251 lines
8.5 KiB
Text
251 lines
8.5 KiB
Text
/-
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Copyright (c) 2014 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, Andrew Zipperer, Haitao Zhang
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Functions between subsets of finite types.
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-/
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import .basic
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import algebra.function
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open function eq.ops
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namespace set
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variables {X Y Z : Type}
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abbreviation eq_on (f1 f2 : X → Y) (a : set X) : Prop :=
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∀₀ x ∈ a, f1 x = f2 x
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/- image -/
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definition image (f : X → Y) (a : set X) : set Y := {y : Y | ∃x, x ∈ a ∧ f x = y}
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notation f `'[`:max a `]` := image f a
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theorem image_eq_image_of_eq_on {f1 f2 : X → Y} {a : set X} (H1 : eq_on f1 f2 a) :
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f1 '[a] = f2 '[a] :=
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setext (take y, iff.intro
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(assume H2,
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obtain x (H3 : x ∈ a ∧ f1 x = y), from H2,
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have H4 : x ∈ a, from and.left H3,
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have H5 : f2 x = y, from (H1 H4)⁻¹ ⬝ and.right H3,
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exists.intro x (and.intro H4 H5))
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(assume H2,
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obtain x (H3 : x ∈ a ∧ f2 x = y), from H2,
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have H4 : x ∈ a, from and.left H3,
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have H5 : f1 x = y, from (H1 H4) ⬝ and.right H3,
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exists.intro x (and.intro H4 H5)))
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theorem in_image {f : X → Y} {a : set X} {x : X} {y : Y}
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(H1 : x ∈ a) (H2 : f x = y) : y ∈ f '[a] :=
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exists.intro x (and.intro H1 H2)
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lemma image_compose (f : Y → Z) (g : X → Y) (a : set X) : (f ∘ g) '[a] = f '[g '[a]] :=
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setext (take z,
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iff.intro
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(assume Hz : z ∈ (f ∘ g) '[a],
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obtain x (Hx₁ : x ∈ a) (Hx₂ : f (g x) = z), from Hz,
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have Hgx : g x ∈ g '[a], from in_image Hx₁ rfl,
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show z ∈ f '[g '[a]], from in_image Hgx Hx₂)
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(assume Hz : z ∈ f '[g '[a]],
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obtain y (Hy₁ : y ∈ g '[a]) (Hy₂ : f y = z), from Hz,
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obtain x (Hz₁ : x ∈ a) (Hz₂ : g x = y), from Hy₁,
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show z ∈ (f ∘ g) '[a], from in_image Hz₁ (Hz₂⁻¹ ▸ Hy₂)))
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/- maps to -/
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definition maps_to [reducible] (f : X → Y) (a : set X) (b : set Y) : Prop := ∀⦃x⦄, x ∈ a → f x ∈ b
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theorem maps_to_of_eq_on {f1 f2 : X → Y} {a : set X} {b : set Y} (eq_on_a : eq_on f1 f2 a)
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(maps_to_f1 : maps_to f1 a b) :
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maps_to f2 a b :=
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take x,
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assume xa : x ∈ a,
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have H : f1 x ∈ b, from maps_to_f1 xa,
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show f2 x ∈ b, from eq_on_a xa ▸ H
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theorem maps_to_compose {g : Y → Z} {f : X → Y} {a : set X} {b : set Y} {c : set Z}
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(H1 : maps_to g b c) (H2 : maps_to f a b) : maps_to (g ∘ f) a c :=
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take x, assume H : x ∈ a, H1 (H2 H)
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/- injectivity -/
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definition inj_on [reducible] (f : X → Y) (a : set X) : Prop :=
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∀⦃x1 x2 : X⦄, x1 ∈ a → x2 ∈ a → f x1 = f x2 → x1 = x2
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theorem inj_on_of_eq_on {f1 f2 : X → Y} {a : set X} (eq_f1_f2 : eq_on f1 f2 a)
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(inj_f1 : inj_on f1 a) :
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inj_on f2 a :=
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take x1 x2 : X,
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assume ax1 : x1 ∈ a,
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assume ax2 : x2 ∈ a,
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assume H : f2 x1 = f2 x2,
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have H' : f1 x1 = f1 x2, from eq_f1_f2 ax1 ⬝ H ⬝ (eq_f1_f2 ax2)⁻¹,
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show x1 = x2, from inj_f1 ax1 ax2 H'
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theorem inj_on_compose {g : Y → Z} {f : X → Y} {a : set X} {b : set Y}
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(fab : maps_to f a b) (Hg : inj_on g b) (Hf: inj_on f a) :
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inj_on (g ∘ f) a :=
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take x1 x2 : X,
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assume x1a : x1 ∈ a,
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assume x2a : x2 ∈ a,
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have fx1b : f x1 ∈ b, from fab x1a,
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have fx2b : f x2 ∈ b, from fab x2a,
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assume H1 : g (f x1) = g (f x2),
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have H2 : f x1 = f x2, from Hg fx1b fx2b H1,
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show x1 = x2, from Hf x1a x2a H2
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theorem inj_on_of_inj_on_of_subset {f : X → Y} {a b : set X} (H1 : inj_on f b) (H2 : a ⊆ b) :
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inj_on f a :=
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take x1 x2 : X, assume (x1a : x1 ∈ a) (x2a : x2 ∈ a),
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assume H : f x1 = f x2,
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show x1 = x2, from H1 (H2 x1a) (H2 x2a) H
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/- surjectivity -/
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definition surj_on [reducible] (f : X → Y) (a : set X) (b : set Y) : Prop := b ⊆ f '[a]
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theorem surj_on_of_eq_on {f1 f2 : X → Y} {a : set X} {b : set Y} (eq_f1_f2 : eq_on f1 f2 a)
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(surj_f1 : surj_on f1 a b) :
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surj_on f2 a b :=
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take y, assume H : y ∈ b,
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obtain x (H1 : x ∈ a ∧ f1 x = y), from surj_f1 H,
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have H2 : x ∈ a, from and.left H1,
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have H3 : f2 x = y, from (eq_f1_f2 H2)⁻¹ ⬝ and.right H1,
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exists.intro x (and.intro H2 H3)
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theorem surj_on_compose {g : Y → Z} {f : X → Y} {a : set X} {b : set Y} {c : set Z}
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(Hg : surj_on g b c) (Hf: surj_on f a b) :
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surj_on (g ∘ f) a c :=
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take z,
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assume zc : z ∈ c,
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obtain y (H1 : y ∈ b ∧ g y = z), from Hg zc,
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obtain x (H2 : x ∈ a ∧ f x = y), from Hf (and.left H1),
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show ∃x, x ∈ a ∧ g (f x) = z, from
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exists.intro x
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(and.intro
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(and.left H2)
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(calc
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g (f x) = g y : {and.right H2}
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... = z : and.right H1))
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/- bijectivity -/
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definition bij_on [reducible] (f : X → Y) (a : set X) (b : set Y) : Prop :=
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maps_to f a b ∧ inj_on f a ∧ surj_on f a b
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theorem bij_on_of_eq_on {f1 f2 : X → Y} {a : set X} {b : set Y} (eqf : eq_on f1 f2 a)
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(H : bij_on f1 a b) : bij_on f2 a b :=
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match H with and.intro Hmap (and.intro Hinj Hsurj) :=
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and.intro
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(maps_to_of_eq_on eqf Hmap)
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(and.intro
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(inj_on_of_eq_on eqf Hinj)
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(surj_on_of_eq_on eqf Hsurj))
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end
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theorem bij_on_compose {g : Y → Z} {f : X → Y} {a : set X} {b : set Y} {c : set Z}
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(Hg : bij_on g b c) (Hf: bij_on f a b) :
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bij_on (g ∘ f) a c :=
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match Hg with and.intro Hgmap (and.intro Hginj Hgsurj) :=
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match Hf with and.intro Hfmap (and.intro Hfinj Hfsurj) :=
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and.intro
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(maps_to_compose Hgmap Hfmap)
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(and.intro
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(inj_on_compose Hfmap Hginj Hfinj)
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(surj_on_compose Hgsurj Hfsurj))
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end
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end
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/- left inverse -/
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-- g is a left inverse to f on a
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definition left_inv_on [reducible] (g : Y → X) (f : X → Y) (a : set X) : Prop :=
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∀₀ x ∈ a, g (f x) = x
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theorem left_inv_on_of_eq_on_left {g1 g2 : Y → X} {f : X → Y} {a : set X} {b : set Y}
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(fab : maps_to f a b) (eqg : eq_on g1 g2 b) (H : left_inv_on g1 f a) : left_inv_on g2 f a :=
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take x,
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assume xa : x ∈ a,
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calc
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g2 (f x) = g1 (f x) : (eqg (fab xa))⁻¹
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... = x : H xa
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theorem left_inv_on_of_eq_on_right {g : Y → X} {f1 f2 : X → Y} {a : set X}
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(eqf : eq_on f1 f2 a) (H : left_inv_on g f1 a) : left_inv_on g f2 a :=
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take x,
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assume xa : x ∈ a,
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calc
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g (f2 x) = g (f1 x) : {(eqf xa)⁻¹}
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... = x : H xa
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theorem inj_on_of_left_inv_on {g : Y → X} {f : X → Y} {a : set X} (H : left_inv_on g f a) :
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inj_on f a :=
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take x1 x2,
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assume x1a : x1 ∈ a,
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assume x2a : x2 ∈ a,
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assume H1 : f x1 = f x2,
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calc
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x1 = g (f x1) : H x1a
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... = g (f x2) : H1
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... = x2 : H x2a
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theorem left_inv_on_compose {f' : Y → X} {g' : Z → Y} {g : Y → Z} {f : X → Y}
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{a : set X} {b : set Y} (fab : maps_to f a b)
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(Hf : left_inv_on f' f a) (Hg : left_inv_on g' g b) : left_inv_on (f' ∘ g') (g ∘ f) a :=
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take x : X,
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assume xa : x ∈ a,
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have fxb : f x ∈ b, from fab xa,
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calc
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f' (g' (g (f x))) = f' (f x) : Hg fxb
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... = x : Hf xa
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/- right inverse -/
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-- g is a right inverse to f on a
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definition right_inv_on [reducible] (g : Y → X) (f : X → Y) (b : set Y) : Prop :=
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left_inv_on f g b
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theorem right_inv_on_of_eq_on_left {g1 g2 : Y → X} {f : X → Y} {a : set X} {b : set Y}
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(eqg : eq_on g1 g2 b) (H : right_inv_on g1 f b) : right_inv_on g2 f b :=
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left_inv_on_of_eq_on_right eqg H
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theorem right_inv_on_of_eq_on_right {g : Y → X} {f1 f2 : X → Y} {a : set X} {b : set Y}
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(gba : maps_to g b a) (eqf : eq_on f1 f2 a) (H : right_inv_on g f1 b) : right_inv_on g f2 b :=
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left_inv_on_of_eq_on_left gba eqf H
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theorem surj_on_of_right_inv_on {g : Y → X} {f : X → Y} {a : set X} {b : set Y}
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(gba : maps_to g b a) (H : right_inv_on g f b) :
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surj_on f a b :=
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take y,
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assume yb : y ∈ b,
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have gya : g y ∈ a, from gba yb,
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have H1 : f (g y) = y, from H yb,
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exists.intro (g y) (and.intro gya H1)
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theorem right_inv_on_compose {f' : Y → X} {g' : Z → Y} {g : Y → Z} {f : X → Y}
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{c : set Z} {b : set Y} (g'cb : maps_to g' c b)
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(Hf : right_inv_on f' f b) (Hg : right_inv_on g' g c) : right_inv_on (f' ∘ g') (g ∘ f) c :=
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left_inv_on_compose g'cb Hg Hf
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theorem eq_on_of_left_inv_of_right_inv {g1 g2 : Y → X} {f : X → Y} {a : set X} {b : set Y}
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(g2ba : maps_to g2 b a) (Hg1 : left_inv_on g1 f a) (Hg2 : right_inv_on g2 f b) : eq_on g1 g2 b :=
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take y,
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assume yb : y ∈ b,
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calc
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g1 y = g1 (f (g2 y)) : {(Hg2 yb)⁻¹}
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... = g2 y : Hg1 (g2ba yb)
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/- inverses -/
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-- g is an inverse to f viewed as a map from a to b
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definition inv_on [reducible] (g : Y → X) (f : X → Y) (a : set X) (b : set Y) : Prop :=
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left_inv_on g f a ∧ right_inv_on g f b
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theorem bij_on_of_inv_on {g : Y → X} {f : X → Y} {a : set X} {b : set Y} (fab : maps_to f a b)
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(gba : maps_to g b a) (H : inv_on g f a b) : bij_on f a b :=
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and.intro fab
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(and.intro
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(inj_on_of_left_inv_on (and.left H))
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(surj_on_of_right_inv_on gba (and.right H)))
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end set
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