# mathlibdocumentation

analysis.calculus.parametric_integral

# Derivatives of integrals depending on parameters #

A parametric integral is a function with shape f = λ x : H, ∫ a : α, F x a ∂μ for some F : H → α → E, where H and E are normed spaces and α is a measured space with measure μ.

We already know from continuous_of_dominated in measure_theory.bochner_integral how to guarantee that f is continuous using the dominated convergence theorem. In this file, we want to express the derivative of f as the integral of the derivative of F with respect to x.

## Main results #

As explained above, all results express the derivative of a parametric integral as the integral of a derivative. The variations come from the assumptions and from the different ways of expressing derivative, especially Fréchet derivatives vs elementary derivative of function of one real variable.

• has_fderiv_at_of_dominated_loc_of_lip: this version assumes F x is ae-measurable for x near x₀, F x₀ is integrable, λ x, F x a has derivative F' a : H →L[ℝ] E at x₀ which is ae-measurable, λ x, F x a is locally Lipschitz near x₀ for almost every a, with a Lipschitz bound which is integrable with respect to a. A subtle point is that the "near x₀" in the last condition has to be uniform in a. This is controlled by a positive number ε.

• has_fderiv_at_of_dominated_of_fderiv_le: this version assume λ x, F x a has derivative F' x a for x near x₀ and F' x is bounded by an integrable function independent from x near x₀.

has_deriv_at_of_dominated_loc_of_lip and has_deriv_at_of_dominated_loc_of_deriv_le are versions of the above two results that assume H = ℝ and use the high-school derivative deriv instead of Fréchet derivative fderiv.

theorem has_fderiv_at_of_dominated_loc_of_lip' {α : Type u_1} {μ : measure_theory.measure α} {E : Type u_2} [normed_group E] [ E] [borel_space E] {H : Type u_3} [normed_group H] [ H] {F : H → α → E} {F' : α → (H →L[] E)} {x₀ : H} {bound : α → } {ε : } (ε_pos : 0 < ε) (hF_meas : ∀ (x : H), x εae_measurable (F x) μ) (hF_int : μ) (hF'_meas : μ) (h_lipsch : ∀ᵐ (a : α) ∂μ, lipschitz_on_with (real.nnabs (bound a)) (λ (x : H), F x a) (metric.ball x₀ ε)) (bound_integrable : μ) (h_diff : ∀ᵐ (a : α) ∂μ, has_fderiv_at (λ (x : H), F x a) (F' a) x₀) :
has_fderiv_at (λ (x : H), (a : α), F x a μ) ( (a : α), F' a μ) x₀

Differentiation under integral of x ↦ ∫ F x a at a given point x₀, assuming F x₀ is integrable, x ↦ F x a is locally Lipschitz on a ball around x₀ for ae a with integrable Lipschitz bound (with a ball radius independent of a), and F x is ae-measurable for x in the same ball. See has_fderiv_at_of_dominated_loc_of_lip for a slightly more general version.

theorem has_fderiv_at_of_dominated_loc_of_lip {α : Type u_1} {μ : measure_theory.measure α} {E : Type u_2} [normed_group E] [ E] [borel_space E] {H : Type u_3} [normed_group H] [ H] {F : H → α → E} {F' : α → (H →L[] E)} {x₀ : H} {bound : α → } {ε : } (ε_pos : 0 < ε) (hF_meas : ∀ᶠ (x : H) in 𝓝 x₀, ae_measurable (F x) μ) (hF_int : μ) (hF'_meas : μ) (h_lip : ∀ᵐ (a : α) ∂μ, lipschitz_on_with (real.nnabs (bound a)) (λ (x : H), F x a) (metric.ball x₀ ε)) (bound_integrable : μ) (h_diff : ∀ᵐ (a : α) ∂μ, has_fderiv_at (λ (x : H), F x a) (F' a) x₀) :
has_fderiv_at (λ (x : H), (a : α), F x a μ) ( (a : α), F' a μ) x₀

Differentiation under integral of x ↦ ∫ F x a at a given point x₀, assuming F x₀ is integrable, x ↦ F x a is locally Lipschitz on a ball around x₀ for ae a (with a ball radius independent of a) with integrable Lipschitz bound, and F x is ae-measurable for x in a possibly smaller neighborhood of x₀.

theorem has_fderiv_at_of_dominated_of_fderiv_le {α : Type u_1} {μ : measure_theory.measure α} {E : Type u_2} [normed_group E] [ E] [borel_space E] {H : Type u_3} [normed_group H] [ H] {F : H → α → E} {F' : H → α → (H →L[] E)} {x₀ : H} {bound : α → } {ε : } (ε_pos : 0 < ε) (hF_meas : ∀ᶠ (x : H) in 𝓝 x₀, ae_measurable (F x) μ) (hF_int : μ) (hF'_meas : ae_measurable (F' x₀) μ) (h_bound : ∀ᵐ (a : α) ∂μ, ∀ (x : H), x εF' x a bound a) (bound_integrable : μ) (h_diff : ∀ᵐ (a : α) ∂μ, ∀ (x : H), x εhas_fderiv_at (λ (x : H), F x a) (F' x a) x) :
has_fderiv_at (λ (x : H), (a : α), F x a μ) ( (a : α), F' x₀ a μ) x₀

Differentiation under integral of x ↦ ∫ F x a at a given point x₀, assuming F x₀ is integrable, x ↦ F x a is differentiable on a ball around x₀ for ae a with derivative norm uniformly bounded by an integrable function (the ball radius is independent of a), and F x is ae-measurable for x in a possibly smaller neighborhood of x₀.

theorem has_deriv_at_of_dominated_loc_of_lip {α : Type u_1} {μ : measure_theory.measure α} {E : Type u_2} [normed_group E] [ E] [borel_space E] {F : α → E} {F' : α → E} {x₀ ε : } (ε_pos : 0 < ε) (hF_meas : ∀ᶠ (x : ) in 𝓝 x₀, ae_measurable (F x) μ) (hF_int : μ) (hF'_meas : μ) {bound : α → } (h_lipsch : ∀ᵐ (a : α) ∂μ, lipschitz_on_with (real.nnabs (bound a)) (λ (x : ), F x a) (metric.ball x₀ ε)) (bound_integrable : μ) (h_diff : ∀ᵐ (a : α) ∂μ, has_deriv_at (λ (x : ), F x a) (F' a) x₀) :
has_deriv_at (λ (x : ), (a : α), F x a μ) ( (a : α), F' a μ) x₀

Derivative under integral of x ↦ ∫ F x a at a given point x₀ : ℝ, assuming F x₀ is integrable, x ↦ F x a is locally Lipschitz on an interval around x₀ for ae a (with interval radius independent of a) with integrable Lipschitz bound, and F x is ae-measurable for x in a possibly smaller neighborhood of x₀.

theorem has_deriv_at_of_dominated_loc_of_deriv_le {α : Type u_1} {μ : measure_theory.measure α} {E : Type u_2} [normed_group E] [ E] [borel_space E] {F F' : α → E} {x₀ ε : } (ε_pos : 0 < ε) (hF_meas : ∀ᶠ (x : ) in 𝓝 x₀, ae_measurable (F x) μ) (hF_int : μ) (hF'_meas : ae_measurable (F' x₀) μ) {bound : α → } (h_bound : ∀ᵐ (a : α) ∂μ, ∀ (x : ), x εF' x a bound a) (bound_integrable : μ) (h_diff : ∀ᵐ (a : α) ∂μ, ∀ (x : ), x εhas_deriv_at (λ (x : ), F x a) (F' x a) x) :
measure_theory.integrable (F' x₀) μ has_deriv_at (λ (n : ), (a : α), F n a μ) ( (a : α), F' x₀ a μ) x₀

Derivative under integral of x ↦ ∫ F x a at a given point x₀ : ℝ, assuming F x₀ is integrable, x ↦ F x a is differentiable on an interval around x₀ for ae a (with interval radius independent of a) with derivative uniformly bounded by an integrable function, and F x is ae-measurable for x in a possibly smaller neighborhood of x₀.