diff --git a/src/simplicial-hott/04-extension-types.rzk.md b/src/simplicial-hott/04-extension-types.rzk.md index ec4d818a..3656eed7 100644 --- a/src/simplicial-hott/04-extension-types.rzk.md +++ b/src/simplicial-hott/04-extension-types.rzk.md @@ -268,7 +268,7 @@ We refer to another form as an "extension extensionality" axiom. Weak extension extensionality implies extension extensionality; this is the context of RS17 Proposition 4.8 (i). We prove this in a series of lemmas. The `ext-projection-temp` function is a (hopefully temporary) helper that explicitly -cases an extension type to a function type. +cases an extension type to a function type. ```rzk #section rs-4-8 @@ -288,27 +288,28 @@ cases an extension type to a function type. #define is-contr-ext-based-paths uses (weak-ext-ext f) : is-contr ((t : ψ ) → (Σ (y : A t) , ((ext-projection-temp) t = y))[ϕ t ↦ (a t , refl)]) - := weak-ext-ext - ( I ) - ( ψ ) - ( ϕ ) - ( \ t → (Σ (y : A t) , ((ext-projection-temp) t = y))) - ( \ t → - is-contr-based-paths (A t ) ((ext-projection-temp) t)) - ( \ t → (a t , refl) ) - -#define is-contr-ext-codomain-based-paths uses (weak-ext-ext f) - : is-contr - ( ( t : ψ) → - ( Σ (y : A t) , (y = ext-projection-temp t)) - [ ϕ t ↦ (a t , refl)]) - := weak-ext-ext - ( I) - ( ψ) - ( ϕ) - ( \ t → (Σ (y : A t) , y = ext-projection-temp t)) - ( \ t → is-contr-codomain-based-paths (A t) (ext-projection-temp t)) - ( \ t → (a t , refl)) + := + weak-ext-ext + ( I ) + ( ψ ) + ( ϕ ) + ( \ t → (Σ (y : A t) , ((ext-projection-temp) t = y))) + ( \ t → + is-contr-based-paths (A t ) ((ext-projection-temp) t)) + ( \ t → (a t , refl) ) + +#define is-contr-ext-codomain-based-paths uses (weak-ext-ext f) + : is-contr + ( ( t : ψ) → + ( Σ (y : A t) , (y = ext-projection-temp t)) [ ϕ t ↦ (a t , refl)]) + := + weak-ext-ext + ( I) + ( ψ) + ( ϕ) + ( \ t → (Σ (y : A t) , y = ext-projection-temp t)) + ( \ t → is-contr-codomain-based-paths (A t) (ext-projection-temp t)) + ( \ t → (a t , refl)) #define is-contr-based-paths-ext uses (weak-ext-ext) : is-contr (Σ (g : (t : ψ ) → A t [ϕ t ↦ a t]) , @@ -347,10 +348,10 @@ The map that defines extension extensionality ((t : ψ ) → (f t = g t) [ϕ t ↦ refl])) := total-map - ( (t : ψ ) → A t [ϕ t ↦ a t]) - ( \ g → (f = g)) - ( \ g → (t : ψ ) → (f t = g t) [ϕ t ↦ refl]) - ( ext-htpy-eq I ψ ϕ A a f) + ( (t : ψ ) → A t [ϕ t ↦ a t]) + ( \ g → (f = g)) + ( \ g → (t : ψ ) → (f t = g t) [ϕ t ↦ refl]) + ( ext-htpy-eq I ψ ϕ A a f) ``` The total bundle version of extension extensionality @@ -370,14 +371,14 @@ The total bundle version of extension extensionality (ext-ext-weak-ext-ext-map I ψ ϕ A a f) := is-equiv-are-contr - ((Σ (g : (t : ψ ) → A t [ϕ t ↦ a t]), (f = g)) ) - ( Σ (g : (t : ψ ) → A t [ϕ t ↦ a t]) , + ( Σ (g : (t : ψ ) → A t [ϕ t ↦ a t]), (f = g) ) + ( Σ (g : (t : ψ ) → A t [ϕ t ↦ a t]) , ((t : ψ ) → (f t = g t) [ϕ t ↦ refl])) - ( is-contr-based-paths - ( (t : ψ ) → A t [ϕ t ↦ a t]) - ( f )) - ( is-contr-based-paths-ext weak-ext-ext I ψ ϕ A a f) - ( ext-ext-weak-ext-ext-map I ψ ϕ A a f) + ( is-contr-based-paths + ( (t : ψ ) → A t [ϕ t ↦ a t]) + ( f )) + ( is-contr-based-paths-ext weak-ext-ext I ψ ϕ A a f) + ( ext-ext-weak-ext-ext-map I ψ ϕ A a f) ``` Finally, using equivalences between families of equivalences and bundles of @@ -412,7 +413,7 @@ extension extensionality that we get by extraccting the fiberwise equivalence. ```rzk title="RS17 Proposition 4.8(i)" #define ext-ext-weak-ext-ext (weak-ext-ext : WeakExtExt) - : ExtExt + : ExtExt := \ I ψ ϕ A a f g → ext-ext-weak-ext-ext' weak-ext-ext I ψ ϕ A a f g ``` @@ -478,25 +479,28 @@ equivalences of extension types. For simplicity, we extend from `#!rzk BOT`. ( b) ( \ t → second (second (second (famequiv t))) (b t)))))) ``` -We have a homotopy extension property. -The following code is another instantiation of casting, necessary for some arguments below. +We have a homotopy extension property. + +The following code is another instantiation of casting, necessary for some +arguments below. ```rzk #define restrict - ( I : CUBE) - ( ψ : I → TOPE) - ( ϕ : ψ → TOPE) - ( A : ψ → U) - ( a : (t : ϕ) → A t) - ( f : (t : ψ) → A t [ϕ t ↦ a t]) - : (t : ψ ) → A t - := f + ( I : CUBE) + ( ψ : I → TOPE) + ( ϕ : ψ → TOPE) + ( A : ψ → U) + ( a : (t : ϕ) → A t) + ( f : (t : ψ) → A t [ϕ t ↦ a t]) + : (t : ψ ) → A t + := f ``` -The homotopy extension property follows from a straightforward application of the axiom of choice -to the point of contraction for weak extension extensionality. +The homotopy extension property follows from a straightforward application of +the axiom of choice to the point of contraction for weak extension +extensionality. ```rzk title="RS17 Proposition 4.10" #define htpy-ext-property @@ -506,29 +510,28 @@ to the point of contraction for weak extension extensionality. ( ϕ : ψ → TOPE) ( A : ψ → U) ( b : (t : ψ) → A t) - ( a : (t : ϕ) → A t) - ( e : (t : ϕ) → a t = b t) - : Σ (a' : (t : ψ) → A t [ϕ t ↦ a t]) , + ( a : (t : ϕ) → A t) + ( e : (t : ϕ) → a t = b t) + : Σ (a' : (t : ψ) → A t [ϕ t ↦ a t]) , ((t : ψ) → (restrict I ψ ϕ A a a' t = b t) [ϕ t ↦ e t]) - := - first - ( axiom-choice + := + first + ( axiom-choice ( I) - ( ψ) - ( ϕ) - ( A) + ( ψ) + ( ϕ) + ( A) ( \ t y → y = b t) - ( a) - ( e)) - ( first - ( weak-ext-ext + ( a) + ( e)) + ( first + ( weak-ext-ext ( I) ( ψ) ( ϕ) ( \ t → (Σ (y : A t) , y = b t)) - ( \ t → is-contr-codomain-based-paths + ( \ t → is-contr-codomain-based-paths ( A t) ( b t)) ( \ t → ( a t , e t) ))) ``` -