If $epsilon_1$ and $epsilon_2$ are 2 elementary families of sets in $X_1$ and $X_2$ respectively then the cross product is an elementary family too.

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I am trying to show that if $epsilon_1$ and $epsilon_2$ are 2 elementary families of sets in $X_1$ and $X_2$ respectively then $$epsilon_1 times epsilon_2=E_1 times E_2 $$ is an elementary family of sets on $X_1 times X_2$.




My book, Folland, defines an elementary family to be a collection
$epsilon$ of subsets of $X$ such that:



1) The empty set is in $epsilon$



2) If $E,F in epsilon$, then $E cap F in epsilon$



3) If $E in epsilon$, then $E^c$ is a finite disjoint union of
members of $epsilon$




For this problem I am trying to show the following:



1) The empty set is in $epsilon_1 times epsilon_2$ is clear since it is in both $epsilon_1$ and $epsilon_2$.



I am not sure about 2) and 3)...



2) I want to show that if $E,F in epsilon_1 times epsilon_2$, then $E cap F in epsilon_1 times epsilon_2$



We know that there exists $E_i,E_m in epsilon_1$ and $E_j,E_n in epsilon_2$ so that $E=E_i times E_j$ and $F=E_m times E_n$. Then, $E cap F=(E_i times E_j) cap (E_m times E_n)$ where both terms in parentheses are in $epsilon_1 times epsilon_2$ by definition. So we have that $E cap F in epsilon_1 times epsilon_2$. I am wondering if this is correct...



3) I want to show that If $E in epsilon_1 times epsilon_2$, then $E^c$ is a finite disjoint union of members of $epsilon_1 times epsilon_2$.



Again, we know that there exists $E_i in epsilon_1$ and $E_j in epsilon_2$ so that $E=E_i times E_j$. I then said that $E^c=(E_i times E_j)^c = (E_i^c times X_1) cup (E_j^c times X_2)$ but I am stuck because $(E_i^c times X_1)$ and $(E_j^c times X_2)$ are not disjoint as far as I understand...



Thank you for any help you can give me.







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  • 1




    In 2, the point is that $Ecap F=(E_icap E_m)times (E_jcap E_n)$.
    – Lord Shark the Unknown
    Aug 27 at 18:19










  • @LordSharktheUnknown why is that? Shouldn't E be made up of some $E_i in X_1$ and some $E_j in X_2$?
    – MathIsHard
    Aug 27 at 18:24










  • Oh I think I misread what you put. Thank you I see what you mean now. :)
    – MathIsHard
    Aug 27 at 18:26














up vote
1
down vote

favorite












I am trying to show that if $epsilon_1$ and $epsilon_2$ are 2 elementary families of sets in $X_1$ and $X_2$ respectively then $$epsilon_1 times epsilon_2=E_1 times E_2 $$ is an elementary family of sets on $X_1 times X_2$.




My book, Folland, defines an elementary family to be a collection
$epsilon$ of subsets of $X$ such that:



1) The empty set is in $epsilon$



2) If $E,F in epsilon$, then $E cap F in epsilon$



3) If $E in epsilon$, then $E^c$ is a finite disjoint union of
members of $epsilon$




For this problem I am trying to show the following:



1) The empty set is in $epsilon_1 times epsilon_2$ is clear since it is in both $epsilon_1$ and $epsilon_2$.



I am not sure about 2) and 3)...



2) I want to show that if $E,F in epsilon_1 times epsilon_2$, then $E cap F in epsilon_1 times epsilon_2$



We know that there exists $E_i,E_m in epsilon_1$ and $E_j,E_n in epsilon_2$ so that $E=E_i times E_j$ and $F=E_m times E_n$. Then, $E cap F=(E_i times E_j) cap (E_m times E_n)$ where both terms in parentheses are in $epsilon_1 times epsilon_2$ by definition. So we have that $E cap F in epsilon_1 times epsilon_2$. I am wondering if this is correct...



3) I want to show that If $E in epsilon_1 times epsilon_2$, then $E^c$ is a finite disjoint union of members of $epsilon_1 times epsilon_2$.



Again, we know that there exists $E_i in epsilon_1$ and $E_j in epsilon_2$ so that $E=E_i times E_j$. I then said that $E^c=(E_i times E_j)^c = (E_i^c times X_1) cup (E_j^c times X_2)$ but I am stuck because $(E_i^c times X_1)$ and $(E_j^c times X_2)$ are not disjoint as far as I understand...



Thank you for any help you can give me.







share|cite|improve this question
















  • 1




    In 2, the point is that $Ecap F=(E_icap E_m)times (E_jcap E_n)$.
    – Lord Shark the Unknown
    Aug 27 at 18:19










  • @LordSharktheUnknown why is that? Shouldn't E be made up of some $E_i in X_1$ and some $E_j in X_2$?
    – MathIsHard
    Aug 27 at 18:24










  • Oh I think I misread what you put. Thank you I see what you mean now. :)
    – MathIsHard
    Aug 27 at 18:26












up vote
1
down vote

favorite









up vote
1
down vote

favorite











I am trying to show that if $epsilon_1$ and $epsilon_2$ are 2 elementary families of sets in $X_1$ and $X_2$ respectively then $$epsilon_1 times epsilon_2=E_1 times E_2 $$ is an elementary family of sets on $X_1 times X_2$.




My book, Folland, defines an elementary family to be a collection
$epsilon$ of subsets of $X$ such that:



1) The empty set is in $epsilon$



2) If $E,F in epsilon$, then $E cap F in epsilon$



3) If $E in epsilon$, then $E^c$ is a finite disjoint union of
members of $epsilon$




For this problem I am trying to show the following:



1) The empty set is in $epsilon_1 times epsilon_2$ is clear since it is in both $epsilon_1$ and $epsilon_2$.



I am not sure about 2) and 3)...



2) I want to show that if $E,F in epsilon_1 times epsilon_2$, then $E cap F in epsilon_1 times epsilon_2$



We know that there exists $E_i,E_m in epsilon_1$ and $E_j,E_n in epsilon_2$ so that $E=E_i times E_j$ and $F=E_m times E_n$. Then, $E cap F=(E_i times E_j) cap (E_m times E_n)$ where both terms in parentheses are in $epsilon_1 times epsilon_2$ by definition. So we have that $E cap F in epsilon_1 times epsilon_2$. I am wondering if this is correct...



3) I want to show that If $E in epsilon_1 times epsilon_2$, then $E^c$ is a finite disjoint union of members of $epsilon_1 times epsilon_2$.



Again, we know that there exists $E_i in epsilon_1$ and $E_j in epsilon_2$ so that $E=E_i times E_j$. I then said that $E^c=(E_i times E_j)^c = (E_i^c times X_1) cup (E_j^c times X_2)$ but I am stuck because $(E_i^c times X_1)$ and $(E_j^c times X_2)$ are not disjoint as far as I understand...



Thank you for any help you can give me.







share|cite|improve this question












I am trying to show that if $epsilon_1$ and $epsilon_2$ are 2 elementary families of sets in $X_1$ and $X_2$ respectively then $$epsilon_1 times epsilon_2=E_1 times E_2 $$ is an elementary family of sets on $X_1 times X_2$.




My book, Folland, defines an elementary family to be a collection
$epsilon$ of subsets of $X$ such that:



1) The empty set is in $epsilon$



2) If $E,F in epsilon$, then $E cap F in epsilon$



3) If $E in epsilon$, then $E^c$ is a finite disjoint union of
members of $epsilon$




For this problem I am trying to show the following:



1) The empty set is in $epsilon_1 times epsilon_2$ is clear since it is in both $epsilon_1$ and $epsilon_2$.



I am not sure about 2) and 3)...



2) I want to show that if $E,F in epsilon_1 times epsilon_2$, then $E cap F in epsilon_1 times epsilon_2$



We know that there exists $E_i,E_m in epsilon_1$ and $E_j,E_n in epsilon_2$ so that $E=E_i times E_j$ and $F=E_m times E_n$. Then, $E cap F=(E_i times E_j) cap (E_m times E_n)$ where both terms in parentheses are in $epsilon_1 times epsilon_2$ by definition. So we have that $E cap F in epsilon_1 times epsilon_2$. I am wondering if this is correct...



3) I want to show that If $E in epsilon_1 times epsilon_2$, then $E^c$ is a finite disjoint union of members of $epsilon_1 times epsilon_2$.



Again, we know that there exists $E_i in epsilon_1$ and $E_j in epsilon_2$ so that $E=E_i times E_j$. I then said that $E^c=(E_i times E_j)^c = (E_i^c times X_1) cup (E_j^c times X_2)$ but I am stuck because $(E_i^c times X_1)$ and $(E_j^c times X_2)$ are not disjoint as far as I understand...



Thank you for any help you can give me.









share|cite|improve this question











share|cite|improve this question




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asked Aug 27 at 18:16









MathIsHard

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  • 1




    In 2, the point is that $Ecap F=(E_icap E_m)times (E_jcap E_n)$.
    – Lord Shark the Unknown
    Aug 27 at 18:19










  • @LordSharktheUnknown why is that? Shouldn't E be made up of some $E_i in X_1$ and some $E_j in X_2$?
    – MathIsHard
    Aug 27 at 18:24










  • Oh I think I misread what you put. Thank you I see what you mean now. :)
    – MathIsHard
    Aug 27 at 18:26












  • 1




    In 2, the point is that $Ecap F=(E_icap E_m)times (E_jcap E_n)$.
    – Lord Shark the Unknown
    Aug 27 at 18:19










  • @LordSharktheUnknown why is that? Shouldn't E be made up of some $E_i in X_1$ and some $E_j in X_2$?
    – MathIsHard
    Aug 27 at 18:24










  • Oh I think I misread what you put. Thank you I see what you mean now. :)
    – MathIsHard
    Aug 27 at 18:26







1




1




In 2, the point is that $Ecap F=(E_icap E_m)times (E_jcap E_n)$.
– Lord Shark the Unknown
Aug 27 at 18:19




In 2, the point is that $Ecap F=(E_icap E_m)times (E_jcap E_n)$.
– Lord Shark the Unknown
Aug 27 at 18:19












@LordSharktheUnknown why is that? Shouldn't E be made up of some $E_i in X_1$ and some $E_j in X_2$?
– MathIsHard
Aug 27 at 18:24




@LordSharktheUnknown why is that? Shouldn't E be made up of some $E_i in X_1$ and some $E_j in X_2$?
– MathIsHard
Aug 27 at 18:24












Oh I think I misread what you put. Thank you I see what you mean now. :)
– MathIsHard
Aug 27 at 18:26




Oh I think I misread what you put. Thank you I see what you mean now. :)
– MathIsHard
Aug 27 at 18:26










1 Answer
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For 3, let $Einvarepsilon_1$ and $E'invarepsilon_2$. Then
$X_1=E_0cup E_1cupcdotscup E_m$ where this is a disjoint union of
elements of $varepsilon_1$ and $E_0=E$. Likewise
$X_2=E_0'cup E_1'cupcdotscup E_n'$ where this is a disjoint union of
elements of $varepsilon_2$ and $E_0'=E'$.



Then $(Etimes E')^c$ is the disjoint union of the $E_itimes E_j'$
for $(i,j)ne(0,0)$.






share|cite|improve this answer




















  • Oh thank you very much. I appreciate the help. :) I see now that my approach wasn't quite correct.
    – MathIsHard
    Aug 27 at 18:28










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1 Answer
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active

oldest

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1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
1
down vote



accepted










For 3, let $Einvarepsilon_1$ and $E'invarepsilon_2$. Then
$X_1=E_0cup E_1cupcdotscup E_m$ where this is a disjoint union of
elements of $varepsilon_1$ and $E_0=E$. Likewise
$X_2=E_0'cup E_1'cupcdotscup E_n'$ where this is a disjoint union of
elements of $varepsilon_2$ and $E_0'=E'$.



Then $(Etimes E')^c$ is the disjoint union of the $E_itimes E_j'$
for $(i,j)ne(0,0)$.






share|cite|improve this answer




















  • Oh thank you very much. I appreciate the help. :) I see now that my approach wasn't quite correct.
    – MathIsHard
    Aug 27 at 18:28














up vote
1
down vote



accepted










For 3, let $Einvarepsilon_1$ and $E'invarepsilon_2$. Then
$X_1=E_0cup E_1cupcdotscup E_m$ where this is a disjoint union of
elements of $varepsilon_1$ and $E_0=E$. Likewise
$X_2=E_0'cup E_1'cupcdotscup E_n'$ where this is a disjoint union of
elements of $varepsilon_2$ and $E_0'=E'$.



Then $(Etimes E')^c$ is the disjoint union of the $E_itimes E_j'$
for $(i,j)ne(0,0)$.






share|cite|improve this answer




















  • Oh thank you very much. I appreciate the help. :) I see now that my approach wasn't quite correct.
    – MathIsHard
    Aug 27 at 18:28












up vote
1
down vote



accepted







up vote
1
down vote



accepted






For 3, let $Einvarepsilon_1$ and $E'invarepsilon_2$. Then
$X_1=E_0cup E_1cupcdotscup E_m$ where this is a disjoint union of
elements of $varepsilon_1$ and $E_0=E$. Likewise
$X_2=E_0'cup E_1'cupcdotscup E_n'$ where this is a disjoint union of
elements of $varepsilon_2$ and $E_0'=E'$.



Then $(Etimes E')^c$ is the disjoint union of the $E_itimes E_j'$
for $(i,j)ne(0,0)$.






share|cite|improve this answer












For 3, let $Einvarepsilon_1$ and $E'invarepsilon_2$. Then
$X_1=E_0cup E_1cupcdotscup E_m$ where this is a disjoint union of
elements of $varepsilon_1$ and $E_0=E$. Likewise
$X_2=E_0'cup E_1'cupcdotscup E_n'$ where this is a disjoint union of
elements of $varepsilon_2$ and $E_0'=E'$.



Then $(Etimes E')^c$ is the disjoint union of the $E_itimes E_j'$
for $(i,j)ne(0,0)$.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Aug 27 at 18:24









Lord Shark the Unknown

88.5k955115




88.5k955115











  • Oh thank you very much. I appreciate the help. :) I see now that my approach wasn't quite correct.
    – MathIsHard
    Aug 27 at 18:28
















  • Oh thank you very much. I appreciate the help. :) I see now that my approach wasn't quite correct.
    – MathIsHard
    Aug 27 at 18:28















Oh thank you very much. I appreciate the help. :) I see now that my approach wasn't quite correct.
– MathIsHard
Aug 27 at 18:28




Oh thank you very much. I appreciate the help. :) I see now that my approach wasn't quite correct.
– MathIsHard
Aug 27 at 18:28

















 

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