238 lines
9.4 KiB
Text
238 lines
9.4 KiB
Text
/-
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Copyright (c) 2015 Jeremy Avigad. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Author: Jeremy Avigad
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Metric spaces.
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-/
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import data.real.division
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open real eq.ops classical
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structure metric_space [class] (M : Type) : Type :=
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(dist : M → M → ℝ)
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(dist_self : ∀ x : M, dist x x = 0)
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(eq_of_dist_eq_zero : ∀ {x y : M}, dist x y = 0 → x = y)
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(dist_comm : ∀ x y : M, dist x y = dist y x)
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(dist_triangle : ∀ x y z : M, dist x y + dist y z ≥ dist x z)
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namespace metric_space
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section metric_space_M
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variables {M : Type} [strucM : metric_space M]
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include strucM
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proposition dist_eq_zero_iff (x y : M) : dist x y = 0 ↔ x = y :=
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iff.intro eq_of_dist_eq_zero (suppose x = y, this ▸ !dist_self)
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proposition dist_nonneg (x y : M) : 0 ≤ dist x y :=
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have dist x y + dist y x ≥ 0, by rewrite -(dist_self x); apply dist_triangle,
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have 2 * dist x y ≥ 0, using this,
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by krewrite [-real.one_add_one, right_distrib, +one_mul, dist_comm at {2}]; apply this,
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nonneg_of_mul_nonneg_left this two_pos
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proposition dist_pos_of_ne {x y : M} (H : x ≠ y) : dist x y > 0 :=
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lt_of_le_of_ne !dist_nonneg (suppose 0 = dist x y, H (iff.mp !dist_eq_zero_iff this⁻¹))
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proposition eq_of_forall_dist_le {x y : M} (H : ∀ ε, ε > 0 → dist x y ≤ ε) : x = y :=
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eq_of_dist_eq_zero (eq_zero_of_nonneg_of_forall_le !dist_nonneg H)
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open nat
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/- convergence of a sequence -/
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definition converges_to_seq (X : ℕ → M) (y : M) : Prop :=
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∀ ⦃ε : ℝ⦄, ε > 0 → ∃ N : ℕ, ∀ ⦃n⦄, n ≥ N → dist (X n) y < ε
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-- the same, with ≤ in place of <; easier to prove, harder to use
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definition converges_to_seq.intro {X : ℕ → M} {y : M}
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(H : ∀ ⦃ε : ℝ⦄, ε > 0 → ∃ N : ℕ, ∀ {n}, n ≥ N → dist (X n) y ≤ ε) :
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converges_to_seq X y :=
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take ε, assume epos : ε > 0,
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have e2pos : ε / 2 > 0, from div_pos_of_pos_of_pos `ε > 0` two_pos,
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obtain N HN, from H e2pos,
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exists.intro N
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(take n, suppose n ≥ N,
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calc
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dist (X n) y ≤ ε / 2 : HN _ `n ≥ N`
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... < ε : div_two_lt_of_pos epos)
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notation X `⟶` y `in` `ℕ` := converges_to_seq X y
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definition converges_seq [class] (X : ℕ → M) : Prop := ∃ y, X ⟶ y in ℕ
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noncomputable definition limit_seq (X : ℕ → M) [H : converges_seq X] : M := some H
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proposition converges_to_limit_seq (X : ℕ → M) [H : converges_seq X] :
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(X ⟶ limit_seq X in ℕ) :=
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some_spec H
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proposition converges_to_seq_unique {X : ℕ → M} {y₁ y₂ : M}
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(H₁ : X ⟶ y₁ in ℕ) (H₂ : X ⟶ y₂ in ℕ) : y₁ = y₂ :=
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eq_of_forall_dist_le
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(take ε, suppose ε > 0,
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have e2pos : ε / 2 > 0, from div_pos_of_pos_of_pos `ε > 0` two_pos,
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obtain N₁ (HN₁ : ∀ {n}, n ≥ N₁ → dist (X n) y₁ < ε / 2), from H₁ e2pos,
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obtain N₂ (HN₂ : ∀ {n}, n ≥ N₂ → dist (X n) y₂ < ε / 2), from H₂ e2pos,
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let N := max N₁ N₂ in
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have dN₁ : dist (X N) y₁ < ε / 2, from HN₁ !le_max_left,
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have dN₂ : dist (X N) y₂ < ε / 2, from HN₂ !le_max_right,
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have dist y₁ y₂ < ε, from calc
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dist y₁ y₂ ≤ dist y₁ (X N) + dist (X N) y₂ : dist_triangle
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... = dist (X N) y₁ + dist (X N) y₂ : dist_comm
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... < ε / 2 + ε / 2 : add_lt_add dN₁ dN₂
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... = ε : add_halves,
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show dist y₁ y₂ ≤ ε, from le_of_lt this)
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proposition eq_limit_of_converges_to_seq {X : ℕ → M} {y : M} (H : X ⟶ y in ℕ) :
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y = @limit_seq M _ X (exists.intro y H) :=
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converges_to_seq_unique H (@converges_to_limit_seq M _ X (exists.intro y H))
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proposition converges_to_seq_constant (y : M) : (λn, y) ⟶ y in ℕ :=
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take ε, assume egt0 : ε > 0,
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exists.intro 0
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(take n, suppose n ≥ 0,
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calc
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dist y y = 0 : !dist_self
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... < ε : egt0)
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proposition converges_to_seq_offset {X : ℕ → M} {y : M} (k : ℕ) (H : X ⟶ y in ℕ) :
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(λ n, X (n + k)) ⟶ y in ℕ :=
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take ε, suppose ε > 0,
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obtain N HN, from H `ε > 0`,
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exists.intro N
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(take n : ℕ, assume ngtN : n ≥ N,
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show dist (X (n + k)) y < ε, from HN (n + k) (le.trans ngtN !le_add_right))
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proposition converges_to_seq_offset_left {X : ℕ → M} {y : M} (k : ℕ) (H : X ⟶ y in ℕ) :
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(λ n, X (k + n)) ⟶ y in ℕ :=
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have aux : (λ n, X (k + n)) = (λ n, X (n + k)), from funext (take n, by rewrite add.comm),
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by+ rewrite aux; exact converges_to_seq_offset k H
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proposition converges_to_seq_offset_succ {X : ℕ → M} {y : M} (H : X ⟶ y in ℕ) :
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(λ n, X (succ n)) ⟶ y in ℕ :=
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converges_to_seq_offset 1 H
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proposition converges_to_seq_of_converges_to_seq_offset
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{X : ℕ → M} {y : M} {k : ℕ} (H : (λ n, X (n + k)) ⟶ y in ℕ) :
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X ⟶ y in ℕ :=
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take ε, suppose ε > 0,
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obtain N HN, from H `ε > 0`,
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exists.intro (N + k)
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(take n : ℕ, assume nge : n ≥ N + k,
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have n - k ≥ N, from nat.le_sub_of_add_le nge,
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have dist (X (n - k + k)) y < ε, from HN (n - k) this,
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show dist (X n) y < ε, using this,
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by rewrite [(nat.sub_add_cancel (le.trans !le_add_left nge)) at this]; exact this)
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proposition converges_to_seq_of_converges_to_seq_offset_left
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{X : ℕ → M} {y : M} {k : ℕ} (H : (λ n, X (k + n)) ⟶ y in ℕ) :
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X ⟶ y in ℕ :=
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have aux : (λ n, X (k + n)) = (λ n, X (n + k)), from funext (take n, by rewrite add.comm),
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by+ rewrite aux at H; exact converges_to_seq_of_converges_to_seq_offset H
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proposition converges_to_seq_of_converges_to_seq_offset_succ
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{X : ℕ → M} {y : M} (H : (λ n, X (succ n)) ⟶ y in ℕ) :
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X ⟶ y in ℕ :=
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@converges_to_seq_of_converges_to_seq_offset M strucM X y 1 H
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proposition converges_to_seq_offset_iff (X : ℕ → M) (y : M) (k : ℕ) :
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((λ n, X (n + k)) ⟶ y in ℕ) ↔ (X ⟶ y in ℕ) :=
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iff.intro converges_to_seq_of_converges_to_seq_offset !converges_to_seq_offset
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proposition converges_to_seq_offset_left_iff (X : ℕ → M) (y : M) (k : ℕ) :
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((λ n, X (k + n)) ⟶ y in ℕ) ↔ (X ⟶ y in ℕ) :=
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iff.intro converges_to_seq_of_converges_to_seq_offset_left !converges_to_seq_offset_left
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proposition converges_to_seq_offset_succ_iff (X : ℕ → M) (y : M) :
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((λ n, X (succ n)) ⟶ y in ℕ) ↔ (X ⟶ y in ℕ) :=
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iff.intro converges_to_seq_of_converges_to_seq_offset_succ !converges_to_seq_offset_succ
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/- cauchy sequences -/
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definition cauchy (X : ℕ → M) : Prop :=
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∀ ε : ℝ, ε > 0 → ∃ N, ∀ m n, m ≥ N → n ≥ N → dist (X m) (X n) < ε
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proposition cauchy_of_converges_seq (X : ℕ → M) [H : converges_seq X] : cauchy X :=
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take ε, suppose ε > 0,
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obtain y (Hy : converges_to_seq X y), from H,
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have e2pos : ε / 2 > 0, from div_pos_of_pos_of_pos `ε > 0` two_pos,
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obtain N₁ (HN₁ : ∀ {n}, n ≥ N₁ → dist (X n) y < ε / 2), from Hy e2pos,
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obtain N₂ (HN₂ : ∀ {n}, n ≥ N₂ → dist (X n) y < ε / 2), from Hy e2pos,
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let N := max N₁ N₂ in
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exists.intro N
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(take m n, suppose m ≥ N, suppose n ≥ N,
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have m ≥ N₁, from le.trans !le_max_left `m ≥ N`,
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have n ≥ N₂, from le.trans !le_max_right `n ≥ N`,
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have dN₁ : dist (X m) y < ε / 2, from HN₁ `m ≥ N₁`,
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have dN₂ : dist (X n) y < ε / 2, from HN₂ `n ≥ N₂`,
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show dist (X m) (X n) < ε, from calc
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dist (X m) (X n) ≤ dist (X m) y + dist y (X n) : dist_triangle
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... = dist (X m) y + dist (X n) y : dist_comm
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... < ε / 2 + ε / 2 : add_lt_add dN₁ dN₂
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... = ε : add_halves)
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end metric_space_M
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/- convergence of a function at a point -/
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section metric_space_M_N
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variables {M N : Type} [strucM : metric_space M] [strucN : metric_space N]
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include strucM strucN
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definition converges_to_at (f : M → N) (y : N) (x : M) :=
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∀ ⦃ε⦄, ε > 0 → ∃ δ, δ > 0 ∧ ∀ x', x ≠ x' ∧ dist x x' < δ → dist (f x') y < ε
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notation f `⟶` y `at` x := converges_to_at f y x
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definition converges_at [class] (f : M → N) (x : M) :=
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∃ y, converges_to_at f y x
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noncomputable definition limit_at (f : M → N) (x : M) [H : converges_at f x] : N :=
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some H
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proposition converges_to_limit_at (f : M → N) (x : M) [H : converges_at f x] :
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(f ⟶ limit_at f x at x) :=
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some_spec H
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definition continuous_at (f : M → N) (x : M) := converges_to_at f (f x) x
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definition continuous (f : M → N) := ∀ x, continuous_at f x
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theorem continuous_at_spec {f : M → N} {x : M} (Hf : continuous_at f x) :
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∀ ⦃ε⦄, ε > 0 → ∃ δ, δ > 0 ∧ ∀ x', dist x x' < δ → dist (f x') (f x) < ε :=
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take ε, suppose ε > 0,
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obtain δ Hδ, from Hf this,
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exists.intro δ (and.intro
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(and.left Hδ)
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(take x', suppose dist x x' < δ,
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if Heq : x = x' then
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by rewrite [Heq, dist_self]; assumption
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else
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(suffices dist x x' < δ, from and.right Hδ x' (and.intro Heq this),
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this)))
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theorem image_seq_converges_of_converges [instance] (X : ℕ → M) [HX : converges_seq X] {f : M → N} (Hf : continuous f) :
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converges_seq (λ n, f (X n)) :=
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begin
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cases HX with xlim Hxlim,
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existsi f xlim,
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rewrite ↑converges_to_seq at *,
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intros ε Hε,
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let Hcont := Hf xlim Hε,
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cases Hcont with δ Hδ,
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cases Hxlim (and.left Hδ) with B HB,
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existsi B,
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intro n Hn,
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cases em (xlim = X n),
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rewrite [a, dist_self],
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assumption,
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apply and.right Hδ,
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split,
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exact a,
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rewrite dist_comm,
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apply HB Hn
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end
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end metric_space_M_N
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end metric_space
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