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(***********************************************************************)
(* v * The Coq Proof Assistant / The Coq Development Team *)
(* <O___,, * INRIA-Rocquencourt & LRI-CNRS-Orsay *)
(* \VV/ *************************************************************)
(* // * This file is distributed under the terms of the *)
(* * GNU Lesser General Public License Version 2.1 *)
(***********************************************************************)
(* $Id$ *)
Require ZArith.
Require Export Wf_nat.
(** Well-founded relations on Z. *)
(** We define the following family of relations on [Z x Z]:
[x (Zwf c) y] iff [c <= x < y]
*)
Definition Zwf := [c:Z][x,y:Z] `c <= x` /\ `c <= y` /\ `x < y`.
(** and we prove that [(Zwf c)] is well founded *)
Section wf_proof.
Variable c : Z.
(** The proof of well-foundness is classic: we do the proof by induction
on a measure in nat, which is here [|x-c|] *)
Local f := [z:Z](absolu (Zminus z c)).
Lemma Zwf_well_founded : (well_founded Z (Zwf c)).
Proof.
Apply well_founded_lt_compat with f:=f.
Unfold Zwf f.
Intros.
Apply absolu_lt.
Unfold Zminus. Split.
Apply Zle_left; Intuition.
Rewrite (Zplus_sym x `-c`). Rewrite (Zplus_sym y `-c`).
Apply Zlt_reg_l; Intuition.
Save.
End wf_proof.
Hints Resolve Zwf_well_founded : datatypes v62.
(** We also define the other family of relations:
[x (Zwf_up c) y] iff [y < x <= c]
*)
Definition Zwf_up := [c:Z][x,y:Z] `y < x <= c`.
(** and we prove that [(Zwf_up c)] is well founded *)
Section wf_proof_up.
Variable c : Z.
(** The proof of well-foundness is classic: we do the proof by induction
on a measure in nat, which is here [|c-x|] *)
Local f := [z:Z](absolu (Zminus c z)).
Lemma Zwf_up_well_founded : (well_founded Z (Zwf_up c)).
Proof.
Apply well_founded_lt_compat with f:=f.
Unfold Zwf_up f.
Intros.
Apply absolu_lt.
Unfold Zminus. Split.
Apply Zle_left; Intuition.
Apply Zlt_reg_l; Unfold Zlt; Rewrite <- Zcompare_Zopp; Intuition.
Save.
End wf_proof_up.
Hints Resolve Zwf_up_well_founded : datatypes v62.
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