dmult_const :
  forall (D:R -> Prop) (f df:R -> R) (x0 a:R),
    D_in f df D x0 -> D_in (fun x:R => a * f x) (fun x:R => a * df x) D x0.

Lemma Dopp :
  forall (D:R -> Prop) (f df:R -> R) (x0:R),
    D_in f df D x0 -> D_in (fun x:R => - f x) (fun x:R => - df x) D x0.

Lemma Dminus :
  forall (D:R -> Prop) (df dg f g:R -> R) (x0:R),
    D_in f df D x0 ->
    D_in g dg D x0 ->
    D_in (fun x:R => f x - g x) (fun x:R => df x - dg x) D x0.

Lemma Dx_pow_n :
  forall (n:nat) (D:R -> Prop) (x0:R),
    D_in (fun x:R => x ^ n) (fun x:R => INR n * x ^ (n - 1)) D x0.

Lemma Dcomp :
  forall (Df Dg:R -> Prop) (df dg f g:R -> R) (x0:R),
    D_in f df Df x0 ->
    D_in g dg Dg (f x0) ->
    D_in (fun x:R => g (f x)) (fun x:R => df x * dg (f x)) (Dgf Df Dg f) x0.

Lemma D_pow_n :
  forall (n:nat) (D:R -> Prop) (x0:R) (expr dexpr:R -> R),
    D_in expr dexpr D x0 ->
    D_in (fun x:R => expr x ^ n)
    (fun x:R => INR n * expr x ^ (n - 1) * dexpr x) (
      Dgf D D expr) x0.