Almost everywhere equal functions #
We build a space of equivalence classes of functions, where two functions are treated as identical
if they are almost everywhere equal. We form the set of equivalence classes under the relation of
being almost everywhere equal, which is sometimes known as the L⁰ space.
To use this space as a basis for the L^p spaces and for the Bochner integral, we consider
equivalence classes of strongly measurable functions (or, equivalently, of almost everywhere
strongly measurable functions.)
See Mathlib/MeasureTheory/Function/L1Space/AEEqFun.lean for L¹ space.
Notation #
- α →ₘ[μ] βis the type of- L⁰space, where- αis a measurable space,- βis a topological space, and- μis a measure on- α.- f : α →ₘ βis a "function" in- L⁰. In comments,- [f]is also used to denote an- L⁰function.- ₘcan be typed as- \_m. Sometimes it is shown as a box if font is missing.
Main statements #
- The linear structure of - L⁰: Addition and scalar multiplication are defined on- L⁰in the natural way, i.e.,- [f] + [g] := [f + g],- c • [f] := [c • f]. So defined,- α →ₘ βinherits the linear structure of- β. For example, if- βis a module, then- α →ₘ βis a module over the same ring.- See - mk_add_mk,- neg_mk,- mk_sub,- smul_mk,- coeFn_add,- coeFn_neg,- coeFn_sub,- coeFn_smul
- The order structure of - L⁰:- ≤can be defined in a similar way:- [f] ≤ [g]if- f a ≤ g afor almost all- ain domain. And- α →ₘ βinherits the preorder and partial order of- β.- TODO: Define - supand- infon- L⁰so that it forms a lattice. It seems that- βmust be a linear order, since otherwise- f ⊔ gmay not be a measurable function.
Implementation notes #
- f.cast: To find a representative of- f : α →ₘ β, use the coercion- (f : α → β), which is implemented as- f.toFun. For each operation- opin- L⁰, there is a lemma called- coe_fn_op, characterizing, say,- (f op g : α → β).
- AEEqFun.mk: To constructs an- L⁰function- α →ₘ βfrom an almost everywhere strongly measurable function- f : α → β, use- ae_eq_fun.mk
- comp: Use- comp g fto get- [g ∘ f]from- g : β → γand- [f] : α →ₘ γwhen- gis continuous. Use- compMeasurableif- gis only measurable (this requires the target space to be second countable).
- comp₂: Use- comp₂ g f₁ f₂to get- [fun a ↦ g (f₁ a) (f₂ a)]. For example,- [f + g]is- comp₂ (+)
Tags #
function space, almost everywhere equal, L⁰, ae_eq_fun
The equivalence relation of being almost everywhere equal for almost everywhere strongly measurable functions.
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The space of equivalence classes of almost everywhere strongly measurable functions, where two
strongly measurable functions are equivalent if they agree almost everywhere, i.e.,
they differ on a set of measure 0.
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- (α →ₘ[μ] β) = Quotient (MeasureTheory.Measure.aeEqSetoid β μ)
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The space of equivalence classes of almost everywhere strongly measurable functions, where two
strongly measurable functions are equivalent if they agree almost everywhere, i.e.,
they differ on a set of measure 0.
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- One or more equations did not get rendered due to their size.
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Construct the equivalence class [f] of an almost everywhere measurable function f, based
on the equivalence relation of being almost everywhere equal.
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- MeasureTheory.AEEqFun.mk f hf = Quotient.mk'' ⟨f, hf⟩
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Coercion from a space of equivalence classes of almost everywhere strongly measurable
functions to functions. We ensure that if f has a constant representative,
then we choose that one.
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- ↑f = if h : ∃ (b : β), f = MeasureTheory.AEEqFun.mk (Function.const α b) ⋯ then Function.const α (Classical.choose h) else MeasureTheory.AEStronglyMeasurable.mk ↑(Quotient.out f) ⋯
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A measurable representative of an AEEqFun [f]
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Composition of an a.e. equal function with a (quasi-)measure-preserving function #
Composition of an almost everywhere equal function and a quasi-measure-preserving function.
See also AEEqFun.compMeasurePreserving.
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- g.compQuasiMeasurePreserving f hf = Quotient.liftOn' g (fun (g : { f : β → γ // MeasureTheory.AEStronglyMeasurable f ν }) => MeasureTheory.AEEqFun.mk (↑g ∘ f) ⋯) ⋯
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Composition of an almost everywhere equal function and a quasi-measure-preserving function.
This is an important special case of AEEqFun.compQuasiMeasurePreserving. We use a separate
definition so that lemmas that need f to be measure preserving can be @[simp] lemmas.
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- g.compMeasurePreserving f hf = g.compQuasiMeasurePreserving f ⋯
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Given a continuous function g : β → γ, and an almost everywhere equal function [f] : α →ₘ β,
return the equivalence class of g ∘ f, i.e., the almost everywhere equal function
[g ∘ f] : α →ₘ γ.
Equations
- MeasureTheory.AEEqFun.comp g hg f = Quotient.liftOn' f (fun (f : { f : α → β // MeasureTheory.AEStronglyMeasurable f μ }) => MeasureTheory.AEEqFun.mk (g ∘ ↑f) ⋯) ⋯
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Given a measurable function g : β → γ, and an almost everywhere equal function [f] : α →ₘ β,
return the equivalence class of g ∘ f, i.e., the almost everywhere equal function
[g ∘ f] : α →ₘ γ. This requires that γ has a second countable topology.
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- MeasureTheory.AEEqFun.compMeasurable g hg f = Quotient.liftOn' f (fun (f' : { f : α → β // MeasureTheory.AEStronglyMeasurable f μ }) => MeasureTheory.AEEqFun.mk (g ∘ ↑f') ⋯) ⋯
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The class of x ↦ (f x, g x).
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- One or more equations did not get rendered due to their size.
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Given a continuous function g : β → γ → δ, and almost everywhere equal functions
[f₁] : α →ₘ β and [f₂] : α →ₘ γ, return the equivalence class of the function
fun a => g (f₁ a) (f₂ a), i.e., the almost everywhere equal function
[fun a => g (f₁ a) (f₂ a)] : α →ₘ γ
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- MeasureTheory.AEEqFun.comp₂ g hg f₁ f₂ = MeasureTheory.AEEqFun.comp (Function.uncurry g) hg (f₁.pair f₂)
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Given a measurable function g : β → γ → δ, and almost everywhere equal functions
[f₁] : α →ₘ β and [f₂] : α →ₘ γ, return the equivalence class of the function
fun a => g (f₁ a) (f₂ a), i.e., the almost everywhere equal function
[fun a => g (f₁ a) (f₂ a)] : α →ₘ γ. This requires δ to have second-countable topology.
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- MeasureTheory.AEEqFun.comp₂Measurable g hg f₁ f₂ = MeasureTheory.AEEqFun.compMeasurable (Function.uncurry g) hg (f₁.pair f₂)
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Interpret f : α →ₘ[μ] β as a germ at ae μ forgetting that f is almost everywhere
strongly measurable.
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- f.toGerm = Quotient.liftOn' f (fun (f : { f : α → β // MeasureTheory.AEStronglyMeasurable f μ }) => ↑↑f) ⋯
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Given a predicate p and an equivalence class [f], return true if p holds of f a
for almost all a
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Given a relation r and equivalence class [f] and [g], return true if r holds of
(f a, g a) for almost all a
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- MeasureTheory.AEEqFun.LiftRel r f g = Filter.Germ.LiftRel r f.toGerm g.toGerm
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- MeasureTheory.AEEqFun.instSup = { max := fun (f g : α →ₘ[μ] β) => MeasureTheory.AEEqFun.comp₂ (fun (x1 x2 : β) => x1 ⊔ x2) ⋯ f g }
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- MeasureTheory.AEEqFun.instInf = { min := fun (f g : α →ₘ[μ] β) => MeasureTheory.AEEqFun.comp₂ (fun (x1 x2 : β) => x1 ⊓ x2) ⋯ f g }
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- One or more equations did not get rendered due to their size.
The equivalence class of a constant function: [fun _ : α => b], based on the equivalence
relation of being almost everywhere equal
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- MeasureTheory.AEEqFun.const α b = MeasureTheory.AEEqFun.mk (fun (x : α) => b) ⋯
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If the measure is nonzero, we can strengthen coeFn_const to get an equality.
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- MeasureTheory.AEEqFun.instInhabited = { default := MeasureTheory.AEEqFun.const α default }
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- MeasureTheory.AEEqFun.instOne = { one := MeasureTheory.AEEqFun.const α 1 }
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- MeasureTheory.AEEqFun.instZero = { zero := MeasureTheory.AEEqFun.const α 0 }
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- MeasureTheory.AEEqFun.instSMul = { smul := fun (c : 𝕜) (f : α →ₘ[μ] γ) => MeasureTheory.AEEqFun.comp (fun (x : γ) => c • x) ⋯ f }
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- MeasureTheory.AEEqFun.instMul = { mul := MeasureTheory.AEEqFun.comp₂ (fun (x1 x2 : γ) => x1 * x2) ⋯ }
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- MeasureTheory.AEEqFun.instAdd = { add := MeasureTheory.AEEqFun.comp₂ (fun (x1 x2 : γ) => x1 + x2) ⋯ }
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- MeasureTheory.AEEqFun.instPowNat = { pow := fun (f : α →ₘ[μ] γ) (n : ℕ) => MeasureTheory.AEEqFun.comp (fun (a : γ) => a ^ n) ⋯ f }
AEEqFun.toGerm as a MonoidHom.
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- MeasureTheory.AEEqFun.toGermMonoidHom = { toFun := MeasureTheory.AEEqFun.toGerm, map_one' := ⋯, map_mul' := ⋯ }
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AEEqFun.toGerm as an AddMonoidHom.
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- MeasureTheory.AEEqFun.toGermAddMonoidHom = { toFun := MeasureTheory.AEEqFun.toGerm, map_zero' := ⋯, map_add' := ⋯ }
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- MeasureTheory.AEEqFun.instPowInt = { pow := fun (f : α →ₘ[μ] γ) (n : ℤ) => MeasureTheory.AEEqFun.comp (fun (a : γ) => a ^ n) ⋯ f }
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- MeasureTheory.AEEqFun.instAddCommGroup = { toAddGroup := MeasureTheory.AEEqFun.instAddGroup, add_comm := ⋯ }
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- MeasureTheory.AEEqFun.instCommGroup = { toGroup := MeasureTheory.AEEqFun.instGroup, mul_comm := ⋯ }
For f : α → ℝ≥0∞, define ∫ [f] to be ∫ f
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- MeasureTheory.AEEqFun.instStarOfContinuousStar = { star := fun (f : α →ₘ[μ] R) => MeasureTheory.AEEqFun.comp star ⋯ f }
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- MeasureTheory.AEEqFun.instInvolutiveStarOfContinuousStar = { toStar := MeasureTheory.AEEqFun.instStarOfContinuousStar, star_involutive := ⋯ }
Positive part of an AEEqFun.
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- f.posPart = MeasureTheory.AEEqFun.comp (fun (x : γ) => max x 0) ⋯ f
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The ae-limit is ae-unique.
The equivalence class of μ-almost-everywhere measurable functions associated to a continuous
map.
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The MulHom from the group of continuous maps from α to β to the group of equivalence
classes of μ-almost-everywhere measurable functions.
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- ContinuousMap.toAEEqFunMulHom μ = { toFun := ContinuousMap.toAEEqFun μ, map_one' := ⋯, map_mul' := ⋯ }
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The AddHom from the group of continuous maps from α to β to the group of
equivalence classes of μ-almost-everywhere measurable functions.
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- ContinuousMap.toAEEqFunAddHom μ = { toFun := ContinuousMap.toAEEqFun μ, map_zero' := ⋯, map_add' := ⋯ }
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The linear map from the group of continuous maps from α to β to the group of equivalence
classes of μ-almost-everywhere measurable functions.
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- ContinuousMap.toAEEqFunLinearMap μ = { toFun := (↑(ContinuousMap.toAEEqFunAddHom μ)).toFun, map_add' := ⋯, map_smul' := ⋯ }