$mathscr{C}={EsubsetOmega:Einmathscr{R}:text{or}:Omega-Einmathscr{R}}$ is an algebra
$begingroup$
Let $mathscr{R}$ be a ring of subsets of $Omega$ and $mathscr{C}={EsubsetOmega:Einmathscr{R}:text{or}:Omega-Einmathscr{R}}$.
Show that $mathscr{C}$ is an algebra.
To show $mathscr{C}$ is an algebra:
i) $emptysetinmathscr{C}$
ii) $Ainmathscr{C}$ and $Binmathscr{C}$ so $Acup Binmathscr{C}$
iii)$Ainmathscr{C}$ and $Binmathscr{C}$ so $Asetminus Binmathscr{C}$
iv)$Ainmathscr{C}$ so $A^c=Omegasetminus Ainmathscr{C}$
For example if I want to check $mathscr{C}$ is closed for unions then:
If $Ainmathscr{R}$ and $Binmathscr{R}$then $Acup Binmathscr{R}$ because $mathscr{R}$ is a ring hence closed for finite unions or $Omegasetminus(Acup B)=(Acup B)^c=A^ccap B^cinmathscr{R}$ once $mathscr{R}$ is closed for finite intersections. This concludes $Acup B$ is in $mathscr{C}$.
Question:
It was pointed out that this argument I wrote is wrong once I does not prove $Acup B$ is in $mathscr{C}$. But I fail to see why? Can someone tell me what is failing?
Thanks in advance!
measure-theory proof-verification elementary-set-theory proof-writing
$endgroup$
add a comment |
$begingroup$
Let $mathscr{R}$ be a ring of subsets of $Omega$ and $mathscr{C}={EsubsetOmega:Einmathscr{R}:text{or}:Omega-Einmathscr{R}}$.
Show that $mathscr{C}$ is an algebra.
To show $mathscr{C}$ is an algebra:
i) $emptysetinmathscr{C}$
ii) $Ainmathscr{C}$ and $Binmathscr{C}$ so $Acup Binmathscr{C}$
iii)$Ainmathscr{C}$ and $Binmathscr{C}$ so $Asetminus Binmathscr{C}$
iv)$Ainmathscr{C}$ so $A^c=Omegasetminus Ainmathscr{C}$
For example if I want to check $mathscr{C}$ is closed for unions then:
If $Ainmathscr{R}$ and $Binmathscr{R}$then $Acup Binmathscr{R}$ because $mathscr{R}$ is a ring hence closed for finite unions or $Omegasetminus(Acup B)=(Acup B)^c=A^ccap B^cinmathscr{R}$ once $mathscr{R}$ is closed for finite intersections. This concludes $Acup B$ is in $mathscr{C}$.
Question:
It was pointed out that this argument I wrote is wrong once I does not prove $Acup B$ is in $mathscr{C}$. But I fail to see why? Can someone tell me what is failing?
Thanks in advance!
measure-theory proof-verification elementary-set-theory proof-writing
$endgroup$
add a comment |
$begingroup$
Let $mathscr{R}$ be a ring of subsets of $Omega$ and $mathscr{C}={EsubsetOmega:Einmathscr{R}:text{or}:Omega-Einmathscr{R}}$.
Show that $mathscr{C}$ is an algebra.
To show $mathscr{C}$ is an algebra:
i) $emptysetinmathscr{C}$
ii) $Ainmathscr{C}$ and $Binmathscr{C}$ so $Acup Binmathscr{C}$
iii)$Ainmathscr{C}$ and $Binmathscr{C}$ so $Asetminus Binmathscr{C}$
iv)$Ainmathscr{C}$ so $A^c=Omegasetminus Ainmathscr{C}$
For example if I want to check $mathscr{C}$ is closed for unions then:
If $Ainmathscr{R}$ and $Binmathscr{R}$then $Acup Binmathscr{R}$ because $mathscr{R}$ is a ring hence closed for finite unions or $Omegasetminus(Acup B)=(Acup B)^c=A^ccap B^cinmathscr{R}$ once $mathscr{R}$ is closed for finite intersections. This concludes $Acup B$ is in $mathscr{C}$.
Question:
It was pointed out that this argument I wrote is wrong once I does not prove $Acup B$ is in $mathscr{C}$. But I fail to see why? Can someone tell me what is failing?
Thanks in advance!
measure-theory proof-verification elementary-set-theory proof-writing
$endgroup$
Let $mathscr{R}$ be a ring of subsets of $Omega$ and $mathscr{C}={EsubsetOmega:Einmathscr{R}:text{or}:Omega-Einmathscr{R}}$.
Show that $mathscr{C}$ is an algebra.
To show $mathscr{C}$ is an algebra:
i) $emptysetinmathscr{C}$
ii) $Ainmathscr{C}$ and $Binmathscr{C}$ so $Acup Binmathscr{C}$
iii)$Ainmathscr{C}$ and $Binmathscr{C}$ so $Asetminus Binmathscr{C}$
iv)$Ainmathscr{C}$ so $A^c=Omegasetminus Ainmathscr{C}$
For example if I want to check $mathscr{C}$ is closed for unions then:
If $Ainmathscr{R}$ and $Binmathscr{R}$then $Acup Binmathscr{R}$ because $mathscr{R}$ is a ring hence closed for finite unions or $Omegasetminus(Acup B)=(Acup B)^c=A^ccap B^cinmathscr{R}$ once $mathscr{R}$ is closed for finite intersections. This concludes $Acup B$ is in $mathscr{C}$.
Question:
It was pointed out that this argument I wrote is wrong once I does not prove $Acup B$ is in $mathscr{C}$. But I fail to see why? Can someone tell me what is failing?
Thanks in advance!
measure-theory proof-verification elementary-set-theory proof-writing
measure-theory proof-verification elementary-set-theory proof-writing
edited Jan 18 at 22:05
Andrés E. Caicedo
65.4k8158249
65.4k8158249
asked Jan 18 at 19:30
Pedro GomesPedro Gomes
1,8262721
1,8262721
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
You have to take sets $A$ and $ B$ from $mathscr{C}$ and not from $mathscr{R}$. Now do some case analysis
Case 1: $Ainmathscr{R}$ and $Binmathscr{R}$ then $Acup Binmathscr{R}implies Acup Binmathscr{C}$
Case 2: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $A-Binmathscr{R}stackrel{?}{implies} Acup Binmathscr{C}$
Case 3: $A^cinmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c= A^ccap B^cinmathscr{R}stackrel{}{implies} Acup Binmathscr{C}$
However, I'm not sure how to deal with case 2.
$endgroup$
$begingroup$
How do I know that if I take a set in $mathscr{C}$, it is in $mathscr{R}$?
$endgroup$
– Pedro Gomes
Jan 18 at 19:39
$begingroup$
I think $Asetminus B$ is closed in $mathscr{R}$ once $mathscr{R}$ is a ring. Thanks for your answer!
$endgroup$
– Pedro Gomes
Jan 18 at 21:23
$begingroup$
Yes, but is it in $mathscr{C}$?
$endgroup$
– greedoid
Jan 19 at 18:06
$begingroup$
Check my answer!
$endgroup$
– Pedro Gomes
Jan 19 at 18:15
add a comment |
$begingroup$
After greedoid answer I am able to post an answer to the question. So we have 4 cases:
1: $Ainmathscr{R}$ and $Binmathscr{R}$ then $Acup Binmathscr{R}implies Acup Binmathscr{C}$
2: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c=A^ccap B^c=B^ccap A^c=B^csetminus Ainmathscr{R}{implies} Acup Binmathscr{C}$
3: $A^cinmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c= A^ccap B^cinmathscr{R}stackrel{}{implies} Acup Binmathscr{C}$
It was intended to show that either $Acup Binmathscr{R}$ or $(Acup B)^cinmathscr{R}$,which are the conditions defined in $mathscr{C}$ for $Acup B$ to belong to $mathscr{C}$. The remaining sections of the exercise are about the same reasoning.
$endgroup$
$begingroup$
Is this true: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $B^csetminus Ainmathscr{R}$
$endgroup$
– greedoid
Jan 19 at 18:20
$begingroup$
Yes it is. Once rings are closed for finite $setminus$ and we admit that $B^cinmathscr{R}$.It does not mean $Binmathscr{R}$ once the latter is a ring.
$endgroup$
– Pedro Gomes
Jan 19 at 18:23
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
You have to take sets $A$ and $ B$ from $mathscr{C}$ and not from $mathscr{R}$. Now do some case analysis
Case 1: $Ainmathscr{R}$ and $Binmathscr{R}$ then $Acup Binmathscr{R}implies Acup Binmathscr{C}$
Case 2: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $A-Binmathscr{R}stackrel{?}{implies} Acup Binmathscr{C}$
Case 3: $A^cinmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c= A^ccap B^cinmathscr{R}stackrel{}{implies} Acup Binmathscr{C}$
However, I'm not sure how to deal with case 2.
$endgroup$
$begingroup$
How do I know that if I take a set in $mathscr{C}$, it is in $mathscr{R}$?
$endgroup$
– Pedro Gomes
Jan 18 at 19:39
$begingroup$
I think $Asetminus B$ is closed in $mathscr{R}$ once $mathscr{R}$ is a ring. Thanks for your answer!
$endgroup$
– Pedro Gomes
Jan 18 at 21:23
$begingroup$
Yes, but is it in $mathscr{C}$?
$endgroup$
– greedoid
Jan 19 at 18:06
$begingroup$
Check my answer!
$endgroup$
– Pedro Gomes
Jan 19 at 18:15
add a comment |
$begingroup$
You have to take sets $A$ and $ B$ from $mathscr{C}$ and not from $mathscr{R}$. Now do some case analysis
Case 1: $Ainmathscr{R}$ and $Binmathscr{R}$ then $Acup Binmathscr{R}implies Acup Binmathscr{C}$
Case 2: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $A-Binmathscr{R}stackrel{?}{implies} Acup Binmathscr{C}$
Case 3: $A^cinmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c= A^ccap B^cinmathscr{R}stackrel{}{implies} Acup Binmathscr{C}$
However, I'm not sure how to deal with case 2.
$endgroup$
$begingroup$
How do I know that if I take a set in $mathscr{C}$, it is in $mathscr{R}$?
$endgroup$
– Pedro Gomes
Jan 18 at 19:39
$begingroup$
I think $Asetminus B$ is closed in $mathscr{R}$ once $mathscr{R}$ is a ring. Thanks for your answer!
$endgroup$
– Pedro Gomes
Jan 18 at 21:23
$begingroup$
Yes, but is it in $mathscr{C}$?
$endgroup$
– greedoid
Jan 19 at 18:06
$begingroup$
Check my answer!
$endgroup$
– Pedro Gomes
Jan 19 at 18:15
add a comment |
$begingroup$
You have to take sets $A$ and $ B$ from $mathscr{C}$ and not from $mathscr{R}$. Now do some case analysis
Case 1: $Ainmathscr{R}$ and $Binmathscr{R}$ then $Acup Binmathscr{R}implies Acup Binmathscr{C}$
Case 2: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $A-Binmathscr{R}stackrel{?}{implies} Acup Binmathscr{C}$
Case 3: $A^cinmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c= A^ccap B^cinmathscr{R}stackrel{}{implies} Acup Binmathscr{C}$
However, I'm not sure how to deal with case 2.
$endgroup$
You have to take sets $A$ and $ B$ from $mathscr{C}$ and not from $mathscr{R}$. Now do some case analysis
Case 1: $Ainmathscr{R}$ and $Binmathscr{R}$ then $Acup Binmathscr{R}implies Acup Binmathscr{C}$
Case 2: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $A-Binmathscr{R}stackrel{?}{implies} Acup Binmathscr{C}$
Case 3: $A^cinmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c= A^ccap B^cinmathscr{R}stackrel{}{implies} Acup Binmathscr{C}$
However, I'm not sure how to deal with case 2.
edited Jan 18 at 20:29
answered Jan 18 at 19:36
greedoidgreedoid
42.4k1153105
42.4k1153105
$begingroup$
How do I know that if I take a set in $mathscr{C}$, it is in $mathscr{R}$?
$endgroup$
– Pedro Gomes
Jan 18 at 19:39
$begingroup$
I think $Asetminus B$ is closed in $mathscr{R}$ once $mathscr{R}$ is a ring. Thanks for your answer!
$endgroup$
– Pedro Gomes
Jan 18 at 21:23
$begingroup$
Yes, but is it in $mathscr{C}$?
$endgroup$
– greedoid
Jan 19 at 18:06
$begingroup$
Check my answer!
$endgroup$
– Pedro Gomes
Jan 19 at 18:15
add a comment |
$begingroup$
How do I know that if I take a set in $mathscr{C}$, it is in $mathscr{R}$?
$endgroup$
– Pedro Gomes
Jan 18 at 19:39
$begingroup$
I think $Asetminus B$ is closed in $mathscr{R}$ once $mathscr{R}$ is a ring. Thanks for your answer!
$endgroup$
– Pedro Gomes
Jan 18 at 21:23
$begingroup$
Yes, but is it in $mathscr{C}$?
$endgroup$
– greedoid
Jan 19 at 18:06
$begingroup$
Check my answer!
$endgroup$
– Pedro Gomes
Jan 19 at 18:15
$begingroup$
How do I know that if I take a set in $mathscr{C}$, it is in $mathscr{R}$?
$endgroup$
– Pedro Gomes
Jan 18 at 19:39
$begingroup$
How do I know that if I take a set in $mathscr{C}$, it is in $mathscr{R}$?
$endgroup$
– Pedro Gomes
Jan 18 at 19:39
$begingroup$
I think $Asetminus B$ is closed in $mathscr{R}$ once $mathscr{R}$ is a ring. Thanks for your answer!
$endgroup$
– Pedro Gomes
Jan 18 at 21:23
$begingroup$
I think $Asetminus B$ is closed in $mathscr{R}$ once $mathscr{R}$ is a ring. Thanks for your answer!
$endgroup$
– Pedro Gomes
Jan 18 at 21:23
$begingroup$
Yes, but is it in $mathscr{C}$?
$endgroup$
– greedoid
Jan 19 at 18:06
$begingroup$
Yes, but is it in $mathscr{C}$?
$endgroup$
– greedoid
Jan 19 at 18:06
$begingroup$
Check my answer!
$endgroup$
– Pedro Gomes
Jan 19 at 18:15
$begingroup$
Check my answer!
$endgroup$
– Pedro Gomes
Jan 19 at 18:15
add a comment |
$begingroup$
After greedoid answer I am able to post an answer to the question. So we have 4 cases:
1: $Ainmathscr{R}$ and $Binmathscr{R}$ then $Acup Binmathscr{R}implies Acup Binmathscr{C}$
2: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c=A^ccap B^c=B^ccap A^c=B^csetminus Ainmathscr{R}{implies} Acup Binmathscr{C}$
3: $A^cinmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c= A^ccap B^cinmathscr{R}stackrel{}{implies} Acup Binmathscr{C}$
It was intended to show that either $Acup Binmathscr{R}$ or $(Acup B)^cinmathscr{R}$,which are the conditions defined in $mathscr{C}$ for $Acup B$ to belong to $mathscr{C}$. The remaining sections of the exercise are about the same reasoning.
$endgroup$
$begingroup$
Is this true: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $B^csetminus Ainmathscr{R}$
$endgroup$
– greedoid
Jan 19 at 18:20
$begingroup$
Yes it is. Once rings are closed for finite $setminus$ and we admit that $B^cinmathscr{R}$.It does not mean $Binmathscr{R}$ once the latter is a ring.
$endgroup$
– Pedro Gomes
Jan 19 at 18:23
add a comment |
$begingroup$
After greedoid answer I am able to post an answer to the question. So we have 4 cases:
1: $Ainmathscr{R}$ and $Binmathscr{R}$ then $Acup Binmathscr{R}implies Acup Binmathscr{C}$
2: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c=A^ccap B^c=B^ccap A^c=B^csetminus Ainmathscr{R}{implies} Acup Binmathscr{C}$
3: $A^cinmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c= A^ccap B^cinmathscr{R}stackrel{}{implies} Acup Binmathscr{C}$
It was intended to show that either $Acup Binmathscr{R}$ or $(Acup B)^cinmathscr{R}$,which are the conditions defined in $mathscr{C}$ for $Acup B$ to belong to $mathscr{C}$. The remaining sections of the exercise are about the same reasoning.
$endgroup$
$begingroup$
Is this true: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $B^csetminus Ainmathscr{R}$
$endgroup$
– greedoid
Jan 19 at 18:20
$begingroup$
Yes it is. Once rings are closed for finite $setminus$ and we admit that $B^cinmathscr{R}$.It does not mean $Binmathscr{R}$ once the latter is a ring.
$endgroup$
– Pedro Gomes
Jan 19 at 18:23
add a comment |
$begingroup$
After greedoid answer I am able to post an answer to the question. So we have 4 cases:
1: $Ainmathscr{R}$ and $Binmathscr{R}$ then $Acup Binmathscr{R}implies Acup Binmathscr{C}$
2: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c=A^ccap B^c=B^ccap A^c=B^csetminus Ainmathscr{R}{implies} Acup Binmathscr{C}$
3: $A^cinmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c= A^ccap B^cinmathscr{R}stackrel{}{implies} Acup Binmathscr{C}$
It was intended to show that either $Acup Binmathscr{R}$ or $(Acup B)^cinmathscr{R}$,which are the conditions defined in $mathscr{C}$ for $Acup B$ to belong to $mathscr{C}$. The remaining sections of the exercise are about the same reasoning.
$endgroup$
After greedoid answer I am able to post an answer to the question. So we have 4 cases:
1: $Ainmathscr{R}$ and $Binmathscr{R}$ then $Acup Binmathscr{R}implies Acup Binmathscr{C}$
2: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c=A^ccap B^c=B^ccap A^c=B^csetminus Ainmathscr{R}{implies} Acup Binmathscr{C}$
3: $A^cinmathscr{R}$ and $B^cinmathscr{R}$ then $(Acup B)^c= A^ccap B^cinmathscr{R}stackrel{}{implies} Acup Binmathscr{C}$
It was intended to show that either $Acup Binmathscr{R}$ or $(Acup B)^cinmathscr{R}$,which are the conditions defined in $mathscr{C}$ for $Acup B$ to belong to $mathscr{C}$. The remaining sections of the exercise are about the same reasoning.
answered Jan 19 at 18:14
Pedro GomesPedro Gomes
1,8262721
1,8262721
$begingroup$
Is this true: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $B^csetminus Ainmathscr{R}$
$endgroup$
– greedoid
Jan 19 at 18:20
$begingroup$
Yes it is. Once rings are closed for finite $setminus$ and we admit that $B^cinmathscr{R}$.It does not mean $Binmathscr{R}$ once the latter is a ring.
$endgroup$
– Pedro Gomes
Jan 19 at 18:23
add a comment |
$begingroup$
Is this true: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $B^csetminus Ainmathscr{R}$
$endgroup$
– greedoid
Jan 19 at 18:20
$begingroup$
Yes it is. Once rings are closed for finite $setminus$ and we admit that $B^cinmathscr{R}$.It does not mean $Binmathscr{R}$ once the latter is a ring.
$endgroup$
– Pedro Gomes
Jan 19 at 18:23
$begingroup$
Is this true: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $B^csetminus Ainmathscr{R}$
$endgroup$
– greedoid
Jan 19 at 18:20
$begingroup$
Is this true: $Ainmathscr{R}$ and $B^cinmathscr{R}$ then $B^csetminus Ainmathscr{R}$
$endgroup$
– greedoid
Jan 19 at 18:20
$begingroup$
Yes it is. Once rings are closed for finite $setminus$ and we admit that $B^cinmathscr{R}$.It does not mean $Binmathscr{R}$ once the latter is a ring.
$endgroup$
– Pedro Gomes
Jan 19 at 18:23
$begingroup$
Yes it is. Once rings are closed for finite $setminus$ and we admit that $B^cinmathscr{R}$.It does not mean $Binmathscr{R}$ once the latter is a ring.
$endgroup$
– Pedro Gomes
Jan 19 at 18:23
add a comment |
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