import Mathlib
import Analysis.Section_6_1
import Analysis.Tags.Section_6_1
import Analysis.Section_6_2
import Analysis.Tags.Section_6_2

namespace Chapter6

-- <CONTEXT>
abbrev _root_.Real.Close (ε x y : ℝ) : Prop := dist x y ≤ ε

@[ext]
structure Sequence where
  m : ℤ
  seq : ℤ → ℝ
  vanish : ∀ n < m, seq n = 0

instance Sequence.instCoeFun : CoeFun Sequence (fun _ ↦ ℤ → ℝ) where
  coe a := a.seq

@[coe]
abbrev Sequence.ofNatFun (a:ℕ → ℝ) : Sequence :=
 {
    m := 0
    seq n := if n ≥ 0 then a n.toNat else 0
    vanish := by simp_all
 }

instance Sequence.instCoe : Coe (ℕ → ℝ) Sequence where
  coe := ofNatFun

abbrev Sequence.mk' (m:ℤ) (a: { n // n ≥ m } → ℝ) : Sequence where
  m := m
  seq n := if h : n ≥ m then a ⟨n, h⟩ else 0
  vanish := by simp_all

abbrev Sequence.from (a:Sequence) (m₁:ℤ) : Sequence := mk' (max a.m m₁) (a ↑·)

abbrev _root_.Real.CloseSeq (ε: ℝ) (a: Chapter6.Sequence) (L:ℝ) : Prop := ∀ n ≥ a.m, ε.Close (a n) L

abbrev _root_.Real.EventuallyClose (ε: ℝ) (a: Chapter6.Sequence) (L:ℝ) : Prop :=
  ∃ N ≥ a.m, ε.CloseSeq (a.from N) L

abbrev Sequence.TendsTo (a:Sequence) (L:ℝ) : Prop :=
  ∀ ε > (0:ℝ), ε.EventuallyClose a L

abbrev Sequence.Convergent (a:Sequence) : Prop := ∃ L, a.TendsTo L

open Classical in
noncomputable abbrev lim (a:Sequence) : ℝ := if h: a.Convergent then h.choose else 0

abbrev Sequence.BoundedBy (a:Sequence) (M:ℝ) : Prop :=
  ∀ n, |a n| ≤ M

abbrev Sequence.IsBounded (a:Sequence) : Prop := ∃ M ≥ 0, a.BoundedBy M

abbrev _root_.EReal.IsFinite (x:EReal) : Prop := ∃ (y:Real), y = x

noncomputable abbrev Sequence.sup (a:Sequence) : EReal := sSup { x | ∃ n ≥ a.m, x = a n }

noncomputable abbrev Sequence.inf (a:Sequence) : EReal := sInf { x | ∃ n ≥ a.m, x = a n }

abbrev Sequence.BddAboveBy (a:Sequence) (M:ℝ) : Prop := ∀ n ≥ a.m, a n ≤ M

abbrev Sequence.BddAbove (a:Sequence) : Prop := ∃ M, a.BddAboveBy M

abbrev Sequence.BddBelowBy (a:Sequence) (M:ℝ) : Prop := ∀ n ≥ a.m, a n ≥ M

abbrev Sequence.BddBelow (a:Sequence) : Prop := ∃ M, a.BddBelowBy M

theorem Sequence.bounded_iff (a:Sequence) : a.IsBounded ↔ a.BddAbove ∧ a.BddBelow := by
  sorry

theorem Sequence.sup_of_bounded {a:Sequence} (h: a.IsBounded) : a.sup.IsFinite := by
  sorry

theorem Sequence.inf_of_bounded {a:Sequence} (h: a.IsBounded) : a.inf.IsFinite := by
  sorry

theorem Sequence.ge_inf {a:Sequence} {n:ℤ} (hn: n ≥ a.m) : a n ≥ a.inf := by
  sorry

theorem Sequence.inf_ge_lower {a:Sequence} {M:EReal} (h: ∀ n ≥ a.m, a n ≥ M) : a.inf ≥ M := by
  sorry

theorem Sequence.exists_between_gt_inf {a:Sequence} {y:EReal} (h: y > a.inf ) :
    ∃ n ≥ a.m, y > a n ∧ a n ≥ a.inf := by
  sorry

abbrev Sequence.IsMonotone (a:Sequence) : Prop := ∀ n ≥ a.m, a (n+1) ≥ a n

abbrev Sequence.IsAntitone (a:Sequence) : Prop := ∀ n ≥ a.m, a (n+1) ≤ a n

theorem Sequence.convergent_of_antitone {a:Sequence} (hbound: a.BddBelow) (hmono: a.IsAntitone) :
    a.Convergent := by
  sorry

theorem Sequence.lim_of_antitone {a:Sequence} (hbound: a.BddBelow) (hmono: a.IsAntitone) :
    lim a = a.inf := by
  sorry

theorem Sequence.convergent_iff_bounded_of_monotone {a:Sequence} (ha: a.IsMonotone) :
    a.Convergent ↔ a.IsBounded := by
  sorry

theorem Sequence.bounded_iff_convergent_of_antitone {a:Sequence} (ha: a.IsAntitone) :
    a.Convergent ↔ a.IsBounded := by
  sorry

noncomputable abbrev Example_6_3_9 (n:ℕ) := ⌊ Real.pi * 10^n ⌋ / (10:ℝ)^n

theorem lim_of_exp {x:ℝ} (hpos: 0 < x) (hbound: x < 1) :
    ((fun (n:ℕ) ↦ x^n):Sequence).Convergent ∧ lim ((fun (n:ℕ) ↦ x^n):Sequence) = 0 := by
  sorry
-- </CONTEXT>
-- <TARGET_THEOREM>
theorem Exercise_6_3_1a : ((fun (n:ℕ) ↦ 1/((n:ℝ)+1)):Sequence).sup = 1 := by
-- </TARGET_THEOREM>

end Chapter6