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Mathlib.MeasureTheory.MeasurableSpace.CountablyGenerated

Countably generated measurable spaces #

We say a measurable space is countably generated if it can be generated by a countable set of sets.

In such a space, we can also build a sequence of finer and finer finite measurable partitions of the space such that the measurable space is generated by the union of all partitions.

Main definitions #

Main statements #

The file also contains measurability results about memPartition, from which the properties of countablePartition are deduced.

We say a measurable space is countably generated if it can be generated by a countable set of sets.

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    A countable set of sets that generate the measurable space. We insert to ensure it is nonempty.

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      A countable sequence of sets generating the measurable space.

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        @[instance 100]

        Any measurable space structure on a countable space is countably generated.

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        We say that a measurable space separates points if for any two distinct points, there is a measurable set containing one but not the other.

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          theorem MeasurableSpace.SeparatesPoints.separates {α : Type u_3} [m : MeasurableSpace α] [self : MeasurableSpace.SeparatesPoints α] (x : α) (y : α) :
          (∀ (s : Set α), MeasurableSet sx sy s)x = y
          theorem MeasurableSpace.separatesPoints_def {α : Type u_1} [MeasurableSpace α] [hs : MeasurableSpace.SeparatesPoints α] {x : α} {y : α} (h : ∀ (s : Set α), MeasurableSet sx sy s) :
          x = y
          theorem MeasurableSpace.exists_measurableSet_of_ne {α : Type u_1} [MeasurableSpace α] [MeasurableSpace.SeparatesPoints α] {x : α} {y : α} (h : x y) :
          ∃ (s : Set α), MeasurableSet s x s ys
          theorem MeasurableSpace.separatesPoints_iff {α : Type u_1} [MeasurableSpace α] :
          MeasurableSpace.SeparatesPoints α ∀ (x y : α), (∀ (s : Set α), MeasurableSet s(x s y s))x = y
          theorem MeasurableSpace.separating_of_generateFrom {α : Type u_1} (S : Set (Set α)) [h : MeasurableSpace.SeparatesPoints α] (x : α) (y : α) :
          (sS, x s y s)x = y

          If the measurable space generated by S separates points, then this is witnessed by sets in S.

          We say that a measurable space is countably separated if there is a countable sequence of measurable sets separating points.

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            @[instance 100]
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            If a measurable space admits a countable separating family of measurable sets, there is a countably generated coarser space which still separates points.

            noncomputable def MeasurableSpace.mapNatBool (α : Type u_1) [MeasurableSpace α] [MeasurableSpace.CountablyGenerated α] (x : α) (n : ) :

            A map from a measurable space to the Cantor space ℕ → Bool induced by a countable sequence of sets generating the measurable space.

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              If a measurable space is countably generated and separates points, it is measure equivalent to some subset of the Cantor space ℕ → Bool (equipped with the product sigma algebra). Note: s need not be measurable, so this map need not be a MeasurableEmbedding to the Cantor Space.

              If a measurable space admits a countable sequence of measurable sets separating points, it admits a measurable injection into the Cantor space ℕ → Bool (equipped with the product sigma algebra).

              theorem MeasurableSpace.measurableSet_succ_memPartition {α : Type u_1} (t : Set α) (n : ) {s : Set α} (hs : s memPartition t n) :
              theorem MeasurableSpace.measurableSet_generateFrom_memPartition_iff {α : Type u_1} (t : Set α) (n : ) (s : Set α) :
              MeasurableSet s ∃ (S : Finset (Set α)), S memPartition t n s = ⋃₀ S
              theorem MeasurableSpace.measurableSet_memPartition {α : Type u_1} [MeasurableSpace α] {t : Set α} (ht : ∀ (n : ), MeasurableSet (t n)) (n : ) {s : Set α} (hs : s memPartition t n) :
              theorem MeasurableSpace.measurableSet_memPartitionSet {α : Type u_1} [MeasurableSpace α] {t : Set α} (ht : ∀ (n : ), MeasurableSet (t n)) (n : ) (a : α) :

              For each n : ℕ, countablePartition α n is a partition of the space in at most 2^n sets. Each partition is finer than the preceeding one. The measurable space generated by the union of all those partitions is the measurable space on α.

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                The set in countablePartition α n to which a : α belongs.

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                  A class registering that either α is countable or β is a countably generated measurable space.

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