Disprove [itex]\sigma[/itex] (singletons in R) = Borel field

[Lily@pie]

Homework Statement

{ E }_{ 1 }:=\{ \left( -\infty ,x \right) :x\in \Re \}

{ E }_{ 2 }:=\{ \left\{ x \right\} :x\in \Re \}

Prove \sigma \left( { E }_{ 1 } \right) =\sigma \left( { E }_{ 2 } \right)

Homework Equations

I know { E }_{ 1 } generates the Borel field

(i.e.) \sigma \left( { E }_{ 1 } \right)=B(ℝ)

The Attempt at a Solution

I know this is wrong as \sigma \left( { E }_{ 2 } \right) \varsubsetneqq \sigma \left( { E }_{ 1 } \right)

Hence my attempt is assume \sigma \left( { E }_{ 1 } \right) \subseteq \sigma \left( { E }_{ 2 } \right)

Let F \in σ(E_{1}) be a non-empty set

As F \in E_{1} \Rightarrow F \in σ(E_{1})

When F \in E_{1}, F = (-\infty, x) for some x \in ℝ

I want to prove that for some x \in ℝ F = (-\infty, x) \notin E_{2}, which means F = (-\infty, x) \notin σ(E_{2}).
It is kind of obvious to me as -∞ \notin ℝ, and x \in ℝ implies that (-\infty, x) cannot be generated by a countable union of singletons.

Hence \sigma \left( { E }_{ 1 } \right) \subseteq \sigma \left( { E }_{ 2 } \right) must be wrong.

But I'm not sure if this statement is strong enough.

Thanks!

 

[Dick]

Homework Statement

{ E }_{ 1 }:=\{ \left( -\infty ,x \right) :x\in \Re \}

{ E }_{ 2 }:=\{ \left\{ x \right\} :x\in \Re \}

Prove \sigma \left( { E }_{ 1 } \right) =\sigma \left( { E }_{ 2 } \right)

Homework Equations

I know { E }_{ 1 } generates the Borel field

(i.e.) \sigma \left( { E }_{ 1 } \right)=B(ℝ)

The Attempt at a Solution

I know this is wrong as \sigma \left( { E }_{ 2 } \right) \varsubsetneqq \sigma \left( { E }_{ 1 } \right)

Hence my attempt is assume \sigma \left( { E }_{ 1 } \right) \subseteq \sigma \left( { E }_{ 2 } \right)

Let F \in σ(E_{1}) be a non-empty set

As F \in E_{1} \Rightarrow F \in σ(E_{1})

When F \in E_{1}, F = (-\infty, x) for some x \in ℝ

I want to prove that for some x \in ℝ F = (-\infty, x) \notin E_{2}, which means F = (-\infty, x) \notin σ(E_{2}).
It is kind of obvious to me as -∞ \notin ℝ, and x \in ℝ implies that (-\infty, x) cannot be generated by a countable union of singletons.

Hence \sigma \left( { E }_{ 1 } \right) \subseteq \sigma \left( { E }_{ 2 } \right) must be wrong.

But I'm not sure if this statement is strong enough.

Thanks!

No, that's not good enough, I hope you know why. Let G be the set consisting of all sets X such that either X is countable or $X^C$ is countable. Can you show that G is a sigma algebra? Now can you show that's exactly the sigma algebra generated by the singletons?

 

[Lily@pie]

No, that's not good enough, I hope you know why. Let G be the set consisting of all sets X such that either X is countable or $X^C$ is countable. Can you show that G is a sigma algebra? Now can you show that's exactly the sigma algebra generated by the singletons?

I could prove that G is a sigma algebra.

However, I'm not quite sure in proving G=σ(singletons)

Can I say G \subseteq {{x}:x\in ℝ}?
Then G=σ(G) \subseteq σ({{x}:x\in ℝ}).

I could also show {{x}:x\in ℝ} \subseteq σ(G)

Hence, G=σ({{x}:x\in ℝ}.

Therefore to show σ( E_{1}) not subset of G,

Let E\in E_{1} be (-∞,x] for some real number x.

since (-∞,x] and (x,∞) are not countable, E \notin G =σ({{x}:x\in ℝ}) .

Am I heading to the correct direction?

Thanks a lot :)

 

[Dick]

I could prove that G is a sigma algebra.

However, I'm not quite sure in proving G=σ(singletons)

Can I say G \subseteq {{x}:x\in ℝ}?
Then G=σ(G) \subseteq σ({{x}:x\in ℝ}).

I could also show {{x}:x\in ℝ} \subseteq σ(G)

Hence, G=σ({{x}:x\in ℝ}.

Therefore to show σ( E_{1}) not subset of G,

Let E\in E_{1} be (-∞,x] for some real number x.

since (-∞,x] and (x,∞) are not countable, E \notin G =σ({{x}:x\in ℝ}) .

Am I heading to the correct direction?

Thanks a lot :)

Given what follows, I'd be interested in hearing the general idea of how you proved G is a sigma algebra. Because the middle part sounds strange. You clearly can't say that G is a subset of the singletons, it's not. There are certainly elements of G that aren't singletons. Just pick an element of G and show that it is in the sigma algebra generated by the singletons. But the end part sounds good.

 

[Lily@pie]

Given what follows, I'd be interested in hearing the general idea of how you proved G is a sigma algebra. Because the middle part sounds strange. You clearly can't say that G is a subset of the singletons, it's not. There are certainly elements of G that aren't singletons. Just pick an element of G and show that it is in the sigma algebra generated by the singletons. But the end part sounds good.

To prove that G is a sigma algebra, I just showed that G satisfy the 3 axioms of sigma algebra.

I admit the middle part is strange as I am not sure.

I've tried the following to prove G = σ({{x}:x\in ℝ})

1st, I will prove that G \subset σ({{x}:x\in ℝ}).

Since an element in G is either countable to the complement is countable, it can be written as a countable union of singletons. Hence the element will be in the sigma field.

However, I really don't know how to show that σ({{x}:x\in ℝ}) \subset G. Can I say that an element in sigma(singleton) can be singleton or the complement of singleton?

 

[Dick]

1st, I will prove that G \subset σ({{x}:x\in ℝ}).

Since an element in G is either countable to the complement is countable, it can be written as a countable union of singletons. Hence the element will be in the sigma field.

However, I really don't know how to show that σ({{x}:x\in ℝ}) \subset G. Can I say that an element in sigma(singleton) can be singleton or the complement of singleton?

Maybe you just aren't making complete statements. It's surely not true that if a set has a countable complement then it's a "countable union of singletons", is it? For the second part, let's call S=σ({{x}:x\in ℝ}). You know that every element of G is in S. And G is a sigma algebra. And the singletons are in G. S is defined to be the SMALLEST sigma algebra containing the singletons. So?

 

[Lily@pie]

Maybe you just aren't making complete statements. It's surely not true that if a set has a countable complement then it's a "countable union of singletons", is it? For the second part, let's call S=σ({{x}:x\in ℝ}). You know that every element of G is in S. And G is a sigma algebra. And the singletons are in G. S is defined to be the SMALLEST sigma algebra containing the singletons. So?

I get the second part now.

This is what I was thinking for the first part,

Let A={{x}:x\in ℝ}
For the first part, I am trying to prove that G \subseteq σ(A)

If F \in G, F is countable or the complement is countable,

If F is countable, we can write F as \stackrel{\bigcup}{x\in F}{x}. F is now a countable union of elements in σ(A), so F \in σ(A).

If F^C is countable, F^{C}=\stackrel{\bigcup}{x\in F^{C}}{x}. F^C is now a countable union of elements in σ(A), so F^{C} \in σ(A) and this implies (F^{C})^{C}=F \in σ(A).

(i.e) G \subseteq σ(A)

Am I heading the right direction? Thanks heaps!

 

[Dick]

I get the second part now.

This is what I was thinking for the first part,

Let A={{x}:x\in ℝ}
For the first part, I am trying to prove that G \subseteq σ(A)

If F \in G, F is countable or the complement is countable,

If F is countable, we can write F as \stackrel{\bigcup}{x\in F}{x}. F is now a countable union of elements in σ(A), so F \in σ(A).

If F^C is countable, F^{C}=\stackrel{\bigcup}{x\in F^{C}}{x}. F^C is now a countable union of elements in σ(A), so F^{C} \in σ(A) and this implies (F^{C})^{C}=F \in σ(A).

(i.e) G \subseteq σ(A)

Am I heading the right direction? Thanks heaps!

Yes, you are. Now if you are confident you have proved that G is a sigma algebra using the axioms, you should be done. Like I said, if you want to explain how that worked, I could look at that part too.