From 7497d4129775d15cdce862a0ac681c6400aabe54 Mon Sep 17 00:00:00 2001 From: Hugo Herbelin Date: Thu, 6 Oct 2016 07:02:24 +0200 Subject: Logic library: Adding a characterization of excluded-middle in term of choice of a representative in a partition of bool. Also move a result about propositional extensionality from ClassicalFacts.v to PropExtensionalityFacts.v, generalizing it by symmetry. Also spotting typos (thanks to Théo). --- doc/stdlib/index-list.html.template | 1 + theories/Logic/ClassicalFacts.v | 66 ++++++++++++++++++- theories/Logic/PropExtensionalityFacts.v | 109 +++++++++++++++++++++++++++++++ 3 files changed, 173 insertions(+), 3 deletions(-) create mode 100644 theories/Logic/PropExtensionalityFacts.v diff --git a/doc/stdlib/index-list.html.template b/doc/stdlib/index-list.html.template index 9216c81fcd..4a685a3b85 100644 --- a/doc/stdlib/index-list.html.template +++ b/doc/stdlib/index-list.html.template @@ -55,6 +55,7 @@ through the Require Import command.

theories/Logic/Description.v theories/Logic/Epsilon.v theories/Logic/IndefiniteDescription.v + theories/Logic/PropExtensionalityFacts.v theories/Logic/FunctionalExtensionality.v theories/Logic/ExtensionalityFacts.v theories/Logic/WeakFan.v diff --git a/theories/Logic/ClassicalFacts.v b/theories/Logic/ClassicalFacts.v index afd64efdf8..021408a37b 100644 --- a/theories/Logic/ClassicalFacts.v +++ b/theories/Logic/ClassicalFacts.v @@ -34,8 +34,11 @@ Table of contents: 3 3. Independence of general premises and drinker's paradox -4. Classical logic and principle of unrestricted minimization +4. Principles equivalent to classical logic +4.1 Classical logic = principle of unrestricted minimization + +4.2 Classical logic = choice of representatives in a partition of bool *) (************************************************************************) @@ -94,12 +97,14 @@ Qed. (** A weakest form of propositional extensionality: extensionality for provable propositions only *) +Require Import PropExtensionalityFacts. + Definition provable_prop_extensionality := forall A:Prop, A -> A = True. Lemma provable_prop_ext : prop_extensionality -> provable_prop_extensionality. Proof. - intros Ext A Ha; apply Ext; split; trivial. + exact PropExt_imp_ProvPropExt. Qed. (************************************************************************) @@ -516,7 +521,7 @@ End Weak_proof_irrelevance_CCI. (** ** Weak excluded-middle *) (** The weak classical logic based on [~~A \/ ~A] is referred to with - name KC in {[ChagrovZakharyaschev97]] + name KC in [[ChagrovZakharyaschev97]] [[ChagrovZakharyaschev97]] Alexander Chagrov and Michael Zakharyaschev, "Modal Logic", Clarendon Press, 1997. @@ -661,6 +666,8 @@ Proof. exists x0; exact Hnot. Qed. +(** * Axioms equivalent to classical logic *) + (** ** Principle of unrestricted minimization *) Require Import Coq.Arith.PeanoNat. @@ -736,3 +743,56 @@ Section Example_of_undecidable_predicate_with_the_minimization_property. Qed. End Example_of_undecidable_predicate_with_the_minimization_property. + +(** ** Choice of representatives in a partition of bool *) + +(** This is similar to Bell's "weak extensional selection principle" in [[Bell]] + + [[Bell]] John L. Bell, Choice principles in intuitionistic set theory, unpublished. +*) + +Require Import RelationClasses. + +Local Notation representative_boolean_partition := + (forall R:bool->bool->Prop, + Equivalence R -> exists f, forall x, R x (f x) /\ forall y, R x y -> f x = f y). + +Theorem representative_boolean_partition_imp_excluded_middle : + representative_boolean_partition -> excluded_middle. +Proof. + intros ReprFunChoice P. + pose (R (b1 b2 : bool) := b1 = b2 \/ P). + assert (Equivalence R). + { split. + - now left. + - destruct 1. now left. now right. + - destruct 1, 1; try now right. left; now transitivity y. } + destruct (ReprFunChoice R H) as (f,Hf). clear H. + destruct (Bool.bool_dec (f true) (f false)) as [Heq|Hneq]. + + left. + destruct (Hf false) as ([Hfalse|HP],_); try easy. + destruct (Hf true) as ([Htrue|HP],_); try easy. + congruence. + + right. intro HP. + destruct (Hf true) as (_,H). apply Hneq, H. now right. +Qed. + +Theorem excluded_middle_imp_representative_boolean_partition : + excluded_middle -> representative_boolean_partition. +Proof. + intros EM R H. + destruct (EM (R true false)). + - exists (fun _ => true). + intros []; firstorder. + - exists (fun b => b). + intro b. split. + + reflexivity. + + destruct b, y; intros HR; easy || now symmetry in HR. +Qed. + +Theorem excluded_middle_iff_representative_boolean_partition : + excluded_middle <-> representative_boolean_partition. +Proof. + split; auto using excluded_middle_imp_representative_boolean_partition, + representative_boolean_partition_imp_excluded_middle. +Qed. diff --git a/theories/Logic/PropExtensionalityFacts.v b/theories/Logic/PropExtensionalityFacts.v new file mode 100644 index 0000000000..7e455dfa1e --- /dev/null +++ b/theories/Logic/PropExtensionalityFacts.v @@ -0,0 +1,109 @@ +(************************************************************************) +(* v * The Coq Proof Assistant / The Coq Development Team *) +(* Proposition extensionality + Propositional functional extensionality + +2.2 Propositional extensionality -> Provable propositional extensionality + +2.3 Propositional extensionality -> Refutable propositional extensionality + +*) + +Set Implicit Arguments. + +(**********************************************************************) +(** * Definitions *) + +(** Propositional extensionality *) + +Local Notation PropositionalExtensionality := + (forall A B : Prop, (A <-> B) -> A = B). + +(** Provable-proposition extensionality *) + +Local Notation ProvablePropositionExtensionality := + (forall A:Prop, A -> A = True). + +(** Refutable-proposition extensionality *) + +Local Notation RefutablePropositionExtensionality := + (forall A:Prop, ~A -> A = False). + +(** Predicate extensionality *) + +Local Notation PredicateExtensionality := + (forall (A:Type) (P Q : A -> Prop), (forall x, P x <-> Q x) -> P = Q). + +(** Propositional functional extensionality *) + +Local Notation PropositionalFunctionalExtensionality := + (forall (A:Type) (P Q : A -> Prop), (forall x, P x = Q x) -> P = Q). + +(**********************************************************************) +(** * Propositional and predicate extensionality *) + +(**********************************************************************) +(** ** Predicate extensionality <-> Propositional extensionality + Propositional functional extensionality *) + +Lemma PredExt_imp_PropExt : PredicateExtensionality -> PropositionalExtensionality. +Proof. + intros Ext A B Equiv. + change A with ((fun _ => A) I). + now rewrite Ext with (P := fun _ : True =>A) (Q := fun _ => B). +Qed. + +Lemma PredExt_imp_PropFunExt : PredicateExtensionality -> PropositionalFunctionalExtensionality. +Proof. + intros Ext A P Q Eq. apply Ext. intros x. now rewrite (Eq x). +Qed. + +Lemma PropExt_and_PropFunExt_imp_PredExt : + PropositionalExtensionality -> PropositionalFunctionalExtensionality -> PredicateExtensionality. +Proof. + intros Ext FunExt A P Q Equiv. + apply FunExt. intros x. now apply Ext. +Qed. + +Theorem PropExt_and_PropFunExt_iff_PredExt : + PropositionalExtensionality /\ PropositionalFunctionalExtensionality <-> PredicateExtensionality. +Proof. + firstorder using PredExt_imp_PropExt, PredExt_imp_PropFunExt, PropExt_and_PropFunExt_imp_PredExt. +Qed. + +(**********************************************************************) +(** ** Propositional extensionality and provable proposition extensionality *) + +Lemma PropExt_imp_ProvPropExt : PropositionalExtensionality -> ProvablePropositionExtensionality. +Proof. + intros Ext A Ha; apply Ext; split; trivial. +Qed. + +(**********************************************************************) +(** ** Propositional extensionality and refutable proposition extensionality *) + +Lemma PropExt_imp_RefutPropExt : PropositionalExtensionality -> RefutablePropositionExtensionality. +Proof. + intros Ext A Ha; apply Ext; split; easy. +Qed. -- cgit v1.2.3