diff --git a/src/hott/08-families-of-maps.rzk.md b/src/hott/08-families-of-maps.rzk.md index dfaaa61..fb65a59 100644 --- a/src/hott/08-families-of-maps.rzk.md +++ b/src/hott/08-families-of-maps.rzk.md @@ -695,7 +695,15 @@ contractible. ## Weak function extensionality implies function extensionality -```rzk title="Rijke, 13.1" +Using the fundamental theorem of identity types, we prove the converse of +`weakfunext-funext`, so we now know that `#!rzk FunExt` is logically equivalent +to `#!rzk WeakFunExt`. We follow the proof in Rijke, section 13.1. + +We first fix one of the two functions and show that `#!rzk WeakFunExt` implies a +version of function extensionality that asserts that a type of "maps together +with homotopies" is contractible. + +```rzk #def prod-eq-pair-dhomotopy ( A : U) ( C : A → U) @@ -765,7 +773,12 @@ contractible. ( A) ( \ x → Σ (c : (C x)) , (f x =_{C x} c)) ( \ x → is-contr-based-paths (C x) (f x))) +``` + +We now use the fundamental theorem of identity types to go from the version for +a fixed f to the total `#!rzk FunExt` axiom. +```rzk #def funext-weirdfunext ( weirdfunext : WeirdFunExt) : FunExt