# Documentation

Mathlib.Topology.MetricSpace.Polish

# Polish spaces #

A topological space is Polish if its topology is second-countable and there exists a compatible complete metric. This is the class of spaces that is well-behaved with respect to measure theory. In this file, we establish the basic properties of Polish spaces.

## Main definitions and results #

• PolishSpace α is a mixin typeclass on a topological space, requiring that the topology is second-countable and compatible with a complete metric. To endow the space with such a metric, use in a proof letI := upgradePolishSpace α. We register an instance from complete second-countable metric spaces to Polish spaces, not the other way around.
• We register that countable products and sums of Polish spaces are Polish.
• IsClosed.polishSpace: a closed subset of a Polish space is Polish.
• IsOpen.polishSpace: an open subset of a Polish space is Polish.
• exists_nat_nat_continuous_surjective: any nonempty Polish space is the continuous image of the fundamental Polish space ℕ → ℕ.

A fundamental property of Polish spaces is that one can put finer topologies, still Polish, with additional properties:

• exists_polishSpace_forall_le: on a topological space, consider countably many topologies t n, all Polish and finer than the original topology. Then there exists another Polish topology which is finer than all the t n.
• IsClopenable s is a property of a subset s of a topological space, requiring that there exists a finer topology, which is Polish, for which s becomes open and closed. We show that this property is satisfied for open sets, closed sets, for complements, and for countable unions. Once Borel-measurable sets are defined in later files, it will follow that any Borel-measurable set is clopenable. Once the Lusin-Souslin theorem is proved using analytic sets, we will even show that a set is clopenable if and only if it is Borel-measurable, see isClopenable_iff_measurableSet.

### Basic properties of Polish spaces #

class PolishSpace (α : Type u_3) [h : ] extends :
• is_open_generated_countable : b,
• complete : m, UniformSpace.toTopologicalSpace = h

A Polish space is a topological space with second countable topology, that can be endowed with a metric for which it is complete. We register an instance from complete second countable metric space to polish space, and not the other way around as this is the most common use case.

To endow a Polish space with a complete metric space structure, do letI := upgradePolishSpace α.

Instances
class UpgradedPolishSpace (α : Type u_3) extends , , :
Type u_3

A convenience class, for a Polish space endowed with a complete metric. No instance of this class should be registered: It should be used as letI := upgradePolishSpace α to endow a Polish space with a complete metric.

Instances
instance polishSpace_of_complete_second_countable {α : Type u_1} [m : ] [h' : ] :
def polishSpaceMetric (α : Type u_3) [] [h : ] :

Construct on a Polish space a metric (compatible with the topology) which is complete.

Instances For
theorem complete_polishSpaceMetric (α : Type u_3) [ht : ] [h : ] :
def upgradePolishSpace (α : Type u_3) [] [] :

This definition endows a Polish space with a complete metric. Use it as: letI := upgradePolishSpace α.

Instances For
instance PolishSpace.t2Space (α : Type u_3) [] [] :
instance PolishSpace.pi_countable {ι : Type u_3} [] {E : ιType u_4} [(i : ι) → TopologicalSpace (E i)] [∀ (i : ι), PolishSpace (E i)] :
PolishSpace ((i : ι) → E i)

A countable product of Polish spaces is Polish.

instance PolishSpace.sigma {ι : Type u_3} [] {E : ιType u_4} [(n : ι) → TopologicalSpace (E n)] [∀ (n : ι), PolishSpace (E n)] :
PolishSpace ((n : ι) × E n)

A countable disjoint union of Polish spaces is Polish.

instance PolishSpace.sum {α : Type u_1} {β : Type u_2} [] [] [] [] :

The disjoint union of two Polish spaces is Polish.

theorem PolishSpace.exists_nat_nat_continuous_surjective (α : Type u_3) [] [] [] :
f,

Any nonempty Polish space is the continuous image of the fundamental space ℕ → ℕ.

theorem ClosedEmbedding.polishSpace {α : Type u_1} {β : Type u_2} [] [] [] {f : αβ} (hf : ) :

Given a closed embedding into a Polish space, the source space is also Polish.

instance PolishSpace.polish_of_countable {α : Type u_1} [] [h : ] [] :

Any countable discrete space is Polish.

theorem Equiv.polishSpace_induced {α : Type u_1} {β : Type u_2} [t : ] [] (f : α β) :

Pulling back a Polish topology under an equiv gives again a Polish topology.

theorem IsClosed.polishSpace {α : Type u_1} [] [] {s : Set α} (hs : ) :

A closed subset of a Polish space is also Polish.

instance PolishSpace.instPolishSpaceUniv {α : Type u_1} [] [] :
PolishSpace Set.univ
def PolishSpace.AuxCopy (α : Type u_3) {ι : Type u_4} (_i : ι) :
Type u_3

A sequence of type synonyms of a given type α, useful in the proof of exists_polishSpace_forall_le to endow each copy with a different topology.

Instances For
theorem PolishSpace.exists_polishSpace_forall_le {α : Type u_1} {ι : Type u_3} [] [t : ] [p : ] (m : ι) (hm : ∀ (n : ι), m n t) (h'm : ∀ (n : ι), ) :
t', (∀ (n : ι), t' m n) t' t

Given a Polish space, and countably many finer Polish topologies, there exists another Polish topology which is finer than all of them.

Porting note: TODO: the topology t' is t ⊓ ⨅ i, m i.

### An open subset of a Polish space is Polish #

To prove this fact, one needs to construct another metric, giving rise to the same topology, for which the open subset is complete. This is not obvious, as for instance (0,1) ⊆ ℝ is not complete for the usual metric of ℝ: one should build a new metric that blows up close to the boundary.

Porting note: definitions and lemmas in this section now take (s : Opens α) instead of {s : Set α} (hs : IsOpen s) so that we can turn various definitions and lemmas into instances. Also, some lemmas used to assume Set.Nonempty sᶜ in Lean 3. In fact, this assumption is not needed, so it was dropped.

def TopologicalSpace.Opens.CompleteCopy {α : Type u_3} [] (s : ) :
Type u_3

A type synonym for a subset s of a metric space, on which we will construct another metric for which it will be complete.

Instances For
instance TopologicalSpace.Opens.CompleteCopy.instDist {α : Type u_1} [] {s : } :

A distance on an open subset s of a metric space, designed to make it complete. It is given by dist' x y = dist x y + |1 / dist x sᶜ - 1 / dist y sᶜ|, where the second term blows up close to the boundary to ensure that Cauchy sequences for dist' remain well inside s.

theorem TopologicalSpace.Opens.CompleteCopy.dist_eq {α : Type u_1} [] {s : } :
dist x y = dist x y + |1 / Metric.infDist (x) (s) - 1 / Metric.infDist (y) (s)|

A metric space structure on a subset s of a metric space, designed to make it complete if s is open. It is given by dist' x y = dist x y + |1 / dist x sᶜ - 1 / dist y sᶜ|, where the second term blows up close to the boundary to ensure that Cauchy sequences for dist' remain well inside s.

Porting note: the definition changed to ensure that the TopologicalSpace structure on TopologicalSpace.Opens.CompleteCopy s is definitionally equal to the original one.

theorem IsOpen.polishSpace {α : Type u_3} [] [] {s : Set α} (hs : ) :

An open subset of a Polish space is also Polish.

### Clopenable sets in Polish spaces #

def PolishSpace.IsClopenable {α : Type u_1} [t : ] (s : Set α) :

A set in a topological space is clopenable if there exists a finer Polish topology for which this set is open and closed. It turns out that this notion is equivalent to being Borel-measurable, but this is nontrivial (see isClopenable_iff_measurableSet).

Instances For
theorem IsClosed.isClopenable {α : Type u_1} [] [] {s : Set α} (hs : ) :

Given a closed set s in a Polish space, one can construct a finer Polish topology for which s is both open and closed.

theorem PolishSpace.IsClopenable.compl {α : Type u_1} [] {s : Set α} (hs : ) :
theorem IsOpen.isClopenable {α : Type u_1} [] [] {s : Set α} (hs : ) :
theorem PolishSpace.IsClopenable.iUnion {α : Type u_1} [t : ] [] {s : Set α} (hs : ∀ (n : ), ) :
PolishSpace.IsClopenable (⋃ (n : ), s n)