# mathlibdocumentation

analysis.calculus.extend_deriv

# Extending differentiability to the boundary

We investigate how differentiable functions inside a set extend to differentiable functions on the boundary. For this, it suffices that the function and its derivative admit limits there. A general version of this statement is given in has_fderiv_at_boundary_of_tendsto_fderiv.

One-dimensional versions, in which one wants to obtain differentiability at the left endpoint or the right endpoint of an interval, are given in has_deriv_at_interval_left_endpoint_of_tendsto_deriv and has_deriv_at_interval_right_endpoint_of_tendsto_deriv. These versions are formulated in terms of the one-dimensional derivative deriv ℝ f.

theorem has_fderiv_at_boundary_of_tendsto_fderiv {E : Type u_1} [normed_group E] [ E] {F : Type u_2} [normed_group F] [ F] {f : E → F} {s : set E} {x : E} {f' : E →L[] F} :
(∀ (y : E), y y)filter.tendsto (λ (y : E), f y) (𝓝[s] x) (𝓝 f') (closure s) x

If a function f is differentiable in a convex open set and continuous on its closure, and its derivative converges to a limit f' at a point on the boundary, then f is differentiable there with derivative f'.

theorem has_deriv_at_interval_left_endpoint_of_tendsto_deriv {E : Type u_1} [normed_group E] [ E] {s : set } {e : E} {a : } {f : → E} :
as 𝓝[] afilter.tendsto (λ (x : ), x) (𝓝[] a) (𝓝 e) (set.Ici a) a

If a function is differentiable on the right of a point a : ℝ, continuous at a, and its derivative also converges at a, then f is differentiable on the right at a.

theorem has_deriv_at_interval_right_endpoint_of_tendsto_deriv {E : Type u_1} [normed_group E] [ E] {s : set } {e : E} {a : } {f : → E} :
as 𝓝[] afilter.tendsto (λ (x : ), x) (𝓝[] a) (𝓝 e) (set.Iic a) a

If a function is differentiable on the left of a point a : ℝ, continuous at a, and its derivative also converges at a, then f is differentiable on the left at a.

theorem has_deriv_at_of_has_deriv_at_of_ne {E : Type u_1} [normed_group E] [ E] {f g : → E} {x : } :
(∀ (y : ), y x (g y) y) (g x) x

If a real function f has a derivative g everywhere but at a point, and f and g are continuous at this point, then g is also the derivative of f at this point.