# Library Coq.Init.Wf

# This module proves the validity of

- well-founded recursion (also known as course of values)
- well-founded induction

from a well-founded ordering on a given set
Set Implicit Arguments.
Require Import Notations.
Require Import Logic.
Require Import Datatypes.
Well-founded induction principle on Prop
Section Well_founded.
Variable A : Type.
Variable R : A -> A -> Prop.
The accessibility predicate is defined to be non-informative (Acc_rect is automatically defined because Acc is a singleton type)
Inductive Acc (x: A) : Prop :=
Acc_intro : (forall y:A, R y x -> Acc y) -> Acc x.
Lemma Acc_inv : forall x:A, Acc x -> forall y:A, R y x -> Acc y.
Global Implicit Arguments Acc_inv [x y] [x].
A relation is well-founded if every element is accessible
Definition well_founded := forall a:A, Acc a.
Well-founded induction on Set and Prop
Hypothesis Rwf : well_founded.
Theorem well_founded_induction_type :
forall P:A -> Type,
(forall x:A, (forall y:A, R y x -> P y) -> P x) -> forall a:A, P a.
Theorem well_founded_induction :
forall P:A -> Set,
(forall x:A, (forall y:A, R y x -> P y) -> P x) -> forall a:A, P a.
Theorem well_founded_ind :
forall P:A -> Prop,
(forall x:A, (forall y:A, R y x -> P y) -> P x) -> forall a:A, P a.
Well-founded fixpoints
Section FixPoint.
Variable P : A -> Type.
Variable F : forall x:A, (forall y:A, R y x -> P y) -> P x.
Fixpoint Fix_F (x:A) (a:Acc x) : P x :=
F (fun (y:A) (h:R y x) => Fix_F (Acc_inv a h)).
Scheme Acc_inv_dep := Induction for Acc Sort Prop.
Lemma Fix_F_eq :
forall (x:A) (r:Acc x),
F (fun (y:A) (p:R y x) => Fix_F (x:=y) (Acc_inv r p)) = Fix_F (x:=x) r.
Definition Fix (x:A) := Fix_F (Rwf x).
Proof that well_founded_induction satisfies the fixpoint equation. It requires an extra property of the functional
Hypothesis
F_ext :
forall (x:A) (f g:forall y:A, R y x -> P y),
(forall (y:A) (p:R y x), f y p = g y p) -> F f = F g.
Lemma Fix_F_inv : forall (x:A) (r s:Acc x), Fix_F r = Fix_F s.
Lemma Fix_eq : forall x:A, Fix x = F (fun (y:A) (p:R y x) => Fix y).
End FixPoint.
End Well_founded.
Well-founded fixpoints over pairs
Section Well_founded_2.
Variables A B : Type.
Variable R : A * B -> A * B -> Prop.
Variable P : A -> B -> Type.
Section FixPoint_2.
Variable
F :
forall (x:A) (x':B),
(forall (y:A) (y':B), R ( y , y' ) ( x , x' ) -> P y y') -> P x x'.
Fixpoint Fix_F_2 (x:A) (x':B) (a:Acc R ( x , x' )) : P x x' :=
F
(fun (y:A) (y':B) (h:R ( y , y' ) ( x , x' )) =>
Fix_F_2 (x:=y) (x':=y') (Acc_inv a ( y , y' ) h)).
End FixPoint_2.
Hypothesis Rwf : well_founded R.
Theorem well_founded_induction_type_2 :
(forall (x:A) (x':B),
(forall (y:A) (y':B), R ( y , y' ) ( x , x' ) -> P y y') -> P x x') ->
forall (a:A) (b:B), P a b.
End Well_founded_2.
Notation Acc_iter := Fix_F (only parsing). Notation Acc_iter_2 := Fix_F_2 (only parsing).