End ext_C1.

Lemma extension_C1_ex_derive (f df : R -> R) (a b : Rbar) :
Rbar_le a b
-> (forall x : R, Rbar_le a x -> Rbar_le x b -> ex_derive f x)
-> forall x : R, ex_derive (extension_C1 f (Derive f) a b) x.