diff --git a/theories/Sets/GCHtoAC.v b/theories/Sets/GCHtoAC.v index e333a03a2d5..37a1727b5ef 100644 --- a/theories/Sets/GCHtoAC.v +++ b/theories/Sets/GCHtoAC.v @@ -8,17 +8,15 @@ From HoTT.Sets Require Import Ordinals Hartogs Powers GCH AC. Open Scope type. -(* The proof of Sierpinski's results that GCH implies AC given in this file consists of 2 ingredients: +(* The proof of Sierpinski's results that GCH implies AC given in this file consists of two ingredients: 1. Adding powers of infinite sets does not increase the cardinality (path_infinite_power). 2. A variant of Cantor's theorem saying that P(X) <= (X + Y) implies P(X) <= Y for large X (Cantor_injects_injects). - Those are used to obtain that cardinality-controlled functions are well-behaved in the presence of GCH (Sierpinski), - from which we obtain by instantiation with the Hartogs number that every set embeds into an ordinal, - which is enough to conclude GCH -> AC (GCH_AC) since the well-ordering theorem implies AC (WO_AC). *) + Those are used to obtain that cardinality-controlled functions are well-behaved in the presence of GCH (Sierpinski), from which we obtain by instantiation with the Hartogs number that every set embeds into an ordinal, which is enough to conclude GCH -> AC (GCH_AC) since the well-ordering theorem implies AC (WO_AC). *) (** * Constructive equivalences *) -(* For the 1. ingredient, we establish a bunch of paths and conclude the wished result by equational reasoning. *) +(* For the first ingredient, we establish a bunch of paths and conclude the desired result by equational reasoning. *) Section Preparation. @@ -63,7 +61,6 @@ Proof. Qed. - (** * Equivalences relying on LEM **) Context {EM : ExcludedMiddle}. @@ -124,7 +121,6 @@ Proof. Qed. - (** * Equivalences on infinite sets *) Lemma path_infinite_unit (X : HSet) : @@ -143,11 +139,9 @@ Proof. Qed. - (** * Variants of Cantors's theorem *) -(* For the 2. ingredient, we give a preliminary version (Cantor_path_inject) to see the idea, - as well as a stronger refinement (Cantor_injects_injects) which is then a mere reformulation. *) +(* For the second ingredient, we give a preliminary version (Cantor_path_inject) to see the idea, as well as a stronger refinement (Cantor_injects_injects) which is then a mere reformulation. *) Context {PR : PropResizing}. @@ -241,7 +235,6 @@ Qed. End Preparation. - (** * Sierpinski's Theorem *) Section Sierpinski. @@ -260,9 +253,7 @@ Hypothesis HN_ninject : forall X, ~ InjectsInto (HN X) X. Variable HN_bound : nat. Hypothesis HN_inject : forall X, InjectsInto (HN X) (power_iterated X HN_bound). -(* This section then concludes the intermediate result that abstractly, - any function HN behaving like the Hartogs number is tamed in the presence of GCH. - Morally we show that X <= HN(X) for all X, we just ensure that X is large enough by considering P(N + X). *) +(* This section then concludes the intermediate result that abstractly, any function HN behaving like the Hartogs number is tamed in the presence of GCH. Morally we show that X <= HN(X) for all X, we just ensure that X is large enough by considering P(N + X). *) Lemma InjectsInto_sum X Y X' Y' : InjectsInto X X' -> InjectsInto Y Y' -> InjectsInto (X + Y) (X' + Y'). @@ -278,8 +269,7 @@ Proof. - apply ap. apply Hg. apply path_sum_inr with X'. apply H. Qed. -(* The main proof is by induction on the cardinality bound for HN. - As the Hartogs number is bounded by P^3(X), we'd actually just need finitely many instances of GCH. *) +(* The main proof is by induction on the cardinality bound for HN. As the Hartogs number is bounded by P^3(X), we'd actually just need finitely many instances of GCH. *) Lemma Sierpinski_step (X : HSet) n : GCH -> infinite X -> powfix X -> InjectsInto (HN X) (power_iterated X n) -> InjectsInto X (HN X).