Theory HLim

(*  Title:      HOL/Nonstandard_Analysis/HLim.thy
    Author:     Jacques D. Fleuriot, University of Cambridge
    Author:     Lawrence C Paulson
*)

section ‹Limits and Continuity (Nonstandard)›

theory HLim
  imports Star
  abbrevs "--->" = "─→⇩N⇩S"
begin

text ‹Nonstandard Definitions.›

definition NSLIM :: "('a::real_normed_vector ⇒ 'b::real_normed_vector) ⇒ 'a ⇒ 'b ⇒ bool"
    ("((_)/ ─(_)/→⇩N⇩S (_))" [60, 0, 60] 60)
  where "f ─a→⇩N⇩S L ⟷ (∀x. x ≠ star_of a ∧ x ≈ star_of a ⟶ ( *f* f) x ≈ star_of L)"

definition isNSCont :: "('a::real_normed_vector ⇒ 'b::real_normed_vector) ⇒ 'a ⇒ bool"
  where  ― ‹NS definition dispenses with limit notions›
    "isNSCont f a ⟷ (∀y. y ≈ star_of a ⟶ ( *f* f) y ≈ star_of (f a))"

definition isNSUCont :: "('a::real_normed_vector ⇒ 'b::real_normed_vector) ⇒ bool"
  where "isNSUCont f ⟷ (∀x y. x ≈ y ⟶ ( *f* f) x ≈ ( *f* f) y)"


subsection ‹Limits of Functions›

lemma NSLIM_I: "(⋀x. x ≠ star_of a ⟹ x ≈ star_of a ⟹ starfun f x ≈ star_of L) ⟹ f ─a→⇩N⇩S L"
  by (simp add: NSLIM_def)

lemma NSLIM_D: "f ─a→⇩N⇩S L ⟹ x ≠ star_of a ⟹ x ≈ star_of a ⟹ starfun f x ≈ star_of L"
  by (simp add: NSLIM_def)

text ‹Proving properties of limits using nonstandard definition.
  The properties hold for standard limits as well!›

lemma NSLIM_mult: "f ─x→⇩N⇩S l ⟹ g ─x→⇩N⇩S m ⟹ (λx. f x * g x) ─x→⇩N⇩S (l * m)"
  for l m :: "'a::real_normed_algebra"
  by (auto simp add: NSLIM_def intro!: approx_mult_HFinite)

lemma starfun_scaleR [simp]: "starfun (λx. f x *⇩R g x) = (λx. scaleHR (starfun f x) (starfun g x))"
  by transfer (rule refl)

lemma NSLIM_scaleR: "f ─x→⇩N⇩S l ⟹ g ─x→⇩N⇩S m ⟹ (λx. f x *⇩R g x) ─x→⇩N⇩S (l *⇩R m)"
  by (auto simp add: NSLIM_def intro!: approx_scaleR_HFinite)

lemma NSLIM_add: "f ─x→⇩N⇩S l ⟹ g ─x→⇩N⇩S m ⟹ (λx. f x + g x) ─x→⇩N⇩S (l + m)"
  by (auto simp add: NSLIM_def intro!: approx_add)

lemma NSLIM_const [simp]: "(λx. k) ─x→⇩N⇩S k"
  by (simp add: NSLIM_def)

lemma NSLIM_minus: "f ─a→⇩N⇩S L ⟹ (λx. - f x) ─a→⇩N⇩S -L"
  by (simp add: NSLIM_def)

lemma NSLIM_diff: "f ─x→⇩N⇩S l ⟹ g ─x→⇩N⇩S m ⟹ (λx. f x - g x) ─x→⇩N⇩S (l - m)"
  by (simp only: NSLIM_add NSLIM_minus diff_conv_add_uminus)

lemma NSLIM_add_minus: "f ─x→⇩N⇩S l ⟹ g ─x→⇩N⇩S m ⟹ (λx. f x + - g x) ─x→⇩N⇩S (l + -m)"
  by (simp only: NSLIM_add NSLIM_minus)

lemma NSLIM_inverse: "f ─a→⇩N⇩S L ⟹ L ≠ 0 ⟹ (λx. inverse (f x)) ─a→⇩N⇩S (inverse L)"
  for L :: "'a::real_normed_div_algebra"
  unfolding NSLIM_def by (metis (no_types) star_of_approx_inverse star_of_simps(6) starfun_inverse)

lemma NSLIM_zero:
  assumes f: "f ─a→⇩N⇩S l"
  shows "(λx. f(x) - l) ─a→⇩N⇩S 0"
proof -
  have "(λx. f x - l) ─a→⇩N⇩S l - l"
    by (rule NSLIM_diff [OF f NSLIM_const])
  then show ?thesis by simp
qed

lemma NSLIM_zero_cancel: 
  assumes "(λx. f x - l) ─x→⇩N⇩S 0"
  shows "f ─x→⇩N⇩S l"
proof -
  have "(λx. f x - l + l) ─x→⇩N⇩S 0 + l"
    by (fast intro: assms NSLIM_const NSLIM_add)
  then show ?thesis
    by simp
qed

lemma NSLIM_const_eq:
  fixes a :: "'a::real_normed_algebra_1"
  assumes "(λx. k) ─a→⇩N⇩S l"
  shows "k = l"
proof -
  have "¬ (λx. k) ─a→⇩N⇩S l" if "k ≠ l"
  proof -
    have "star_of a + of_hypreal ε ≈ star_of a"
      by (simp add: approx_def)
    then show ?thesis
      using epsilon_not_zero that by (force simp add: NSLIM_def)
  qed
  with assms show ?thesis by metis
qed

lemma NSLIM_unique: "f ─a→⇩N⇩S l ⟹ f ─a→⇩N⇩S M ⟹ l = M"
  for a :: "'a::real_normed_algebra_1"
  by (drule (1) NSLIM_diff) (auto dest!: NSLIM_const_eq)

lemma NSLIM_mult_zero: "f ─x→⇩N⇩S 0 ⟹ g ─x→⇩N⇩S 0 ⟹ (λx. f x * g x) ─x→⇩N⇩S 0"
  for f g :: "'a::real_normed_vector ⇒ 'b::real_normed_algebra"
  by (drule NSLIM_mult) auto

lemma NSLIM_self: "(λx. x) ─a→⇩N⇩S a"
  by (simp add: NSLIM_def)


subsubsection ‹Equivalence of \<^term>‹filterlim› and \<^term>‹NSLIM››

lemma LIM_NSLIM:
  assumes f: "f ─a→ L"
  shows "f ─a→⇩N⇩S L"
proof (rule NSLIM_I)
  fix x
  assume neq: "x ≠ star_of a"
  assume approx: "x ≈ star_of a"
  have "starfun f x - star_of L ∈ Infinitesimal"
  proof (rule InfinitesimalI2)
    fix r :: real
    assume r: "0 < r"
    from LIM_D [OF f r] obtain s
      where s: "0 < s" and less_r: "⋀x. x ≠ a ⟹ norm (x - a) < s ⟹ norm (f x - L) < r"
      by fast
    from less_r have less_r':
      "⋀x. x ≠ star_of a ⟹ hnorm (x - star_of a) < star_of s ⟹
        hnorm (starfun f x - star_of L) < star_of r"
      by transfer
    from approx have "x - star_of a ∈ Infinitesimal"
      by (simp only: approx_def)
    then have "hnorm (x - star_of a) < star_of s"
      using s by (rule InfinitesimalD2)
    with neq show "hnorm (starfun f x - star_of L) < star_of r"
      by (rule less_r')
  qed
  then show "starfun f x ≈ star_of L"
    by (unfold approx_def)
qed

lemma NSLIM_LIM:
  assumes f: "f ─a→⇩N⇩S L"
  shows "f ─a→ L"
proof (rule LIM_I)
  fix r :: real
  assume r: "0 < r"
  have "∃s>0. ∀x. x ≠ star_of a ∧ hnorm (x - star_of a) < s ⟶
    hnorm (starfun f x - star_of L) < star_of r"
  proof (rule exI, safe)
    show "0 < ε"
      by (rule epsilon_gt_zero)
  next
    fix x
    assume neq: "x ≠ star_of a"
    assume "hnorm (x - star_of a) < ε"
    with Infinitesimal_epsilon have "x - star_of a ∈ Infinitesimal"
      by (rule hnorm_less_Infinitesimal)
    then have "x ≈ star_of a"
      by (unfold approx_def)
    with f neq have "starfun f x ≈ star_of L"
      by (rule NSLIM_D)
    then have "starfun f x - star_of L ∈ Infinitesimal"
      by (unfold approx_def)
    then show "hnorm (starfun f x - star_of L) < star_of r"
      using r by (rule InfinitesimalD2)
  qed
  then show "∃s>0. ∀x. x ≠ a ∧ norm (x - a) < s ⟶ norm (f x - L) < r"
    by transfer
qed

theorem LIM_NSLIM_iff: "f ─x→ L ⟷ f ─x→⇩N⇩S L"
  by (blast intro: LIM_NSLIM NSLIM_LIM)


subsection ‹Continuity›

lemma isNSContD: "isNSCont f a ⟹ y ≈ star_of a ⟹ ( *f* f) y ≈ star_of (f a)"
  by (simp add: isNSCont_def)

lemma isNSCont_NSLIM: "isNSCont f a ⟹ f ─a→⇩N⇩S (f a)"
  by (simp add: isNSCont_def NSLIM_def)

lemma NSLIM_isNSCont: "f ─a→⇩N⇩S (f a) ⟹ isNSCont f a"
  by (force simp add: isNSCont_def NSLIM_def)

text ‹NS continuity can be defined using NS Limit in
  similar fashion to standard definition of continuity.›
lemma isNSCont_NSLIM_iff: "isNSCont f a ⟷ f ─a→⇩N⇩S (f a)"
  by (blast intro: isNSCont_NSLIM NSLIM_isNSCont)

text ‹Hence, NS continuity can be given in terms of standard limit.›
lemma isNSCont_LIM_iff: "(isNSCont f a) = (f ─a→ (f a))"
  by (simp add: LIM_NSLIM_iff isNSCont_NSLIM_iff)

text ‹Moreover, it's trivial now that NS continuity
  is equivalent to standard continuity.›
lemma isNSCont_isCont_iff: "isNSCont f a ⟷ isCont f a"
  by (simp add: isCont_def) (rule isNSCont_LIM_iff)

text ‹Standard continuity ‹⟹› NS continuity.›
lemma isCont_isNSCont: "isCont f a ⟹ isNSCont f a"
  by (erule isNSCont_isCont_iff [THEN iffD2])

text ‹NS continuity ‹⟹› Standard continuity.›
lemma isNSCont_isCont: "isNSCont f a ⟹ isCont f a"
  by (erule isNSCont_isCont_iff [THEN iffD1])


text ‹Alternative definition of continuity.›

text ‹Prove equivalence between NS limits --
  seems easier than using standard definition.›
lemma NSLIM_at0_iff: "f ─a→⇩N⇩S L ⟷ (λh. f (a + h)) ─0→⇩N⇩S L"
proof
  assume "f ─a→⇩N⇩S L"
  then show "(λh. f (a + h)) ─0→⇩N⇩S L"
    by (simp add: NSLIM_def) (metis (no_types) add_cancel_left_right approx_add_left_iff starfun_lambda_cancel)
next
  assume *: "(λh. f (a + h)) ─0→⇩N⇩S L"
  show "f ─a→⇩N⇩S L"
  proof (clarsimp simp: NSLIM_def)
    fix x
    assume "x ≠ star_of a" "x ≈ star_of a"
    then have "(*f* (λh. f (a + h))) (- star_of a + x) ≈ star_of L"
      by (metis (no_types, lifting) "*" NSLIM_D add.right_neutral add_minus_cancel approx_minus_iff2 star_zero_def)
    then show "(*f* f) x ≈ star_of L"
      by (simp add: starfun_lambda_cancel)
  qed
qed

lemma isNSCont_minus: "isNSCont f a ⟹ isNSCont (λx. - f x) a"
  by (simp add: isNSCont_def)

lemma isNSCont_inverse: "isNSCont f x ⟹ f x ≠ 0 ⟹ isNSCont (λx. inverse (f x)) x"
  for f :: "'a::real_normed_vector ⇒ 'b::real_normed_div_algebra"
  using NSLIM_inverse NSLIM_isNSCont isNSCont_NSLIM by blast

lemma isNSCont_const [simp]: "isNSCont (λx. k) a"
  by (simp add: isNSCont_def)

lemma isNSCont_abs [simp]: "isNSCont abs a"
  for a :: real
  by (auto simp: isNSCont_def intro: approx_hrabs simp: starfun_rabs_hrabs)


subsection ‹Uniform Continuity›

lemma isNSUContD: "isNSUCont f ⟹ x ≈ y ⟹ ( *f* f) x ≈ ( *f* f) y"
  by (simp add: isNSUCont_def)

lemma isUCont_isNSUCont:
  fixes f :: "'a::real_normed_vector ⇒ 'b::real_normed_vector"
  assumes f: "isUCont f"
  shows "isNSUCont f"
  unfolding isNSUCont_def
proof safe
  fix x y :: "'a star"
  assume approx: "x ≈ y"
  have "starfun f x - starfun f y ∈ Infinitesimal"
  proof (rule InfinitesimalI2)
    fix r :: real
    assume r: "0 < r"
    with f obtain s where s: "0 < s"
      and less_r: "⋀x y. norm (x - y) < s ⟹ norm (f x - f y) < r"
      by (auto simp add: isUCont_def dist_norm)
    from less_r have less_r':
      "⋀x y. hnorm (x - y) < star_of s ⟹ hnorm (starfun f x - starfun f y) < star_of r"
      by transfer
    from approx have "x - y ∈ Infinitesimal"
      by (unfold approx_def)
    then have "hnorm (x - y) < star_of s"
      using s by (rule InfinitesimalD2)
    then show "hnorm (starfun f x - starfun f y) < star_of r"
      by (rule less_r')
  qed
  then show "starfun f x ≈ starfun f y"
    by (unfold approx_def)
qed

lemma isNSUCont_isUCont:
  fixes f :: "'a::real_normed_vector ⇒ 'b::real_normed_vector"
  assumes f: "isNSUCont f"
  shows "isUCont f"
  unfolding isUCont_def dist_norm
proof safe
  fix r :: real
  assume r: "0 < r"
  have "∃s>0. ∀x y. hnorm (x - y) < s ⟶ hnorm (starfun f x - starfun f y) < star_of r"
  proof (rule exI, safe)
    show "0 < ε"
      by (rule epsilon_gt_zero)
  next
    fix x y :: "'a star"
    assume "hnorm (x - y) < ε"
    with Infinitesimal_epsilon have "x - y ∈ Infinitesimal"
      by (rule hnorm_less_Infinitesimal)
    then have "x ≈ y"
      by (unfold approx_def)
    with f have "starfun f x ≈ starfun f y"
      by (simp add: isNSUCont_def)
    then have "starfun f x - starfun f y ∈ Infinitesimal"
      by (unfold approx_def)
    then show "hnorm (starfun f x - starfun f y) < star_of r"
      using r by (rule InfinitesimalD2)
  qed
  then show "∃s>0. ∀x y. norm (x - y) < s ⟶ norm (f x - f y) < r"
    by transfer
qed

end