89 lines
2.5 KiB
Text
89 lines
2.5 KiB
Text
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/-
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Copyright (c) 2017 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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Authors: Jeremy Avigad
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-/
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import types.trunc
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open funext eq trunc is_trunc
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definition set (X : Type) := X → Prop
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section
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open trunc_index
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definition propext {p q : Prop} (h : p ↔ q) : p = q :=
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tua (equiv_of_iff_of_is_prop h)
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end
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definition tempty {n : trunc_index} : (n.+1)-Type := trunctype.mk empty _
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namespace set
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variable {X : Type}
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/- membership and subset -/
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definition mem (x : X) (a : set X) := a x
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infix ∈ := mem
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notation a ∉ b := ¬ mem a b
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theorem ext {a b : set X} (H : ∀x, x ∈ a ↔ x ∈ b) : a = b :=
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eq_of_homotopy (take x, propext (H x))
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definition subset (a b : set X) : Prop := Prop.mk (∀⦃x⦄, x ∈ a → x ∈ b) _
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infix ⊆ := subset
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definition superset (s t : set X) : Prop := t ⊆ s
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infix ⊇ := superset
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theorem subset.refl (a : set X) : a ⊆ a := take x, assume H, H
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theorem subset.trans {a b c : set X} (subab : a ⊆ b) (subbc : b ⊆ c) : a ⊆ c :=
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take x, assume ax, subbc (subab ax)
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theorem subset.antisymm {a b : set X} (h₁ : a ⊆ b) (h₂ : b ⊆ a) : a = b :=
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ext (λ x, iff.intro (λ ina, h₁ ina) (λ inb, h₂ inb))
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-- an alterantive name
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theorem eq_of_subset_of_subset {a b : set X} (h₁ : a ⊆ b) (h₂ : b ⊆ a) : a = b :=
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subset.antisymm h₁ h₂
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theorem mem_of_subset_of_mem {s₁ s₂ : set X} {a : X} : s₁ ⊆ s₂ → a ∈ s₁ → a ∈ s₂ :=
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assume h₁ h₂, h₁ _ h₂
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/- empty set -/
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definition empty : set X := λx, tempty
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notation `∅` := empty
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theorem not_mem_empty (x : X) : ¬ (x ∈ ∅) :=
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assume H : x ∈ ∅, H
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theorem mem_empty_eq (x : X) : x ∈ ∅ = tempty := rfl
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/-
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theorem eq_empty_of_forall_not_mem {s : set X} (H : ∀ x, x ∉ s) : s = ∅ :=
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ext (take x, iff.intro
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(assume xs, absurd xs (H x))
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(assume xe, absurd xe (not_mem_empty _)))
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theorem ne_empty_of_mem {s : set X} {x : X} (H : x ∈ s) : s ≠ ∅ :=
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begin intro Hs, rewrite Hs at H, apply not_mem_empty _ H end
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theorem empty_subset (s : set X) : ∅ ⊆ s :=
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take x, assume H, false.elim H
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theorem eq_empty_of_subset_empty {s : set X} (H : s ⊆ ∅) : s = ∅ :=
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subset.antisymm H (empty_subset s)
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theorem subset_empty_iff (s : set X) : s ⊆ ∅ ↔ s = ∅ :=
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iff.intro eq_empty_of_subset_empty (take xeq, by rewrite xeq; apply subset.refl ∅)
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lemma bounded_forall_empty_iff {P : X → Prop} :
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(∀₀x∈∅, P x) ↔ true :=
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iff.intro (take H, true.intro) (take H, by contradiction)
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-/
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end set
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