Bochner integral of a function against the composition and the composition-products of two kernels #
We prove properties of the composition and the composition-product of two kernels.
If κ is a kernel from α to β and η is a kernel from β to γ, we can form their
composition η ∘ₖ κ : Kernel α γ. We proved in ProbabilityTheory.Kernel.lintegral_comp that it
verifies ∫⁻ c, f c ∂((η ∘ₖ κ) a) = ∫⁻ b, ∫⁻ c, f c ∂(η b) ∂(κ a). In this file, we
prove the same equality for the Bochner integral.
If κ is an s-finite kernel from α to β and η is an s-finite kernel from α × β to γ,
we can form their composition-product κ ⊗ₖ η : Kernel α (β × γ).
We proved in ProbabilityTheory.Kernel.lintegral_compProd that it
verifies ∫⁻ bc, f bc ∂((κ ⊗ₖ η) a) = ∫⁻ b, ∫⁻ c, f (b, c) ∂(η (a, b)) ∂(κ a). In this file, we
prove the same equality for the Bochner integral.
Main statements #
ProbabilityTheory.integral_compProd: the integral against the composition-product is∫ z, f z ∂((κ ⊗ₖ η) a) = ∫ x, ∫ y, f (x, y) ∂(η (a, x)) ∂(κ a).ProbabilityTheory.integral_comp: the integral against the composition is∫⁻ z, f z ∂((η ∘ₖ κ) a) = ∫⁻ x, ∫⁻ y, f y ∂(η x) ∂(κ a).
Implementation details #
This file is to a large extent a copy of part of Mathlib/MeasureTheory/Integral/Prod.lean.
The product of two measures is a particular case of composition-product of kernels and
it turns out that once the measurability of the Lebesgue integral of a kernel is proved,
almost all proofs about integrals against products of measures extend with minimal modifications
to the composition-product of two kernels.
The composition of kernels can also be expressed easily with the composition-product and therefore the proofs about the composition are only simplified versions of the ones for the composition-product. However it is necessary to do all the proofs once again because the composition-product requires s-finiteness while the composition does not.