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Module mathcomp.analysis.derive

From HB Require Import structures.
From mathcomp Require Import all_ssreflect ssralg ssrnum matrix interval.
From mathcomp Require Import mathcomp_extra boolp classical_sets functions.
From mathcomp Require Import reals signed topology prodnormedzmodule normedtype.
From mathcomp Require Import landau forms.


Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Import Order.TTheory GRing.Theory Num.Theory.
Import numFieldNormedType.Exports.

Local Open Scope ring_scope.
Local Open Scope classical_set_scope.

Reserved Notation "''d' f x" (at level 0, f at level 0, x at level 0,
  format "''d' f x").
Reserved Notation "'is_diff' F" (at level 0, F at level 0,
  format "'is_diff' F").
Reserved Notation "''J' f p" (at level 10, p, f at next level,
  format "''J' f p").
Reserved Notation "''D_' v f" (at level 10, v, f at next level,
  format "''D_' v f").
Reserved Notation "''D_' v f c" (at level 10, v, f at next level,
  format "''D_' v f c").
Reserved Notation "f ^` ()" (at level 8, format "f ^` ()").
Reserved Notation "f ^` ( n )" (at level 8, format "f ^` ( n )").

Section Differential.
Context {K : numDomainType} {V W : normedModType K}.

Definition diff (F : filter_on V) (_ : phantom (set (set V)) F) (f : V -> W) :=
  (get (fun (df : {linear V -> W}) => continuous df /\ forall x,
      f x = f (lim F) + df (x - lim F) +o_(x \near F) (x - lim F))).

Local Notation "''d' f x" := (@diff _ (Phantom _ (nbhs x)) f).

Fact diff_key : forall T, T -> unit

Proof.

by constructor. Qed.

CoInductive differentiable_def (f : V -> W) (x : filter_on V)
  (phF : phantom (set (set V)) x) : Prop := DifferentiableDef of
  (continuous ('d f x) /\
  f = cst (f (lim x)) + 'd f x \o center (lim x) +o_x (center (lim x))).

Local Notation differentiable f F :=
  (@differentiable_def f _ (Phantom _ (nbhs F))).

Class is_diff_def (x : filter_on V) (Fph : phantom (set (set V)) x) (f : V -> W)
  (df : V -> W) := DiffDef {
    ex_diff : differentiable f x ;
    diff_val : 'd f x = df :> (V -> W)
  }.
Hint Mode is_diff_def - - ! - : typeclass_instances.

Lemma diffP (F : filter_on V) (f : V -> W) :
  differentiable f F <->
  continuous ('d f F) /\
  (forall x, f x = f (lim F) + 'd f F (x - lim F) +o_(x \near F) (x - lim F)).

Proof.

by split=> [[] |]; last constructor; rewrite funeqE. Qed.

Lemma diff_continuous (x : filter_on V) (f : V -> W) :
  differentiable f x -> continuous ('d f x).

Proof.

by move=> /diffP []. Qed.

Hint Extern 0 (continuous _) => exact: diff_continuous : core.

Lemma diffE (F : filter_on V) (f : V -> W) :
  differentiable f F ->
  forall x, f x = f (lim F) + 'd f F (x - lim F) +o_(x \near F) (x - lim F).

Proof.

by move=> /diffP []. Qed.

Lemma littleo_center0 (x : V) (f : V -> W) (e : V -> V) :
  [o_x e of f] = [o_ 0 (e \o shift x) of f \o shift x] \o center x.

Proof.

rewrite /the_littleo /insubd /=; have [g /= _ <-{f}|/asboolP Nfe] /= := insubP.
  rewrite insubT //= ?comp_shiftK //; apply/asboolP => _/posnumP[eps].
  rewrite [\forall x \near _, _ <= _](near_shift x) sub0r; near=> y.
  by rewrite /= subrK; near: y; have /eqoP := littleo_eqo g; apply.
rewrite insubF //; apply/asboolP => fe; apply: Nfe => _/posnumP[eps].
by rewrite [\forall x \near _, _ <= _](near_shift 0) subr0; apply: fe.
Unshelve. all: by end_near. Qed.

End Differential.

Section Differential_numFieldType.
Context {K : numFieldType } {V W : normedModType K}.

Local Notation differentiable f F :=
  (@differentiable_def _ _ _ f _ (Phantom _ (nbhs F))).
Local Notation "''d' f x" := (@diff _ _ _ _ (Phantom _ (nbhs x)) f).
Hint Extern 0 (continuous _) => exact: diff_continuous : core.

Lemma diff_locallyxP (x : V) (f : V -> W) :
  differentiable f x <-> continuous ('d f x) /\
  forall h, f (h + x) = f x + 'd f x h +o_(h \near 0) h.

Proof.

split=> [dxf|[dfc dxf]].
  split => //; apply: eqaddoEx => h; have /diffE -> := dxf.
  rewrite lim_id // addrK; congr (_ + _); rewrite littleo_center0 /= addrK.
  by congr ('o); rewrite funeqE => k /=; rewrite addrK.
apply/diffP; split=> //; apply: eqaddoEx; move=> y.
rewrite lim_id // -[in LHS](subrK x y) dxf; congr (_ + _).
rewrite -(comp_centerK x id) -[X in the_littleo _ _ _ X](comp_centerK x).
by rewrite -[_ (y - x)]/((_ \o (center x)) y) -littleo_center0.
Qed.

Lemma diff_locallyx (x : V) (f : V -> W) : differentiable f x ->
  forall h, f (h + x) = f x + 'd f x h +o_(h \near 0) h.

Proof.

by move=> /diff_locallyxP []. Qed.

Lemma diff_locallyxC (x : V) (f : V -> W) : differentiable f x ->
  forall h, f (x + h) = f x + 'd f x h +o_(h \near 0) h.

Proof.

by move=> ?; apply/eqaddoEx => h; rewrite [x + h]addrC diff_locallyx. Qed.

Lemma diff_locallyP (x : V) (f : V -> W) :
  differentiable f x <->
  continuous ('d f x) /\ (f \o shift x = cst (f x) + 'd f x +o_ 0 id).

Proof.

by apply: iff_trans (diff_locallyxP _ _) _; rewrite funeqE. Qed.

Lemma diff_locally (x : V) (f : V -> W) : differentiable f x ->
  (f \o shift x = cst (f x) + 'd f x +o_ 0 id).

Proof.

by move=> /diff_locallyP []. Qed.

End Differential_numFieldType.

Notation "''d' f F" := (@diff _ _ _ _ (Phantom _ (nbhs F)) f).
Notation differentiable f F := (@differentiable_def _ _ _ f _ (Phantom _ (nbhs F))).

Notation "'is_diff' F" := (is_diff_def (Phantom _ (nbhs F))).
#[global] Hint Extern 0 (differentiable _ _) => solve[apply: ex_diff] : core.
#[global] Hint Extern 0 ({for _, continuous _}) => exact: diff_continuous : core.

Lemma differentiableP (R : numDomainType) (V W : normedModType R) (f : V -> W) x :
  differentiable f x -> is_diff x f ('d f x).

Proof.

by move=> ?; apply: DiffDef. Qed.

Section jacobian.

Definition jacobian n m (R : numFieldType) (f : 'rV[R]_n.+1 -> 'rV[R]_m.+1) p :=
  lin1_mx ('d f p).

End jacobian.

Notation "''J' f p" := (jacobian f p).

Section DifferentialR.

Context {R : numFieldType} {V W : normedModType R}.

Lemma differentiable_continuous (x : V) (f : V -> W) :
  differentiable f x -> {for x, continuous f}.

Proof.

move=> /diff_locallyP [dfc]; rewrite -addrA.
rewrite (littleo_bigO_eqo (cst (1 : R))); last first.
  by apply/eqOP; near=> k; rewrite /cst [`|1|]normr1 mulr1; near do by [].
rewrite addfo; first by move=> /eqolim; rewrite cvg_comp_shift add0r.
by apply/eqolim0P; apply: (cvg_trans (dfc 0)); rewrite linear0.
Unshelve. all: by end_near. Qed.

Section littleo_lemmas.
Variables (X Y Z : normedModType R).

Lemma normm_littleo x (f : X -> Y) : `| [o_(x \near x) (1 : R) of f x]| = 0.

Proof.

rewrite /cst /=; have [e /(_ (`|e x|/2) _)/nbhs_singleton /=] := littleo.
rewrite pmulr_lgt0 // [`|1|]normr1 mulr1 [leLHS]splitr gerDr pmulr_lle0 //.
by move=> /implyP; case : real_ltgtP; rewrite ?realE ?normrE //= lexx.
Qed.

Lemma littleo_lim0 (f : X -> Y) (h : _ -> Z) (x : X) :
  f @ x --> 0 -> [o_x f of h] x = 0.

Proof.

move/eqolim0P => ->; have [k /(_ _ [gt0 of 1 : R])/=] := littleo.
by move=> /nbhs_singleton; rewrite mul1r normm_littleo normr_le0 => /eqP.
Qed.

End littleo_lemmas.

Section diff_locally_converse_tentative.
Lemma diff_locally_converse_part1 (f : R -> R) (a b x : R) :
  f \o shift x = cst a + b *: idfun +o_ 0 id -> f x = a.

Proof.

rewrite funeqE => /(_ 0) /=; rewrite add0r => ->.
by rewrite -[LHS]/(_ 0 + _ 0 + _ 0) /cst [X in a + X]scaler0 littleo_lim0 ?addr0.
Qed.

End diff_locally_converse_tentative.

Definition derive (f : V -> W) a v :=
  lim ((fun h => h^-1 *: ((f \o shift a) (h *: v) - f a)) @ 0^').

Local Notation "''D_' v f" := (derive f ^~ v).
Local Notation "''D_' v f c" := (derive f c v).

Definition derivable (f : V -> W) a v :=
  cvg ((fun h => h^-1 *: ((f \o shift a) (h *: v) - f a)) @ 0^').

Class is_derive (a v : V) (f : V -> W) (df : W) := DeriveDef {
  ex_derive : derivable f a v;
  derive_val : 'D_v f a = df
}.

Lemma derivable_nbhs (f : V -> W) a v :
  derivable f a v ->
  (fun h => (f \o shift a) (h *: v)) = (cst (f a)) +
    (fun h => h *: ('D_v f a)) +o_ (nbhs 0) id.

Proof.

move=> df; apply/eqaddoP => _/posnumP[e].
rewrite -nbhs_nearE nbhs_simpl /= dnbhsE; split; last first.
  rewrite /at_point opprD -![(_ + _ : _ -> _) _]/(_ + _) scale0r add0r.
  by rewrite addrA subrr add0r normrN scale0r !normr0 mulr0.
have /eqolimP := df.
move=> /eqaddoP /(_ e%:num) /(_ [gt0 of e%:num]).
apply: filter_app; rewrite /= !near_simpl near_withinE; near=> h => hN0.
rewrite /= opprD -![(_ + _ : _ -> _) _]/(_ + _) -![(- _ : _ -> _) _]/(- _).
rewrite /cst /= [`|1|]normr1 mulr1 => dfv.
rewrite addrA -[X in X + _]scale1r -(@mulVf _ h) //.
rewrite mulrC -scalerA -scalerBr normrZ.
rewrite -ler_pdivlMl; last by rewrite normr_gt0.
by rewrite mulrCA mulVf ?mulr1; last by rewrite normr_eq0.
Unshelve. all: by end_near. Qed.

Lemma derivable_nbhsP (f : V -> W) a v :
  derivable f a v <->
  (fun h => (f \o shift a) (h *: v)) = (cst (f a)) +
    (fun h => h *: ('D_v f a)) +o_ (nbhs 0) id.

Proof.

split; first exact: derivable_nbhs.
move=> df; apply/cvg_ex; exists ('D_v f a).
apply/(@eqolimP _ _ _ (dnbhs_filter_on _))/eqaddoP => _/posnumP[e].
have /eqaddoP /(_ e%:num) /(_ [gt0 of e%:num]) := df.
rewrite /= !(near_simpl, near_withinE); apply: filter_app; near=> h.
rewrite /= opprD -![(_ + _ : _ -> _) _]/(_ + _) -![(- _ : _ -> _) _]/(- _).
rewrite /cst /= [`|1|]normr1 mulr1 addrA => dfv hN0.
rewrite -[X in _ - X]scale1r -(@mulVf _ h) //.
rewrite -scalerA -scalerBr normrZ normfV ler_pdivrMl ?normr_gt0 //.
by rewrite mulrC.
Unshelve. all: by end_near. Qed.

Lemma derivable_nbhsx (f : V -> W) a v :
  derivable f a v -> forall h, f (a + h *: v) = f a + h *: 'D_v f a
  +o_(h \near (nbhs 0)) h.

Proof.

move=> /derivable_nbhs; rewrite funeqE => df.
by apply: eqaddoEx => h; have /= := (df h); rewrite addrC => ->.
Qed.

Lemma derivable_nbhsxP (f : V -> W) a v :
  derivable f a v <-> forall h, f (a + h *: v) = f a + h *: 'D_v f a
  +o_(h \near nbhs 0) h.

Proof.

split; first exact: derivable_nbhsx.
move=> df; apply/derivable_nbhsP; apply/eqaddoE; rewrite funeqE => h.
by rewrite /= addrC df.
Qed.

End DifferentialR.

Notation "''D_' v f" := (derive f ^~ v).
Notation "''D_' v f c" := (derive f c v).
#[global] Hint Extern 0 (derivable _ _ _) => solve[apply: ex_derive] : core.

Section DifferentialR_numFieldType.
Context {R : numFieldType} {V W : normedModType R}.

Lemma deriveE (f : V -> W) (a v : V) :
  differentiable f a -> 'D_v f a = 'd f a v.

Proof.

rewrite /derive => /diff_locally -> /=; set k := 'o _.
evar (g : R -> W); rewrite [X in X @ _](_ : _ = g) /=; last first.
  rewrite funeqE=> h; rewrite !scalerDr scalerN /cst /=.
  by rewrite addrC !addrA addNr add0r linearZ /= scalerA /g.
apply: cvg_lim => //.
pose g1 : R -> W := fun h => (h^-1 * h) *: 'd f a v.
pose g2 : R -> W := fun h : R => h^-1 *: k (h *: v ).
rewrite (_ : g = g1 + g2) ?funeqE // -(addr0 (_ _ v)); apply: cvgD.
  rewrite -(scale1r (_ _ v)); apply: cvgZl => /= X [e e0].
  rewrite /ball_ /= => eX.
  apply/nbhs_ballP.
  by exists e => //= x _ x0; apply eX; rewrite mulVr // ?unitfE //= subrr normr0.
rewrite /g2.
have [->|v0] := eqVneq v 0.
  rewrite (_ : (fun _ => _) = cst 0); first exact: cvg_cst.
  by rewrite funeqE => ?; rewrite scaler0 /k littleo_lim0 // scaler0.
apply/cvgrPdist_lt => e e0.
rewrite nearE /=; apply/nbhs_ballP.
have /(littleoP [littleo of k]) /nbhs_ballP[i i0 Hi] : 0 < e / (2 * `|v|).
  by rewrite divr_gt0 // pmulr_rgt0 // normr_gt0.
exists (i / `|v|); first by rewrite /= divr_gt0 // normr_gt0.
move=> /= j; rewrite /ball /= /ball_ add0r normrN.
rewrite ltr_pdivlMr ?normr_gt0 // => jvi j0.
rewrite add0r normrN normrZ -ltr_pdivlMl ?normr_gt0 ?invr_neq0 //.
have /Hi/le_lt_trans -> // : ball 0 i (j *: v).
   by rewrite -ball_normE/= add0r normrN (le_lt_trans _ jvi) // normrZ.
rewrite -(mulrC e) -mulrA -ltr_pdivlMl // mulrA mulVr ?unitfE ?gt_eqF //.
rewrite normrV ?unitfE // div1r invrK ltr_pdivrMl; last first.
  by rewrite pmulr_rgt0 // normr_gt0.
rewrite normrZ mulrC -mulrA.
by rewrite ltr_pMl ?ltr1n // pmulr_rgt0 ?normm_gt0 // normr_gt0.
Qed.

End DifferentialR_numFieldType.

Section DifferentialR2.
Variable R : numFieldType.
Implicit Type (V : normedModType R).

Lemma derivemxE m n (f : 'rV[R]_m.+1 -> 'rV[R]_n.+1) (a v : 'rV[R]_m.+1) :
  differentiable f a -> 'D_ v f a = v *m jacobian f a.

Proof.

by move=> /deriveE->; rewrite /jacobian mul_rV_lin1. Qed.

Definition derive1 V (f : R -> V) (a : R) :=
   lim ((fun h => h^-1 *: (f (h + a) - f a)) @ 0^').

Local Notation "f ^` ()" := (derive1 f).

Lemma derive1E V (f : R -> V) a : f^`() a = 'D_1 f a.

Proof.

rewrite /derive1 /derive; set d := (fun _ : R => _); set d' := (fun _ : R => _).
by suff -> : d = d' by []; rewrite funeqE=> h; rewrite /d /d' /= [h%:A](mulr1).
Qed.

Lemma derive1E' V f a : differentiable (f : R -> V) a ->
  f^`() a = 'd f a 1.

Proof.

by move=> ?; rewrite derive1E deriveE. Qed.

Definition derive1n V n (f : R -> V) := iter n (@derive1 V) f.

Local Notation "f ^` ( n )" := (derive1n n f).

Lemma derive1n0 V (f : R -> V) : f^`(0) = f.

Proof.

by []. Qed.

Lemma derive1n1 V (f : R -> V) : f^`(1) = f^`().

Proof.

by []. Qed.

Lemma derive1nS V (f : R -> V) n : f^`(n.+1) = f^`(n)^`().

Proof.

by []. Qed.

Lemma derive1Sn V (f : R -> V) n : f^`(n.+1) = f^`()^`(n).

Proof.

exact: iterSr. Qed.

End DifferentialR2.

Notation "f ^` ()" := (derive1 f).
Notation "f ^` ( n )" := (derive1n n f).

Section DifferentialR3.
Variable R : numFieldType.

Fact dcst (V W : normedModType R) (a : W) (x : V) : continuous (0 : V -> W) /\
  cst a \o shift x = cst (cst a x) + \0 +o_ 0 id.

Proof.

split; first exact: cst_continuous.
apply/eqaddoE; rewrite addr0 funeqE => ? /=; rewrite -[LHS]addr0; congr (_ + _).
by rewrite littleoE; last exact: littleo0_subproof.
Qed.

Variables (V W : normedModType R).

Fact dadd (f g : V -> W) x :
  differentiable f x -> differentiable g x ->
  continuous ('d f x \+ 'd g x) /\
  (f + g) \o shift x = cst ((f + g) x) + ('d f x \+ 'd g x) +o_ 0 id.

Proof.

move=> df dg; split => [?|]; do ?exact: continuousD.
apply/(@eqaddoE R); rewrite funeqE => y /=; rewrite -[(f + g) _]/(_ + _).
by rewrite ![_ (_ + x)]diff_locallyx// addrACA addox addrACA.
Qed.

Fact dopp (f : V -> W) x :
  differentiable f x -> continuous (- ('d f x : V -> W)) /\
  (- f) \o shift x = cst (- f x) \- 'd f x +o_ 0 id.

Proof.

move=> df; split; first by move=> ?; apply: continuousN.
apply/eqaddoE; rewrite funeqE => y /=.
by rewrite -[(- f) _]/(- (_ _)) diff_locallyx// !opprD oppox.
Qed.

Lemma is_diff_eq (V' W' : normedModType R) (f f' g : V' -> W') (x : V') :
  is_diff x f f' -> f' = g -> is_diff x f g.

Proof.

by move=> ? <-. Qed.

Fact dscale (f : V -> W) k x :
  differentiable f x -> continuous (k \*: 'd f x) /\
  (k *: f) \o shift x = cst ((k *: f) x) + k \*: 'd f x +o_ 0 id.

Proof.

move=> df; split; first by move=> ?; apply: continuousZr.
apply/eqaddoE; rewrite funeqE => y /=.
by rewrite -[(k *: f) _]/(_ *: _) diff_locallyx // !scalerDr scaleox.
Qed.

Fact dscalel (k : V -> R) (f : W) x :
  differentiable k x ->
  continuous (fun z : V => 'd k x z *: f) /\
  (fun z => k z *: f) \o shift x =
    cst (k x *: f) + (fun z => 'd k x z *: f) +o_ 0 id.

Proof.

move=> df; split.
  move=> ?; exact/continuousZl/diff_continuous.
apply/eqaddoE; rewrite funeqE => y /=.
by rewrite diff_locallyx //= !scalerDl scaleolx.
Qed.

Fact dlin (V' W' : normedModType R) (f : {linear V' -> W'}) x :
  continuous f -> f \o shift x = cst (f x) + f +o_ 0 id.

Proof.

move=> df; apply: eqaddoE; rewrite funeqE => y /=.
rewrite linearD addrC -[LHS]addr0; congr (_ + _).
by rewrite littleoE; last exact: littleo0_subproof. (*fixme*)
Qed.

Lemma compoO_eqo (U V' W' : normedModType R) (f : U -> V')
  (g : V' -> W') :
  [o_ 0 id of g] \o [O_ 0 id of f] =o_ 0 id.

Proof.

apply/eqoP => _ /posnumP[e].
have /bigO_exP [_ /posnumP[k]] := bigOP [bigO of [O_ 0 id of f]].
have := littleoP [littleo of [o_ 0 id of g]].
move=> /(_ (e%:num / k%:num)) /(_ _) /nbhs_ballP [//|_ /posnumP[d] hd].
apply: filter_app; near=> x => leOxkx; apply: le_trans (hd _ _) _; last first.
  rewrite -ler_pdivlMl //; apply: le_trans leOxkx _.
  by rewrite invf_div mulrA -[_ / _ * _]mulrA mulVf // mulr1.
by rewrite -ball_normE /= distrC subr0 (le_lt_trans leOxkx) // -ltr_pdivlMl.
Unshelve. all: by end_near. Qed.

Lemma compoO_eqox (U V' W' : normedModType R) (f : U -> V')
  (g : V' -> W') :
  forall x : U, [o_ 0 id of g] ([O_ 0 id of f] x) =o_(x \near 0) x.

Proof.

by move=> x; rewrite -[LHS]/((_ \o _) x) compoO_eqo. Qed.

Lemma compOo_eqo (U V' W' : normedModType R) (f : U -> V') (g : V' -> W') :
  [O_ 0 id of g] \o [o_ 0 id of f] =o_ 0 id.

Proof.

apply/eqoP => _ /posnumP[e].
have /bigO_exP [_ /posnumP[k]] := bigOP [bigO of [O_ 0 id of g]].
move=> /nbhs_ballP [_ /posnumP[d] hd].
have ekgt0 : e%:num / k%:num > 0 by [].
have /(_ _ ekgt0) := littleoP [littleo of [o_ 0 id of f]].
apply: filter_app; near=> x => leoxekx; apply: le_trans (hd _ _) _; last first.
  by rewrite -ler_pdivlMl // mulrA [_^-1 * _]mulrC.
by rewrite -ball_normE /= distrC subr0 (le_lt_trans leoxekx)// -ltr_pdivlMl //.
Unshelve. all: by end_near. Qed.

End DifferentialR3.

Section DifferentialR3_numFieldType.
Variable R : numFieldType.

Lemma littleo_linear0 (V W : normedModType R) (f : {linear V -> W}) :
  (f : V -> W) =o_ 0 id -> f = cst 0 :> (V -> W).

Proof.

move/eqoP => oid.
rewrite funeqE => x; apply/eqP; have [|xn0] := real_le0P (normr_real x).
  by rewrite normr_le0 => /eqP ->; rewrite linear0.
rewrite -normr_le0 -(mul0r `|x|) -ler_pdivrMr //.
apply/ler_gtP => _ /posnumP[e]; rewrite ler_pdivrMr //.
have /oid /nbhs_ballP [_ /posnumP[d] dfe] := !! gt0 e.
set k := ((d%:num / 2) / (PosNum xn0)%:num)^-1.
rewrite -{1}(@scalerKV _ _ k _ x) /k // linearZZ normrZ.
rewrite -ler_pdivlMl; last by rewrite gtr0_norm.
rewrite mulrCA (@le_trans _ _ (e%:num * `|k^-1 *: x|)) //; last first.
  by rewrite ler_pM // normrZ normfV.
apply: dfe; rewrite -ball_normE /= sub0r normrN normrZ.
rewrite invrK -ltr_pdivlMr // ger0_norm // ltr_pdivrMr //.
by rewrite -mulrA mulVf ?lt0r_neq0 // mulr1 [ltRHS]splitr ltrDl.
Qed.

Lemma diff_unique (V W : normedModType R) (f : V -> W)
  (df : {linear V -> W}) x :
  continuous df -> f \o shift x = cst (f x) + df +o_ 0 id ->
  'd f x = df :> (V -> W).

Proof.

move=> dfc dxf; apply/subr0_eq; rewrite -[LHS]/(_ \- _).
apply/littleo_linear0/eqoP/eq_some_oP => /=; rewrite funeqE => y /=.
have hdf h : (f \o shift x = cst (f x) + h +o_ 0 id) ->
    h = f \o shift x - cst (f x) +o_ 0 id.
  move=> hdf; apply: eqaddoE.
  rewrite hdf addrAC -!addrA addrC !addrA subrK.
  rewrite -[LHS]addr0 -addrA; congr (_ + _).
  by apply/eqP; rewrite eq_sym addrC addr_eq0 oppo.
rewrite (hdf _ dxf).
suff /diff_locally /hdf -> : differentiable f x.
  by rewrite opprD addrCA -(addrA (_ - _)) addKr oppox addox.
apply/diffP => /=.
apply: (@getPex _ (fun (df : {linear V -> W}) => continuous df /\
  forall y, f y = f (lim (nbhs x)) + df (y - lim (nbhs x))
                  +o_(y \near x) (y - lim (nbhs x)))).
exists df; split=> //; apply: eqaddoEx => z.
rewrite (hdf _ dxf) !addrA lim_id // /(_ \o _) /= subrK [f _ + _]addrC addrK.
rewrite -addrA -[LHS]addr0; congr (_ + _).
apply/eqP; rewrite eq_sym addrC addr_eq0 oppox; apply/eqP.
by rewrite littleo_center0 (comp_centerK x id) -[- _ in RHS](comp_centerK x).
Qed.

Lemma diff_cst (V W : normedModType R) a x : ('d (cst a) x : V -> W) = 0.

Proof.

by apply/diff_unique; have [] := dcst a x. Qed.

Variables (V W : normedModType R).

Lemma differentiable_cst (W' : normedModType R) (a : W') (x : V) :
  differentiable (cst a) x.

Proof.

by apply/diff_locallyP; rewrite diff_cst; have := dcst a x. Qed.

Global Instance is_diff_cst (a : W) (x : V) : is_diff x (cst a) 0.

Proof.

exact: DiffDef (differentiable_cst _ _) (diff_cst _ _). Qed.

Lemma diffD (f g : V -> W) x :
  differentiable f x -> differentiable g x ->
  'd (f + g) x = 'd f x \+ 'd g x :> (V -> W).

Proof.

by move=> df dg; apply/diff_unique; have [] := dadd df dg. Qed.

Lemma differentiableD (f g : V -> W) x :
  differentiable f x -> differentiable g x -> differentiable (f + g) x.

Proof.

by move=> df dg; apply/diff_locallyP; rewrite diffD //; have := dadd df dg.
Qed.

Global Instance is_diffD (f g df dg : V -> W) x :
  is_diff x f df -> is_diff x g dg -> is_diff x (f + g) (df + dg).

Proof.

move=> dfx dgx; apply: DiffDef; first exact: differentiableD.
by rewrite diffD // !diff_val.
Qed.

Lemma differentiable_sum n (f : 'I_n -> V -> W) (x : V) :
  (forall i, differentiable (f i) x) -> differentiable (\sum_(i < n) f i) x.

Proof.

by elim/big_ind : _ => // ? ? g h ?; apply: differentiableD; [exact:g|exact:h].
Qed.

Lemma diffN (f : V -> W) x :
  differentiable f x -> 'd (- f) x = - ('d f x : V -> W) :> (V -> W).

Proof.

move=> df; rewrite -[RHS]/(@GRing.opp _ \o _); apply/diff_unique;
by have [] := dopp df.
Qed.

Lemma differentiableN (f : V -> W) x :
  differentiable f x -> differentiable (- f) x.

Proof.

by move=> df; apply/diff_locallyP; rewrite diffN //; have := dopp df.
Qed.

Global Instance is_diffN (f df : V -> W) x :
  is_diff x f df -> is_diff x (- f) (- df).

Proof.

move=> dfx; apply: DiffDef; first exact: differentiableN.
by rewrite diffN // diff_val.
Qed.

Global Instance is_diffB (f g df dg : V -> W) x :
  is_diff x f df -> is_diff x g dg -> is_diff x (f - g) (df - dg).

Proof.

by move=> dfx dgx; apply: is_diff_eq. Qed.

Lemma diffB (f g : V -> W) x :
  differentiable f x -> differentiable g x ->
  'd (f - g) x = 'd f x \- 'd g x :> (V -> W).

Proof.

by move=> /differentiableP df /differentiableP dg; rewrite diff_val. Qed.

Lemma differentiableB (f g : V -> W) x :
  differentiable f x -> differentiable g x -> differentiable (f \- g) x.

Proof.

by move=> /differentiableP df /differentiableP dg. Qed.

Lemma diffZ (f : V -> W) k x :
  differentiable f x -> 'd (k *: f) x = k \*: 'd f x :> (V -> W).

Proof.

by move=> df; apply/diff_unique; have [] := dscale k df. Qed.

Lemma differentiableZ (f : V -> W) k x :
  differentiable f x -> differentiable (k *: f) x.

Proof.

by move=> df; apply/diff_locallyP; rewrite diffZ //; have := dscale k df.
Qed.

Global Instance is_diffZ (f df : V -> W) k x :
  is_diff x f df -> is_diff x (k *: f) (k *: df).

Proof.

move=> dfx; apply: DiffDef; first exact: differentiableZ.
by rewrite diffZ // diff_val.
Qed.

Lemma diffZl (k : V -> R) (f : W) x : differentiable k x ->
  'd (fun z => k z *: f) x = (fun z => 'd k x z *: f) :> (_ -> _).

Proof.

move=> df; set g := RHS; have glin : linear g.
  by move=> a u v; rewrite /g linearP /= scalerDl -scalerA.
pose glM := GRing.isLinear.Build _ _ _ _ _ glin.
pose gL : {linear _ -> _} := HB.pack g glM.
by apply:(@diff_unique _ _ _ gL); have [] := dscalel f df.
Qed.

Lemma differentiableZl (k : V -> R) (f : W) x :
  differentiable k x -> differentiable (fun z => k z *: f) x.

Proof.

by move=> df; apply/diff_locallyP; rewrite diffZl //; have [] := dscalel f df.
Qed.

Lemma diff_lin (V' W' : normedModType R) (f : {linear V' -> W'}) x :
  continuous f -> 'd f x = f :> (V' -> W').

Proof.

by move=> fcont; apply/diff_unique => //; apply: dlin. Qed.

Lemma linear_differentiable (V' W' : normedModType R) (f : {linear V' -> W'})
  x : continuous f -> differentiable f x.

Proof.

by move=> fcont; apply/diff_locallyP; rewrite diff_lin //; have := dlin x fcont.
Qed.

Global Instance is_diff_id (x : V) : is_diff x id id.

Proof.

apply: DiffDef.
  by apply: (@linear_differentiable _ _ idfun) => ? //.
by rewrite (@diff_lin _ _ idfun) // => ? //.
Qed.

Global Instance is_diff_scaler (k : R) (x : V) : is_diff x ( *:%R k) ( *:%R k).

Proof.

apply: DiffDef; first exact/linear_differentiable/scaler_continuous.
by rewrite diff_lin //; apply: scaler_continuous.
Qed.

Global Instance is_diff_scalel (k : R) (x : V) :
  is_diff k ( *:%R ^~ x) ( *:%R ^~ x).

Proof.

have sx_lin : linear ( *:%R ^~ x : [the lmodType R of R : Type] -> _).
  by move=> u y z; rewrite scalerDl scalerA.
pose sxlM := GRing.isLinear.Build _ _ _ _ _ sx_lin.
pose sxL : {linear _ -> _} := HB.pack ( *:%R ^~ x) sxlM.
have -> : *:%R ^~ x = sxL by rewrite funeqE.
apply: DiffDef; first exact/linear_differentiable/scalel_continuous.
by rewrite diff_lin //; apply: scalel_continuous.
Qed.

Lemma differentiable_coord m n (M : 'M[R]_(m.+1, n.+1)) i j :
  differentiable (fun N : 'M[R]_(m.+1, n.+1) => N i j : R ) M.

Proof.

have @f : {linear 'M[R]_(m.+1, n.+1) -> R}.
  by exists (fun N : 'M[R]_(_, _) => N i j); do 2![eexists]; do ?[constructor];
     rewrite ?mxE// => ? *; rewrite ?mxE//; move=> ?; rewrite !mxE.
rewrite (_ : (fun _ => _) = f) //; exact/linear_differentiable/coord_continuous.
Qed.

Lemma linear_lipschitz (V' W' : normedModType R) (f : {linear V' -> W'}) :
  continuous f -> exists2 k, k > 0 & forall x, `|f x| <= k * `|x|.

Proof.

move=> /(_ 0); rewrite /continuous_at linear0 => /(_ _ (nbhsx_ballx _ _ ltr01)).
move=> /nbhs_ballP [_ /posnumP[e] he]; exists (2 / e%:num) => // x.
have [|xn0] := real_le0P (normr_real x).
  by rewrite normr_le0 => /eqP->; rewrite linear0 !normr0 mulr0.
set k := 2 / e%:num * (PosNum xn0)%:num.
have kn0 : k != 0 by rewrite /k.
have abskgt0 : `|k| > 0 by rewrite normr_gt0.
rewrite -[x in leLHS](scalerKV kn0) linearZZ normrZ -ler_pdivlMl //.
suff /he : ball 0 e%:num (k^-1 *: x).
  rewrite -ball_normE /= distrC subr0 => /ltW /le_trans; apply.
  by rewrite ger0_norm /k // mulVf.
rewrite -ball_normE /= distrC subr0 normrZ.
rewrite normfV ger0_norm /k // invrM ?unitfE // mulrAC mulVf //.
by rewrite invf_div mul1r [ltRHS]splitr; apply: ltr_pwDr.
Qed.

Lemma linear_eqO (V' W' : normedModType R) (f : {linear V' -> W'}) :
  continuous f -> (f : V' -> W') =O_ 0 id.

Proof.

move=> /linear_lipschitz [k kgt0 flip]; apply/eqO_exP; exists k => //.
exact: filterE.
Qed.

Lemma diff_eqO (V' W' : normedModType R) (x : filter_on V') (f : V' -> W') :
  differentiable f x -> ('d f x : V' -> W') =O_ 0 id.

Proof.

by move=> /diff_continuous /linear_eqO; apply. Qed.

Lemma compOo_eqox (U V' W' : normedModType R) (f : U -> V') (g : V' -> W') x :
  [O_ 0 id of g] ([o_ 0 id of f] x) =o_(x \near 0) x.

Proof.

by rewrite -[LHS]/((_ \o _) x) compOo_eqo. Qed.

Fact dcomp (U V' W' : normedModType R) (f : U -> V') (g : V' -> W') x :
  differentiable f x -> differentiable g (f x) ->
  continuous ('d g (f x) \o 'd f x) /\ forall y,
  g (f (y + x)) = g (f x) + ('d g (f x) \o 'd f x) y +o_(y \near 0) y.

Proof.

move=> df dg; split; first by move=> ?; apply: continuous_comp.
apply: eqaddoEx => y; rewrite diff_locallyx// -addrA diff_locallyxC// linearD.
rewrite addrA -[LHS]addrA; congr (_ + _ + _).
rewrite diff_eqO // ['d f x : _ -> _]diff_eqO //.
by rewrite {2}eqoO addOx compOo_eqox compoO_eqox addox.
Qed.

Lemma diff_comp (U V' W' : normedModType R) (f : U -> V') (g : V' -> W') x :
  differentiable f x -> differentiable g (f x) ->
  'd (g \o f) x = 'd g (f x) \o 'd f x :> (U -> W').

Proof.

by move=> df dg; apply/diff_unique; have [? /funext] := dcomp df dg. Qed.

Lemma differentiable_comp (U V' W' : normedModType R) (f : U -> V')
  (g : V' -> W') x : differentiable f x -> differentiable g (f x) ->
  differentiable (g \o f) x.

Proof.

move=> df dg; apply/diff_locallyP; rewrite diff_comp //;
by have [? /funext]:= dcomp df dg.
Qed.

Global Instance is_diff_comp (U V' W' : normedModType R) (f df : U -> V')
  (g dg : V' -> W') x : is_diff x f df -> is_diff (f x) g dg ->
  is_diff x (g \o f) (dg \o df) | 99.

Proof.

move=> dfx dgfx; apply: DiffDef; first exact: differentiable_comp.
by rewrite diff_comp // !diff_val.
Qed.

Lemma bilinear_schwarz (U V' W' : normedModType R)
  (f : {bilinear U -> V' -> W'}) : continuous (fun p => f p.1 p.2) ->
  exists2 k, k > 0 & forall u v, `|f u v| <= k * `|u| * `|v|.

Proof.

move=> /(_ 0); rewrite /continuous_at linear0r => /(_ _ (nbhsx_ballx _ _ ltr01)).
move=> /nbhs_ballP [_ /posnumP[e] he]; exists ((2 / e%:num) ^+2) => // u v.
have [|un0] := real_le0P (normr_real u).
  by rewrite normr_le0 => /eqP->; rewrite linear0l !normr0 mulr0 mul0r.
have [|vn0] := real_le0P (normr_real v).
  by rewrite normr_le0 => /eqP->; rewrite linear0r !normr0 mulr0.
rewrite -[`|u|]/((PosNum un0)%:num) -[`|v|]/((PosNum vn0)%:num).
set ku := 2 / e%:num * (PosNum un0)%:num.
set kv := 2 / e%:num * (PosNum vn0)%:num.
rewrite -[X in f X](@scalerKV _ _ ku) /ku // linearZl_LR normrZ.
rewrite gtr0_norm // -ler_pdivlMl //.
rewrite -[X in f _ X](@scalerKV _ _ kv) /kv // linearZr_LR normrZ.
rewrite gtr0_norm // -ler_pdivlMl //.
suff /he : ball 0 e%:num (ku^-1 *: u, kv^-1 *: v).
  rewrite -ball_normE /= distrC subr0 => /ltW /le_trans; apply.
  rewrite ler_pdivlMl 1?pmulr_lgt0// mulr1 ler_pdivlMl 1?pmulr_lgt0//.
  by rewrite mulrA [ku * _]mulrAC expr2.
rewrite -ball_normE /= distrC subr0.
have -> : (ku^-1 *: u, kv^-1 *: v) =
  (e%:num / 2) *: ((PosNum un0)%:num ^-1 *: u, (PosNum vn0)%:num ^-1 *: v).
  rewrite invrM ?unitfE // [kv ^-1]invrM ?unitfE //.
  rewrite mulrC -[_ *: u]scalerA [X in X *: v]mulrC -[_ *: v]scalerA.
  by rewrite invf_div.
rewrite normrZ ger0_norm // -mulrA gtr_pMr // ltr_pdivrMl // mulr1.
by rewrite prod_normE/= !normrZ !normfV !normr_id !mulVf ?gt_eqF// maxxx ltr1n.
Qed.

Lemma bilinear_eqo (U V' W' : normedModType R) (f : {bilinear U -> V' -> W'}) :
  continuous (fun p => f p.1 p.2) -> (fun p => f p.1 p.2) =o_ 0 id.

Proof.

move=> fc; have [_ /posnumP[k] fschwarz] := bilinear_schwarz fc.
apply/eqoP=> _ /posnumP[e]; near=> x; rewrite (le_trans (fschwarz _ _))//.
rewrite ler_pM ?pmulr_rge0 //; last by rewrite num_le_max /= lexx orbT.
rewrite -ler_pdivlMl //.
suff : `|x| <= k%:num ^-1 * e%:num by apply: le_trans; rewrite num_le_max /= lexx.
near: x; rewrite !near_simpl; apply/nbhs_le_nbhs_norm.
by exists (k%:num ^-1 * e%:num) => //= ? /=; rewrite /= distrC subr0 => /ltW.
Unshelve. all: by end_near. Qed.

Fact dbilin (U V' W' : normedModType R) (f : {bilinear U -> V' -> W'}) p :
  continuous (fun p => f p.1 p.2) ->
  continuous (fun q => (f p.1 q.2 + f q.1 p.2)) /\
  (fun q => f q.1 q.2) \o shift p = cst (f p.1 p.2) +
    (fun q => f p.1 q.2 + f q.1 p.2) +o_ 0 id.

Proof.

move=> fc; split=> [q|].
  by apply: (@continuousD _ _ _ (fun q => f p.1 q.2) (fun q => f q.1 p.2));
    move=> A /(fc (_.1, _.2)) /= /nbhs_ballP [_ /posnumP[e] fpqe_A];
    apply/nbhs_ballP; exists e%:num => //= r [? ?]; exact: (fpqe_A (_.1, _.2)).
apply/eqaddoE; rewrite funeqE => q /=.
rewrite linearDl !linearDr addrA addrC.
rewrite -[f q.1 _ + _ + _]addrA [f q.1 _ + _]addrC addrA [f q.1 _ + _]addrC.
by congr (_ + _); rewrite -[LHS]/((fun p => f p.1 p.2) q) bilinear_eqo.
Qed.

Lemma diff_bilin (U V' W' : normedModType R) (f : {bilinear U -> V' -> W'}) p :
  continuous (fun p => f p.1 p.2) -> 'd (fun q => f q.1 q.2) p =
  (fun q => f p.1 q.2 + f q.1 p.2) :> (U * V' -> W').

Proof.

pose d q := f p.1 q.2 + f q.1 p.2.
move=> fc; have lind : linear d.
  by move=> ? ? ?; rewrite /d linearPr linearPl scalerDr addrACA.
pose dlM := GRing.isLinear.Build _ _ _ _ _ lind.
pose dL : {linear _ -> _} := HB.pack d dlM.
rewrite -/d -[d]/(dL : _ -> _).
by apply/diff_unique; have [] := dbilin p fc.
Qed.

Lemma differentiable_bilin (U V' W' : normedModType R)
  (f : {bilinear U -> V' -> W'}) p :
  continuous (fun p => f p.1 p.2) -> differentiable (fun p => f p.1 p.2) p.

Proof.

by move=> fc; apply/diff_locallyP; rewrite diff_bilin //; apply: dbilin p fc.
Qed.

Lemma mulr_is_bilinear :
  bilinear_for
    (GRing.Scale.Law.clone _ _ *:%R _) (GRing.Scale.Law.clone _ _ *:%R _)
    (@GRing.mul R).

Proof.

split=> [u'|u] a x y /=.
- by rewrite mulrDl scalerAl.
- by rewrite mulrDr scalerAr.
Qed.

HB.instance Definition _ := bilinear_isBilinear.Build R R R R _ _ (@GRing.mul R)
  mulr_is_bilinear.

Global Instance is_diff_mulr (p : R * R) :
  is_diff p (fun q => q.1 * q.2) (fun q => p.1 * q.2 + q.1 * p.2).

Proof.

apply: DiffDef; last by rewrite diff_bilin // => ?; apply: mul_continuous.
by apply: differentiable_bilin =>?; apply: mul_continuous.
Qed.

Lemma eqo_pair (U V' W' : normedModType R) (F : filter_on U)
  (f : U -> V') (g : U -> W') :
  (fun t => ([o_F id of f] t, [o_F id of g] t)) =o_F id.

Proof.

apply/eqoP => _/posnumP[e]; near=> x; rewrite num_ge_max /=.
by apply/andP; split; near: x; apply: littleoP.
Unshelve. all: by end_near. Qed.

Fact dpair (U V' W' : normedModType R) (f : U -> V') (g : U -> W') x :
  differentiable f x -> differentiable g x ->
  continuous (fun y => ('d f x y, 'd g x y)) /\
  (fun y => (f y, g y)) \o shift x = cst (f x, g x) +
  (fun y => ('d f x y, 'd g x y)) +o_ 0 id.

Proof.

move=> df dg; split=> [?|]; first by apply: cvg_pair; apply: diff_continuous.
apply/eqaddoE; rewrite funeqE => y /=.
rewrite ![_ (_ + x)]diff_locallyx//.
(* fixme *)
have -> : forall h e, (f x + 'd f x y + [o_ 0 id of h] y,
  g x + 'd g x y + [o_ 0 id of e] y) =
  (f x, g x) + ('d f x y, 'd g x y) +
  ([o_ 0 id of h] y, [o_ 0id of e] y) by [].
by congr (_ + _); rewrite -[LHS]/((fun y => (_ y, _ y)) y) eqo_pair.
Qed.

Lemma diff_pair (U V' W' : normedModType R) (f : U -> V') (g : U -> W') x :
  differentiable f x -> differentiable g x -> 'd (fun y => (f y, g y)) x =
  (fun y => ('d f x y, 'd g x y)) :> (U -> V' * W').

Proof.

move=> df dg.
pose d y := ('d f x y, 'd g x y).
have lin_pair : linear d by move=> ???; rewrite /d !linearPZ.
pose pairlM := GRing.isLinear.Build _ _ _ _ _ lin_pair.
pose pairL : {linear _ -> _} := HB.pack d pairlM.
rewrite -/d -[d]/(pairL : _ -> _).
by apply: diff_unique; have [] := dpair df dg.
Qed.

Lemma differentiable_pair (U V' W' : normedModType R) (f : U -> V')
  (g : U -> W') x : differentiable f x -> differentiable g x ->
  differentiable (fun y => (f y, g y)) x.

Proof.

by move=> df dg; apply/diff_locallyP; rewrite diff_pair //; apply: dpair.
Qed.

Global Instance is_diff_pair (U V' W' : normedModType R) (f df : U -> V')
  (g dg : U -> W') x : is_diff x f df -> is_diff x g dg ->
   is_diff x (fun y => (f y, g y)) (fun y => (df y, dg y)).

Proof.

move=> dfx dgx; apply: DiffDef; first exact: differentiable_pair.
by rewrite diff_pair // !diff_val.
Qed.

Global Instance is_diffM (f g df dg : V -> R) x :
  is_diff x f df -> is_diff x g dg -> is_diff x (f * g) (f x *: dg + g x *: df).

Proof.

move=> dfx dgx.
have -> : f * g = (fun p => p.1 * p.2) \o (fun y => (f y, g y)) by [].
apply: is_diff_eq.
by rewrite funeqE => ?; rewrite /= [_ * g _]mulrC.
Qed.

Lemma diffM (f g : V -> R) x :
  differentiable f x -> differentiable g x ->
  'd (f * g) x = f x \*: 'd g x + g x \*: 'd f x :> (V -> R).

Proof.

by move=> /differentiableP df /differentiableP dg; rewrite diff_val. Qed.

Lemma differentiableM (f g : V -> R) x :
  differentiable f x -> differentiable g x -> differentiable (f * g) x.

Proof.

by move=> /differentiableP df /differentiableP dg. Qed.

Fact dinv (x : R) :
  x != 0 -> continuous (fun h : R => - x ^- 2 *: h) /\
  (fun x => x^-1)%R \o shift x = cst (x^-1)%R +
  (fun h : R => - x ^- 2 *: h) +o_ 0 id.

Proof.

move=> xn0; suff: continuous (fun h : R => - (1 / x) ^+ 2 *: h) /\
  (fun x => 1 / x ) \o shift x = cst (1 / x) +
  (fun h : R => - (1 / x) ^+ 2 *: h) +o_ 0 id.
  rewrite !mul1r !GRing.exprVn.
  rewrite (_ : (fun x => x^-1) = (fun x => 1 / x ))//.
  by rewrite funeqE => y; rewrite mul1r.
split; first by move=> ?; apply: continuousZr.
apply/eqaddoP => _ /posnumP[e]; near=> h.
rewrite -[(_ + _ : R -> R) h]/(_ + _) -[(- _ : R -> R) h]/(- _) /=.
rewrite opprD scaleNr opprK /cst /=.
rewrite -[- _]mulr1 -[X in - _ * X](mulfVK xn0) mulrA mulNr -expr2 mulNr.
rewrite [- _ + _]addrC -mulrBr.
rewrite -[X in X + _]mulr1 -[X in 1 / _ * X](@mulfVK _ (x ^+ 2)); last first.
  by rewrite sqrf_eq0.
rewrite mulrA mulf_div mulr1.
have hDx_neq0 : h + x != 0.
  near: h; rewrite !nbhs_simpl; apply/nbhs_normP.
  exists `|x|; first by rewrite /= normr_gt0.
  move=> h /=; rewrite /= distrC subr0 -subr_gt0 => lthx.
  rewrite -(normr_gt0 (h + x)) addrC -[h]opprK.
  apply: lt_le_trans (ler_dist_dist _ _).
  by rewrite ger0_norm normrN //; apply: ltW.
rewrite addrC -[X in X * _]mulr1 -{2}[1](@mulfVK _ (h + x)) //.
rewrite mulrA expr_div_n expr1n mulf_div mulr1 [_ ^+ 2 * _]mulrC -mulrA.
rewrite -mulrDr mulrBr [1 / _ * _]mulrC normrM.
rewrite mulrDl mulrDl opprD addrACA addrA [x * _]mulrC expr2 2!subrK.
rewrite div1r normfV [X in _ / X]normrM invfM [X in _ * X]mulrC.
rewrite mulrA mulrAC ler_pdivrMr ?normr_gt0 ?mulf_neq0 //.
rewrite mulrAC ler_pdivrMr ?normr_gt0 //.
have : `|h * h| <= `|x / 2| * (e%:num * `|x * x| * `|h|).
  rewrite !mulrA; near: h; exists (`|x / 2| * e%:num * `|x * x|).
    by rewrite /= !pmulr_rgt0 // normr_gt0 mulf_neq0.
  by move=> h /ltW; rewrite distrC subr0 [`|h * _|]normrM => /ler_pM; apply.
move=> /le_trans -> //; rewrite [leLHS]mulrC ler_pM ?mulr_ge0 //.
near: h; exists (`|x| / 2); first by rewrite /= divr_gt0 ?normr_gt0.
move=> h; rewrite /= distrC subr0 => lthhx; rewrite addrC -[h]opprK.
apply: le_trans (@ler_dist_dist _ R _ _).
rewrite normrN [leRHS]ger0_norm; last first.
  rewrite subr_ge0; apply: ltW; apply: lt_le_trans lthhx _.
  by rewrite ler_pdivrMr // -{1}(mulr1 `|x|) ler_pM // ler1n.
rewrite lerBrDr -lerBrDl (splitr `|x|).
by rewrite normrM normfV (@ger0_norm _ 2) // -addrA subrr addr0; apply: ltW.
Unshelve. all: by end_near. Qed.

Lemma diff_Rinv (x : R) : x != 0 ->
  'd GRing.inv x = (fun h : R => - x ^- 2 *: h) :> (R -> R).

Proof.

move=> xn0; have -> : (fun h : R => - x ^- 2 *: h) = ( *:%R (- x ^- 2)) by [].
by apply: diff_unique; have [] := dinv xn0.
Qed.

Lemma differentiable_Rinv (x : R) : x != 0 ->
  differentiable (GRing.inv : R -> R) x.

Proof.

by move=> xn0; apply/diff_locallyP; rewrite diff_Rinv //; apply: dinv.
Qed.

Lemma diffV (f : V -> R) x : differentiable f x -> f x != 0 ->
  'd (fun y => (f y)^-1) x = - (f x) ^- 2 \*: 'd f x :> (V -> R).

Proof.

move=> df fxn0.
by rewrite [LHS](diff_comp df (differentiable_Rinv fxn0)) diff_Rinv.
Qed.

Lemma differentiableV (f : V -> R) x :
  differentiable f x -> f x != 0 -> differentiable (fun y => (f y)^-1) x.

Proof.

by move=> df fxn0; apply: differentiable_comp _ (differentiable_Rinv fxn0).
Qed.

Global Instance is_diffX (f df : V -> R) n x :
  is_diff x f df -> is_diff x (f ^+ n.+1) (n.+1%:R * f x ^+ n *: df).

Proof.

move=> dfx; elim: n => [|n ihn]; first by rewrite expr1 expr0 mulr1 scale1r.
rewrite exprS; apply: is_diff_eq.
rewrite scalerA mulrCA -exprS -scalerDl.
by rewrite [in LHS]mulr_natl exprfctE -mulrSr mulr_natl.
Qed.

Lemma differentiableX (f : V -> R) n x :
  differentiable f x -> differentiable (f ^+ n.+1) x.

Proof.

by move=> /differentiableP. Qed.

Lemma diffX (f : V -> R) n x :
  differentiable f x ->
  'd (f ^+ n.+1) x = n.+1%:R * f x ^+ n \*: 'd f x :> (V -> R).

Proof.

by move=> /differentiableP df; rewrite diff_val. Qed.

End DifferentialR3_numFieldType.

Section DeriveRU.
Variables (R : numFieldType) (U : normedModType R).
Implicit Types f : R -> U.

Let der1 f x : derivable f x 1 ->
  f \o shift x = cst (f x) + ( *:%R^~ (f^`() x)) +o_ 0 id.

Proof.

move=> df; apply/eqaddoE; have /derivable_nbhsP := df.
have -> : (fun h => (f \o shift x) h%:A) = f \o shift x.
  by rewrite funeqE=> ?; rewrite [_%:A]mulr1.
by rewrite derive1E =>->.
Qed.

Lemma deriv1E f x : derivable f x 1 -> 'd f x = ( *:%R^~ (f^`() x)) :> (R -> U).

Proof.

pose d (h : R) := h *: f^`() x.
move=> df; have lin_scal : linear d by move=> ???; rewrite /d scalerDl scalerA.
pose scallM := GRing.isLinear.Build _ _ _ _ _ lin_scal.
pose scalL : {linear _ -> _} := HB.pack d scallM.
rewrite -/d -[d]/(scalL : _ -> _).
by apply: diff_unique; [apply: scalel_continuous|apply: der1].
Qed.

Lemma diff1E f x :
  differentiable f x -> 'd f x = (fun h => h *: f^`() x) :> (R -> U).

Proof.

pose d (h : R) := h *: 'd f x 1.
move=> df; have lin_scal : linear d by move=> ? ? ?; rewrite /d scalerDl scalerA.
pose scallM := GRing.isLinear.Build _ _ _ _ _ lin_scal.
pose scalL : {linear _ -> _} := HB.pack d scallM.
have -> : (fun h => h *: f^`() x) = scalL by rewrite derive1E'.
apply: diff_unique; first exact: scalel_continuous.
apply/eqaddoE; have /diff_locally -> := df; congr (_ + _ + _).
by rewrite funeqE => h /=; rewrite -{1}[h]mulr1 linearZ.
Qed.

Lemma derivable1_diffP f x : derivable f x 1 <-> differentiable f x.

Proof.

split=> dfx.
  by apply/diff_locallyP; rewrite deriv1E //; split;
    [apply: scalel_continuous|apply: der1].
apply/derivable_nbhsP/eqaddoE.
have -> : (fun h => (f \o shift x) h%:A) = f \o shift x.
  by rewrite funeqE=> ?; rewrite [_%:A]mulr1.
by have /diff_locally := dfx; rewrite diff1E // derive1E =>->.
Qed.

Lemma derivable_within_continuous f (i : interval R) :
  {in i, forall x, derivable f x 1} -> {within [set` i], continuous f}.

Proof.

move=> di; apply/continuous_in_subspaceT => z /[1!inE] zA.
apply/differentiable_continuous; rewrite -derivable1_diffP.
by apply: di; rewrite inE.
Qed.

End DeriveRU.

Section DeriveVW.
Variables (R : numFieldType) (V W : normedModType R).
Implicit Types f : V -> W.

Lemma derivable1P f x v :
  derivable f x v <-> derivable (fun h : R => f (h *: v + x)) 0 1.

Proof.

rewrite /derivable; set g1 := fun h => h^-1 *: _; set g2 := fun h => h^-1 *: _.
suff -> : g1 = g2 by [].
by rewrite funeqE /g1 /g2 => h /=; rewrite addr0 scale0r add0r [_%:A]mulr1.
Qed.

Lemma derivableP f x v : derivable f x v -> is_derive x v f ('D_v f x).

Proof.

by move=> df; apply: DeriveDef. Qed.

Lemma diff_derivable f a v : differentiable f a -> derivable f a v.

Proof.

move=> dfa; apply/derivable1P/derivable1_diffP.
by apply: differentiable_comp; rewrite ?scale0r ?add0r.
Qed.

Global Instance is_derive_cst (a : W) (x v : V) : is_derive x v (cst a) 0.

Proof.

apply: DeriveDef; last by rewrite deriveE // diff_val.
exact/diff_derivable.
Qed.

Lemma derivable_cst (w : W) (x v : V) : derivable (cst w) x v.

Proof.

exact/diff_derivable. Qed.

Lemma is_derive_eq f (x v : V) (df f' : W) :
  is_derive x v f f' -> f' = df -> is_derive x v f df.

Proof.

by move=> ? <-. Qed.

End DeriveVW.
Arguments derivable_cst {R V W}.

Section Derive_lemmasVW.
Variables (R : numFieldType) (V W : normedModType R).
Implicit Types f g : V -> W.

Fact der_add f g (x v : V) : derivable f x v -> derivable g x v ->
  (fun h => h^-1 *: (((f + g) \o shift x) (h *: v) - (f + g) x)) @
  0^' --> 'D_v f x + 'D_v g x.

Proof.

move=> df dg.
evar (fg : R -> W); rewrite [X in X @ _](_ : _ = fg) /=; last first.
  rewrite funeqE => h.
  by rewrite !scalerDr scalerN scalerDr opprD addrACA -!scalerBr /fg.
exact: cvgD.
Qed.

Lemma deriveD f g (x v : V) : derivable f x v -> derivable g x v ->
  'D_v (f + g) x = 'D_v f x + 'D_v g x.

Proof.

by move=> df dg; apply: cvg_lim (der_add df dg). Qed.

Lemma derivableD f g (x v : V) :
  derivable f x v -> derivable g x v -> derivable (f + g) x v.

Proof.

move=> df dg; apply/cvg_ex; exists (derive f x v + derive g x v).
exact: der_add.
Qed.

Global Instance is_deriveD f g (x v : V) (df dg : W) :
  is_derive x v f df -> is_derive x v g dg -> is_derive x v (f + g) (df + dg).

Proof.

move=> dfx dgx; apply: DeriveDef; first exact: derivableD.
by rewrite deriveD // !derive_val.
Qed.

Global Instance is_derive_sum n (h : 'I_n -> V -> W) (x v : V)
  (dh : 'I_n -> W) : (forall i, is_derive x v (h i) (dh i)) ->
  is_derive x v (\sum_(i < n) h i) (\sum_(i < n) dh i).

Proof.

by elim/big_ind2 : _ => // [|] *; [exact: is_derive_cst|exact: is_deriveD].
Qed.

Lemma derivable_sum n (h : 'I_n -> V -> W) (x v : V) :
  (forall i, derivable (h i) x v) -> derivable (\sum_(i < n) h i) x v.

Proof.

move=> df; suff : forall i, is_derive x v (h i) ('D_v (h i) x) by [].
by move=> ?; apply: derivableP.
Qed.

Lemma derive_sum n (h : 'I_n -> V -> W) (x v : V) :
  (forall i, derivable (h i) x v) ->
  'D_v (\sum_(i < n) h i) x = \sum_(i < n) 'D_v (h i) x.

Proof.

move=> df; suff dfx : forall i, is_derive x v (h i) ('D_v (h i) x).
  by rewrite derive_val.
by move=> ?; apply: derivableP.
Qed.

Fact der_opp f (x v : V) : derivable f x v ->
  (fun h => h^-1 *: (((- f) \o shift x) (h *: v) - (- f) x)) @
  0^' --> - 'D_v f x.

Proof.

move=> df; evar (g : R -> W); rewrite [X in X @ _](_ : _ = g) /=; last first.
  by rewrite funeqE => h; rewrite !scalerDr !scalerN -opprD -scalerBr /g.
exact: cvgN.
Qed.

Lemma deriveN f (x v : V) : derivable f x v ->
  'D_v (- f) x = - 'D_v f x.

Proof.

by move=> df; apply: cvg_lim (der_opp df). Qed.

Lemma derivableN f (x v : V) :
  derivable f x v -> derivable (- f) x v.

Proof.

by move=> df; apply/cvg_ex; exists (- 'D_v f x); apply: der_opp. Qed.

Global Instance is_deriveN f (x v : V) (df : W) :
  is_derive x v f df -> is_derive x v (- f) (- df).

Proof.

move=> dfx; apply: DeriveDef; first exact: derivableN.
by rewrite deriveN // derive_val.
Qed.

Global Instance is_deriveB f g (x v : V) (df dg : W) :
  is_derive x v f df -> is_derive x v g dg -> is_derive x v (f - g) (df - dg).

Proof.

by move=> ??; apply: is_derive_eq. Qed.

Lemma deriveB f g (x v : V) : derivable f x v -> derivable g x v ->
  'D_v (f - g) x = 'D_v f x - 'D_v g x.

Proof.

by move=> /derivableP df /derivableP dg; rewrite derive_val. Qed.

Lemma derivableB f g (x v : V) :
  derivable f x v -> derivable g x v -> derivable (f - g) x v.

Proof.

by move=> /derivableP df /derivableP dg. Qed.

Fact der_scal f (k : R) (x v : V) : derivable f x v ->
  (fun h => h^-1 *: ((k \*: f \o shift x) (h *: v) - (k \*: f) x)) @
  0^' --> k *: 'D_v f x.

Proof.

move=> df; evar (h : R -> W); rewrite [X in X @ _](_ : _ = h) /=; last first.
  rewrite funeqE => r.
  by rewrite scalerBr !scalerA mulrC -!scalerA -!scalerBr /h.
exact: cvgZr.
Qed.

Lemma deriveZ f (k : R) (x v : V) : derivable f x v ->
  'D_v (k \*: f) x = k *: 'D_v f x.

Proof.

by move=> df; apply: cvg_lim (der_scal df). Qed.

Lemma derivableZ f (k : R) (x v : V) :
  derivable f x v -> derivable (k \*: f) x v.

Proof.

by move=> df; apply/cvg_ex; exists (k *: 'D_v f x); apply: der_scal.
Qed.

Global Instance is_deriveZ f (k : R) (x v : V) (df : W) :
  is_derive x v f df -> is_derive x v (k \*: f) (k *: df).

Proof.

move=> dfx; apply: DeriveDef; first exact: derivableZ.
by rewrite deriveZ // derive_val.
Qed.

Lemma derive_cst (k : R) (x v : V) : 'D_v (cst k) x = 0.

Proof.

by rewrite derive_val. Qed.

End Derive_lemmasVW.

Section derive_id.
Variables (R : numFieldType) (V : normedModType R).

Lemma derivable_id (x v : V) : derivable id x v.

Proof.

exact/diff_derivable. Qed.

Global Instance is_derive_id (x v : V) : is_derive x v id v.

Proof.

apply: (DeriveDef (@derivable_id _ _)).
rewrite deriveE// (@diff_lin _ _ _ idfun)//=.
by rewrite /continuous_at.
Qed.

Global Instance is_deriveNid (x v : V) : is_derive x v -%R (- v).

Proof.

exact: is_deriveN. Qed.

Lemma derive_id (x v : V) : 'D_v id x = v.

Proof.

by have /derivableP := @derivable_id x v; rewrite derive_val. Qed.

End derive_id.

Section Derive_lemmasVR.
Variables (R : numFieldType) (V : normedModType R).
Implicit Types f g : V -> R.

Fact der_mult f g (x v : V) :
  derivable f x v -> derivable g x v ->
  (fun h => h^-1 *: (((f * g) \o shift x) (h *: v) - (f * g) x)) @
  0^' --> f x *: 'D_v g x + g x *: 'D_v f x.

Proof.

move=> df dg.
evar (fg : R -> R); rewrite [X in X @ _](_ : _ = fg) /=; last first.
  rewrite funeqE => h.
  have -> : (f * g) (h *: v + x) - (f * g) x =
    f (h *: v + x) *: (g (h *: v + x) - g x) + g x *: (f (h *: v + x) - f x).
    by rewrite !scalerBr -addrA ![g x *: _]mulrC addKr.
  rewrite scalerDr scalerA mulrC -scalerA.
  by rewrite [_ *: (g x *: _)]scalerA mulrC -scalerA /fg.
apply: cvgD; last exact: cvgZr df.
apply: cvg_comp2 (@mul_continuous _ (_, _)) => /=; last exact: dg.
suff : {for 0, continuous (fun h : R => f(h *: v + x))}.
  by move=> /continuous_withinNx; rewrite scale0r add0r.
exact/differentiable_continuous/derivable1_diffP/(derivable1P _ _ _).1.
Qed.

Lemma deriveM f g (x v : V) : derivable f x v -> derivable g x v ->
  'D_v (f * g) x = f x *: 'D_v g x + g x *: 'D_v f x.

Proof.

by move=> df dg; apply: cvg_lim (der_mult df dg). Qed.

Lemma derivableM f g (x v : V) :
  derivable f x v -> derivable g x v -> derivable (f * g) x v.

Proof.

move=> df dg; apply/cvg_ex; exists (f x *: 'D_v g x + g x *: 'D_v f x).
exact: der_mult.
Qed.

Lemma deriveMr f (r : R) (x v : V) :
  derivable f x v -> 'D_v (r \o* f) x = (r * 'D_v f x)%R.

Proof.

move/deriveM => /(_ _ (derivable_cst _ _ _)) ->.
by rewrite derive_cst scaler0 add0r.
Qed.

Lemma deriveMl f (r : R) (x v : V) :
  derivable f x v -> 'D_v (r \*o f) x = (r * 'D_v f x)%R.

Proof.

by move=> fxv; rewrite -deriveMr// mul_funC. Qed.

Global Instance is_deriveM f g (x v : V) (df dg : R) :
  is_derive x v f df -> is_derive x v g dg ->
  is_derive x v (f * g) (f x *: dg + g x *: df).

Proof.

move=> dfx dgx; apply: DeriveDef; first exact: derivableM.
by rewrite deriveM // !derive_val.
Qed.

Global Instance is_deriveX f n (x v : V) (df : R) :
  is_derive x v f df -> is_derive x v (f ^+ n.+1) ((n.+1%:R * f x ^+n) *: df).

Proof.

move=> dfx; elim: n => [|n ihn]; first by rewrite expr1 expr0 mulr1 scale1r.
rewrite exprS; apply: is_derive_eq.
rewrite scalerA -scalerDl mulrCA -[f x * _]exprS.
by rewrite [in LHS]mulr_natl exprfctE -mulrSr mulr_natl.
Qed.

Lemma derivableX f n (x v : V) : derivable f x v -> derivable (f ^+ n) x v.

Proof.

by case: n => [_|n /derivableP]; [rewrite expr0|]. Qed.

Lemma deriveX f n (x v : V) : derivable f x v ->
  'D_v (f ^+ n.+1) x = (n.+1%:R * f x ^+ n) *: 'D_v f x.

Proof.

by move=> /derivableP df; rewrite derive_val. Qed.

Fact der_inv f (x v : V) : f x != 0 -> derivable f x v ->
  (fun h => h^-1 *: (((fun y => (f y)^-1) \o shift x) (h *: v) - (f x)^-1)) @
  0^' --> - (f x) ^-2 *: 'D_v f x.

Proof.

move=> fxn0 df.
have /derivable1P/derivable1_diffP/differentiable_continuous := df.
move=> /continuous_withinNx; rewrite scale0r add0r => fc.
have fn0 : 0^' [set h | f (h *: v + x) != 0].
  apply: (fc [set x | x != 0]); exists `|f x|; first by rewrite /= normr_gt0.
  move=> y; rewrite /= => yltfx.
  by apply/eqP => y0; move: yltfx; rewrite y0 subr0 ltxx.
have : (fun h => - ((f x)^-1 * (f (h *: v + x))^-1) *:
  (h^-1 *: (f (h *: v + x) - f x))) @ 0^' -->
  - (f x) ^- 2 *: 'D_v f x.
  by apply: cvgM => //; apply: cvgN; rewrite expr2 invfM; apply: cvgM;
     [exact: cvg_cst| exact: cvgV].
apply: cvg_trans => A [_/posnumP[e] /= Ae].
move: fn0; apply: filter_app; near=> h => /= fhvxn0.
have he : ball 0 e%:num (h : R) by near: h; exists e%:num => /=.
have hn0 : h != 0 by near: h; exists e%:num => /=.
suff <- :
  - ((f x)^-1 * (f (h *: v + x))^-1) *: (h^-1 *: (f (h *: v + x) - f x)) =
  h^-1 *: ((f (h *: v + x))^-1 - (f x)^-1) by exact: Ae.
rewrite scalerA mulrC -scalerA; congr (_ *: _).
apply/eqP; rewrite scaleNr eqr_oppLR opprB scalerBr.
rewrite -scalerA [_ *: f _]mulVf // [_%:A]mulr1.
by rewrite mulrC -scalerA [_ *: f _]mulVf // [_%:A]mulr1.
Unshelve. all: by end_near. Qed.

Lemma deriveV f x v : f x != 0 -> derivable f x v ->
  'D_v (fun y => (f y)^-1) x = - (f x) ^- 2 *: 'D_v f x.

Proof.

by move=> fxn0 df; apply: cvg_lim (der_inv fxn0 df). Qed.

Lemma derivableV f (x v : V) :
  f x != 0 -> derivable f x v -> derivable (fun y => (f y)^-1) x v.

Proof.

move=> df dg; apply/cvg_ex; exists (- (f x) ^- 2 *: 'D_v f x).
exact: der_inv.
Qed.

End Derive_lemmasVR.

Lemma derive1_cst {R : numFieldType} (k : R) t : (cst k)^`() t = 0.

Proof.

by rewrite derive1E derive_cst. Qed.

Lemma exprn_derivable {R : numFieldType} n (x : R) v :
  derivable (@GRing.exp R ^~ n) x v.

Proof.

elim: n => [/=|n ih]; first by rewrite (_ : _ ^~ _ = 1).
rewrite (_ : _ ^~ _ = (fun x => x * x ^+ n)); last first.
  by apply/funext => y; rewrite exprS.
by apply: derivableM; first exact: derivable_id.
Qed.

Lemma exp_derive {R : numFieldType} n x v :
  'D_v (@GRing.exp R ^~ n.+1) x = n.+1%:R *: x ^+ n *: v.

Proof.

have /= := @deriveX R R id n x v (@derivable_id _ _ _ _).
by rewrite fctE => ->; rewrite derive_id.
Qed.

Lemma exp_derive1 {R : numFieldType} n x :
  (@GRing.exp R ^~ n.+1)^`() x = n.+1%:R *: x ^+ n.

Proof.

by rewrite derive1E exp_derive [LHS]mulr1. Qed.

Lemma EVT_max (R : realType) (f : R -> R) (a b : R) :
  a <= b -> {within `[a, b], continuous f} -> exists2 c, c \in `[a, b]%R &
  forall t, t \in `[a, b]%R -> f t <= f c.

Proof.

move=> leab fcont; set imf := f @` `[a, b].
have imf_sup : has_sup imf.
  split; first by exists (f a); apply/imageP; rewrite /= in_itv /= lexx.
  have [M [Mreal imfltM]] : bounded_set (f @` `[a, b]).
    by apply/compact_bounded/continuous_compact => //; exact: segment_compact.
  exists (M + 1) => y /imfltM yleM.
  by rewrite (le_trans _ (yleM _ _)) ?ler_norm ?ltrDl.
have [|imf_ltsup] := pselect (exists2 c, c \in `[a, b]%R & f c = sup imf).
  move=> [c cab fceqsup]; exists c => // t tab; rewrite fceqsup.
  by apply/sup_upper_bound => //; exact/imageP.
have {}imf_ltsup t : t \in `[a, b]%R -> f t < sup imf.
  move=> tab; case: (ltrP (f t) (sup imf)) => // supleft.
  rewrite falseE; apply: imf_ltsup; exists t => //; apply/eqP.
  by rewrite eq_le supleft andbT sup_upper_bound//; exact/imageP.
pose g t : R := (sup imf - f t)^-1.
have invf_continuous : {within `[a, b], continuous g}.
  rewrite continuous_subspace_in => t tab; apply: cvgV => //=.
    by rewrite subr_eq0 gt_eqF // imf_ltsup //; rewrite inE in tab.
  by apply: cvgD; [exact: cst_continuous | apply: cvgN; exact: (fcont t)].
have /ex_strict_bound_gt0 [k k_gt0 /= imVfltk] : bounded_set (g @` `[a, b]).
  apply/compact_bounded/continuous_compact; last exact: segment_compact.
  exact: invf_continuous.
have [_ [t tab <-]] : exists2 y, imf y & sup imf - k^-1 < y.
  by apply: sup_adherent => //; rewrite invr_gt0.
rewrite ltrBlDr -ltrBlDl.
suff : sup imf - f t > k^-1 by move=> /ltW; rewrite leNgt => /negbTE ->.
rewrite -[ltRHS]invrK ltf_pV2// ?qualifE/= ?invr_gt0 ?subr_gt0 ?imf_ltsup//.
by rewrite (le_lt_trans (ler_norm _) _) ?imVfltk//; exact: imageP.
Qed.

Lemma EVT_min (R : realType) (f : R -> R) (a b : R) :
  a <= b -> {within `[a, b], continuous f} -> exists2 c, c \in `[a, b]%R &
  forall t, t \in `[a, b]%R -> f c <= f t.

Proof.

move=> leab fcont.
have /(EVT_max leab) [c clr fcmax] : {within `[a, b], continuous (- f)}.
  by move=> ?; apply: continuousN => ?; exact: fcont.
by exists c => // ? /fcmax; rewrite lerN2.
Qed.

Lemma cvg_at_rightE (R : numFieldType) (V : normedModType R) (f : R -> V) x :
  cvg (f @ x^') -> lim (f @ x^') = lim (f @ at_right x).

Proof.

move=> cvfx; apply/Logic.eq_sym.
apply: (@cvg_lim _ _ _ (at_right _)) => // A /cvfx /nbhs_ballP [_ /posnumP[e] xe_A].
by exists e%:num => //= y xe_y; rewrite lt_def => /andP [xney _]; apply: xe_A.
Qed.

Arguments cvg_at_rightE {R V} f x.

Lemma cvg_at_leftE (R : numFieldType) (V : normedModType R) (f : R -> V) x :
  cvg (f @ x^') -> lim (f @ x^') = lim (f @ at_left x).

Proof.

move=> cvfx; apply/Logic.eq_sym.
apply: (@cvg_lim _ _ _ (at_left _)) => // A /cvfx /nbhs_ballP [_ /posnumP[e] xe_A].
exists e%:num => //= y xe_y; rewrite lt_def => /andP [xney _].
by apply: xe_A => //; rewrite eq_sym.
Qed.

Arguments cvg_at_leftE {R V} f x.

Lemma __deprecated__le0r_cvg_map (R : realFieldType) (T : topologicalType)
  (F : set_system T) (FF : ProperFilter F) (f : T -> R) :
  (\forall x \near F, 0 <= f x) -> cvg (f @ F) -> 0 <= lim (f @ F).

Proof.

by move=> ? ?; rewrite limr_ge. Qed.

#[deprecated(since="mathcomp-analysis 0.6.0",
  note="generalized by `limr_ge`")]
Notation le0r_cvg_map := __deprecated__le0r_cvg_map (only parsing).

Lemma __deprecated__ler0_cvg_map (R : realFieldType) (T : topologicalType)
  (F : set_system T) (FF : ProperFilter F) (f : T -> R) :
  (\forall x \near F, f x <= 0) -> cvg (f @ F) -> lim (f @ F) <= 0.

Proof.

by move=> ? ?; rewrite limr_le. Qed.

#[deprecated(since="mathcomp-analysis 0.6.0",
  note="generalized by `limr_le`")]
Notation ler0_cvg_map := __deprecated__ler0_cvg_map (only parsing).

Lemma __deprecated__ler_cvg_map (R : realFieldType) (T : topologicalType)
  (F : set_system T) (FF : ProperFilter F) (f g : T -> R) :
  (\forall x \near F, f x <= g x) -> cvg (f @ F) -> cvg (g @ F) ->
  lim (f @ F) <= lim (g @ F).

Proof.

by move=> ? ? ?; rewrite ler_lim. Qed.

#[deprecated(since="mathcomp-analysis 0.6.0",
  note="subsumed by `ler_lim`")]
Notation ler_cvg_map := __deprecated__ler_cvg_map (only parsing).

Lemma derive1_at_max (R : realFieldType) (f : R -> R) (a b c : R) :
  a <= b -> (forall t, t \in `]a, b[%R -> derivable f t 1) -> c \in `]a, b[%R ->
  (forall t, t \in `]a, b[%R -> f t <= f c) -> is_derive c 1 f 0.

Proof.

move=> leab fdrvbl cab cmax; apply: DeriveDef; first exact: fdrvbl.
apply/eqP; rewrite eq_le; apply/andP; split.
  rewrite ['D_1 f c]cvg_at_rightE; last exact: fdrvbl.
  apply: limr_le.
    have /fdrvbl dfc := cab.
    rewrite -(cvg_at_rightE (fun h : R => h^-1 *: ((f \o shift c) _ - f c))) //.
    apply: cvg_trans dfc; apply: cvg_app.
    move=> A [e egt0 Ae]; exists e => // x xe xgt0; apply: Ae => //.
    exact/lt0r_neq0.
  near=> h; apply: mulr_ge0_le0.
    by rewrite invr_ge0; apply: ltW; near: h; exists 1 => /=.
  rewrite subr_le0 [_%:A]mulr1; apply: cmax; near: h.
  exists (b - c); first by rewrite /= subr_gt0 (itvP cab).
  move=> h; rewrite /= distrC subr0 /= in_itv /= -ltrBrDr.
  move=> /(le_lt_trans (ler_norm _)) -> /ltr_pDl -> //.
  by rewrite (itvP cab).
rewrite ['D_1 f c]cvg_at_leftE; last exact: fdrvbl.
apply: limr_ge.
  have /fdrvbl dfc := cab; rewrite -(cvg_at_leftE (fun h => h^-1 *: ((f \o shift c) _ - f c))) //.
  apply: cvg_trans dfc; apply: cvg_app.
  move=> A [e egt0 Ae]; exists e => // x xe xgt0; apply: Ae => //.
  exact/ltr0_neq0.
near=> h; apply: mulr_le0.
  by rewrite invr_le0; apply: ltW; near: h; exists 1 => /=.
rewrite subr_le0 [_%:A]mulr1; apply: cmax; near: h.
exists (c - a); first by rewrite /= subr_gt0 (itvP cab).
move=> h; rewrite /= distrC subr0.
move=> /ltr_normlP []; rewrite ltrBrDl ltrBlDl in_itv /= => -> _.
by move=> /ltr_nDl -> //; rewrite (itvP cab).
Unshelve. all: by end_near. Qed.

Lemma derive1_at_min (R : realFieldType) (f : R -> R) (a b c : R) :
  a <= b -> (forall t, t \in `]a, b[%R -> derivable f t 1) -> c \in `]a, b[%R ->
  (forall t, t \in `]a, b[%R -> f c <= f t) -> is_derive c 1 f 0.

Proof.

move=> leab fdrvbl cab cmin; apply: DeriveDef; first exact: fdrvbl.
apply/eqP; rewrite -oppr_eq0; apply/eqP.
rewrite -deriveN; last exact: fdrvbl.
suff df : is_derive c 1 (- f) 0 by rewrite derive_val.
apply: derive1_at_max leab _ (cab) _ => t tab; first exact/derivableN/fdrvbl.
by rewrite lerN2; apply: cmin.
Qed.

Lemma Rolle (R : realType) (f : R -> R) (a b : R) :
  a < b -> (forall x, x \in `]a, b[%R -> derivable f x 1) ->
  {within `[a, b], continuous f} -> f a = f b ->
  exists2 c, c \in `]a, b[%R & is_derive c 1 f 0.

Proof.

move=> ltab fdrvbl fcont faefb.
have [cmax cmaxab fcmax] := EVT_max (ltW ltab) fcont.
have [cmaxeaVb|] := boolP (cmax \in [set a; b]); last first.
  rewrite notin_setE => /not_orP[/eqP cnea /eqP cneb].
  have {}cmaxab : cmax \in `]a, b[%R.
    by rewrite in_itv /= !lt_def !(itvP cmaxab) cnea eq_sym cneb.
  exists cmax => //; apply: derive1_at_max (ltW ltab) fdrvbl cmaxab _ => t tab.
  by apply: fcmax; rewrite in_itv /= !ltW // (itvP tab).
have [cmin cminab fcmin] := EVT_min (ltW ltab) fcont.
have [cmineaVb|] := boolP (cmin \in [set a; b]); last first.
  rewrite notin_setE => /not_orP[/eqP cnea /eqP cneb].
  have {}cminab : cmin \in `]a, b[%R.
    by rewrite in_itv /= !lt_def !(itvP cminab) cnea eq_sym cneb.
  exists cmin => //; apply: derive1_at_min (ltW ltab) fdrvbl cminab _ => t tab.
  by apply: fcmin; rewrite in_itv /= !ltW // (itvP tab).
have midab : (a + b) / 2 \in `]a, b[%R by apply: mid_in_itvoo.
exists ((a + b) / 2) => //; apply: derive1_at_max (ltW ltab) fdrvbl (midab) _.
move=> t tab.
suff fcst s : s \in `]a, b[%R -> f s = f cmax by rewrite !fcst.
move=> sab.
apply/eqP; rewrite eq_le fcmax; last by rewrite in_itv /= !ltW ?(itvP sab).
suff -> : f cmax = f cmin by rewrite fcmin // in_itv /= !ltW ?(itvP sab).
by move: cmaxeaVb cmineaVb; rewrite 2!inE => -[|] -> [|] ->.
Qed.

Lemma MVT (R : realType) (f df : R -> R) (a b : R) :
  a < b -> (forall x, x \in `]a, b[%R -> is_derive x 1 f (df x)) ->
  {within `[a, b], continuous f} ->
  exists2 c, c \in `]a, b[%R & f b - f a = df c * (b - a).

Proof.

move=> altb fdrvbl fcont.
set g := f + (- ( *:%R^~ ((f b - f a) / (b - a)) : R -> R)).
have gdrvbl x : x \in `]a, b[%R -> derivable g x 1.
  by move=> /fdrvbl dfx; apply: derivableB => //; exact/derivable1_diffP.
have gcont : {within `[a, b], continuous g}.
  move=> x; apply: continuousD _ ; first by move=>?; exact: fcont.
  by apply/continuousN/continuous_subspaceT=> ?; exact: scalel_continuous.
have gaegb : g a = g b.
  rewrite /g -![(_ - _ : _ -> _) _]/(_ - _).
  apply/eqP; rewrite -subr_eq /= opprK addrAC -addrA -scalerBl.
  rewrite [_ *: _]mulrA mulrC mulrA mulVf.
    by rewrite mul1r addrCA subrr addr0.
  by apply: lt0r_neq0; rewrite subr_gt0.
have [c cab dgc0] := Rolle altb gdrvbl gcont gaegb.
exists c; first exact: cab.
have /fdrvbl dfc := cab; move/@derive_val: dgc0; rewrite deriveB //.
move/eqP; rewrite [X in _ - X]deriveE // derive_val diff_val scale1r subr_eq0.
move/eqP->; rewrite -mulrA mulVf ?mulr1 //; apply: lt0r_neq0.
by rewrite subr_gt0.
Qed.

Lemma MVT_segment (R : realType) (f df : R -> R) (a b : R) :
  a <= b -> (forall x, x \in `]a, b[%R -> is_derive x 1 f (df x)) ->
  {within `[a, b], continuous f} ->
  exists2 c, c \in `[a, b]%R & f b - f a = df c * (b - a).

Proof.

move=> leab fdrvbl fcont; case: ltgtP leab => // [altb|aeb]; last first.
  by exists a; [rewrite inE/= aeb lexx|rewrite aeb !subrr mulr0].
have [c cab D] := MVT altb fdrvbl fcont.
by exists c => //; rewrite in_itv /= ltW (itvP cab).
Qed.

Lemma ler0_derive1_nincr (R : realType) (f : R -> R) (a b : R) :
  (forall x, x \in `]a, b[%R -> derivable f x 1) ->
  (forall x, x \in `]a, b[%R -> f^`() x <= 0) ->
  {within `[a,b], continuous f} ->
  forall x y, a <= x -> x <= y -> y <= b -> f y <= f x.

Proof.

move=> fdrvbl dfle0 ctsf x y leax lexy leyb; rewrite -subr_ge0.
case: ltgtP lexy => // [xlty|->]; last by rewrite subrr.
have itvW : {subset `[x, y]%R <= `[a, b]%R}.
  by apply/subitvP; rewrite /<=%O /= /<=%O /= leyb leax.
have itvWlt : {subset `]x, y[%R <= `]a, b[%R}.
  by apply subitvP; rewrite /<=%O /= /<=%O /= leyb leax.
have fdrv z : z \in `]x, y[%R -> is_derive z 1 f (f^`()z).
  rewrite in_itv/= => /andP[xz zy]; apply: DeriveDef; last by rewrite derive1E.
  by apply: fdrvbl; rewrite in_itv/= (le_lt_trans _ xz)// (lt_le_trans zy).
have [] := @MVT _ f (f^`()) x y xlty fdrv.
  apply: (@continuous_subspaceW _ _ _ `[a,b]); first exact: itvW.
  by rewrite continuous_subspace_in.
move=> t /itvWlt dft dftxy _; rewrite -oppr_le0 opprB dftxy.
by apply: mulr_le0_ge0 => //; [exact: dfle0|by rewrite subr_ge0 ltW].
Qed.

Lemma le0r_derive1_ndecr (R : realType) (f : R -> R) (a b : R) :
  (forall x, x \in `]a, b[%R -> derivable f x 1) ->
  (forall x, x \in `]a, b[%R -> 0 <= f^`() x) ->
  {within `[a,b], continuous f} ->
  forall x y, a <= x -> x <= y -> y <= b -> f x <= f y.

Proof.

move=> fdrvbl dfge0 fcont x y; rewrite -[f _ <= _]lerN2.
apply (@ler0_derive1_nincr _ (- f)) => t tab; first exact/derivableN/fdrvbl.
  rewrite derive1E deriveN; last exact: fdrvbl.
  by rewrite oppr_le0 -derive1E; apply: dfge0.
by apply: continuousN; exact: fcont.
Qed.

Lemma derive1_comp (R : realFieldType) (f g : R -> R) x :
  derivable f x 1 -> derivable g (f x) 1 ->
  (g \o f)^`() x = g^`() (f x) * f^`() x.

Proof.

move=> /derivable1_diffP df /derivable1_diffP dg.
rewrite derive1E'; last exact/differentiable_comp.
rewrite diff_comp // !derive1E' //= -[X in 'd _ _ X = _]mulr1.
by rewrite [LHS]linearZ mulrC.
Qed.

Lemma trigger_derive (R : realType) (f : R -> R) x x1 y1 :
  is_derive x (1 : R) f x1 -> x1 = y1 -> is_derive x 1 f y1.

Proof.

by move=> Hi <-. Qed.