26 lines
1.5 KiB
Text
26 lines
1.5 KiB
Text
import macros
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-- Well-founded relation definition
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-- We are essentially saying that a relation R is well-founded
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-- if every non-empty "set" P, has a R-minimal element
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definition wf {A : (Type U)} (R : A → A → Bool) : Bool
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:= ∀ P, (∃ w, P w) → ∃ min, P min ∧ ∀ b, R b min → ¬ P b
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-- Well-founded induction theorem
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theorem wf_induction {A : (Type U)} {R : A → A → Bool} {P : A → Bool} (Hwf : wf R) (iH : ∀ x, (∀ y, R y x → P y) → P x)
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: ∀ x, P x
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:= by_contradiction (assume N : ¬ ∀ x, P x,
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obtain (w : A) (Hw : ¬ P w), from not_forall_elim N,
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-- The main "trick" is to define Q x as ¬ P x.
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-- Since R is well-founded, there must be a R-minimal element r s.t. Q r (which is ¬ P r)
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let Q : A → Bool := λ x, ¬ P x,
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Qw : ∃ w, Q w := exists_intro w Hw,
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Qwf : ∃ min, Q min ∧ ∀ b, R b min → ¬ Q b := Hwf Q Qw
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in obtain (r : A) (Hr : Q r ∧ ∀ b, R b r → ¬ Q b), from Qwf,
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-- Using the inductive hypothesis iH and Hr, we show P r, and derive the contradiction.
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let s1 : ∀ b, R b r → P b := take b : A, assume H : R b r,
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-- We are using Hr to derive ¬ ¬ P b
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not_not_elim (and_elimr Hr b H),
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s2 : P r := iH r s1,
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s3 : ¬ P r := and_eliml Hr
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in absurd s2 s3)
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