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The Coq Proof Assistant

# Library Coq.Logic.Diaconescu

Diaconescu showed that the Axiom of Choice entails Excluded-Middle in topoi [Diaconescu75]. Lacas and Werner adapted the proof to show that the axiom of choice in equivalence classes entails Excluded-Middle in Type Theory [LacasWerner99].
Three variants of Diaconescu's result in type theory are shown below.
A. A proof that the relational form of the Axiom of Choice + Extensionality for Predicates entails Excluded-Middle (by Hugo Herbelin)
B. A proof that the relational form of the Axiom of Choice + Proof Irrelevance entails Excluded-Middle for Equality Statements (by Benjamin Werner)
C. A proof that extensional Hilbert epsilon's description operator entails excluded-middle (taken from Bell [Bell93])
See also [Carlström04] for a discussion of the connection between the Extensional Axiom of Choice and Excluded-Middle
[Diaconescu75] Radu Diaconescu, Axiom of Choice and Complementation, in Proceedings of AMS, vol 51, pp 176-178, 1975.
[LacasWerner99] Samuel Lacas, Benjamin Werner, Which Choices imply the excluded middle?, preprint, 1999.
[Bell93] John L. Bell, Hilbert's epsilon operator and classical logic, Journal of Philosophical Logic, 22: 1-18, 1993
[Carlström04] Jesper Carlström, EM + Ext + AC_int is equivalent to AC_ext, Mathematical Logic Quaterly, vol 50(3), pp 236-240, 2004.
Require ClassicalFacts ChoiceFacts.

# Pred. Ext. + Rel. Axiom of Choice -> Excluded-Middle

Section PredExt_RelChoice_imp_EM.
The axiom of extensionality for predicates
Definition PredicateExtensionality :=
forall P Q:bool -> Prop, (forall b:bool, P b <-> Q b) -> P = Q.
From predicate extensionality we get propositional extensionality hence proof-irrelevance
Import ClassicalFacts.
Variable pred_extensionality : PredicateExtensionality.
Lemma prop_ext : forall A B:Prop, (A <-> B) -> A = B.
Lemma proof_irrel : forall (A:Prop) (a1 a2:A), a1 = a2.
From proof-irrelevance and relational choice, we get guarded relational choice
Import ChoiceFacts.
Variable rel_choice : RelationalChoice.
Lemma guarded_rel_choice : GuardedRelationalChoice.
The form of choice we need: there is a functional relation which chooses an element in any non empty subset of bool
Import Bool.
Lemma AC_bool_subset_to_bool :
exists R : (bool -> Prop) -> bool -> Prop,
(forall P:bool -> Prop,
(exists b : bool, P b) ->
exists b : bool, P b /\ R P b /\ (forall b':bool, R P b' -> b = b')).
The proof of the excluded middle Remark: P could have been in Set or Type
Theorem pred_ext_and_rel_choice_imp_EM : forall P:Prop, P \/ ~ P.
End PredExt_RelChoice_imp_EM.

# Proof-Irrel. + Rel. Axiom of Choice -> Excl.-Middle for Equality

This is an adaptation of Diaconescu's theorem, exploiting the form of extensionality provided by proof-irrelevance
Section ProofIrrel_RelChoice_imp_EqEM.
Import ChoiceFacts.
Variable rel_choice : RelationalChoice.
Variable proof_irrelevance : forall P:Prop , forall x y:P, x=y.
Let a1 and a2 be two elements in some type A
Variable A :Type.
Variables a1 a2 : A.
We build the subset A' of A made of a1 and a2
Definition A' := @sigT A (fun x => x=a1 \/ x=a2).
Definition a1':A'.
Defined.
Definition a2':A'.
Defined.
By proof-irrelevance, projection is a retraction
Lemma projT1_injective : a1=a2 -> a1'=a2'.
But from the actual proofs of being in A', we can assert in the proof-irrelevant world the existence of relevant boolean witnesses
Lemma decide : forall x:A', exists y:bool ,
(projT1 x = a1 /\ y = true ) \/ (projT1 x = a2 /\ y = false).
Thanks to the axiom of choice, the boolean witnesses move from the propositional world to the relevant world
Theorem proof_irrel_rel_choice_imp_eq_dec : a1=a2 \/ ~a1=a2.
An alternative more concise proof can be done by directly using the guarded relational choice
Lemma proof_irrel_rel_choice_imp_eq_dec' : a1=a2 \/ ~a1=a2.
End ProofIrrel_RelChoice_imp_EqEM.

# Extensional Hilbert's epsilon description operator -> Excluded-Middle

Proof sketch from Bell [Bell93] (with thanks to P. Castéran)
Local Notation inhabited A := A (only parsing).
Section ExtensionalEpsilon_imp_EM.
Variable epsilon : forall A : Type, inhabited A -> (A -> Prop) -> A.
Hypothesis epsilon_spec :
forall (A:Type) (i:inhabited A) (P:A->Prop),
(exists x, P x) -> P (epsilon A i P).
Hypothesis epsilon_extensionality :
forall (A:Type) (i:inhabited A) (P Q:A->Prop),
(forall a, P a <-> Q a) -> epsilon A i P = epsilon A i Q.
Local Notation eps := (epsilon bool true) (only parsing).
Theorem extensional_epsilon_imp_EM : forall P:Prop, P \/ ~ P.
End ExtensionalEpsilon_imp_EM.