The intersection of the associated primes of a reduced ring
$begingroup$
Let $R $ be a commutative ring with identity. Recall that a prime ideal is called associated prime ideal whenever it is the annihilator of a nonzero element. Also a ring is called reduced whenever has no nonzero nilpotent.
Why is zero the intersection of all associated prime ideals of a reduced ring?
abstract-algebra algebraic-geometry ring-theory commutative-algebra
$endgroup$
add a comment |
$begingroup$
Let $R $ be a commutative ring with identity. Recall that a prime ideal is called associated prime ideal whenever it is the annihilator of a nonzero element. Also a ring is called reduced whenever has no nonzero nilpotent.
Why is zero the intersection of all associated prime ideals of a reduced ring?
abstract-algebra algebraic-geometry ring-theory commutative-algebra
$endgroup$
$begingroup$
a nonzero element.... of $R$? Associated primes are usually defined with respect to a given module $M$. Are we supposed to assume $M=R$ or something?
$endgroup$
– rschwieb
Jan 11 at 17:04
$begingroup$
Yes. A prime ideal is a associated prime ideal of an $R $-module, if there exists a non zero element of that module such that the prime ideal is its annihilator.
$endgroup$
– Deroty
Jan 11 at 17:35
$begingroup$
I already knew everything after "yes", and it is not relevant to my question. Does your "yes" mean you are confirming that you are talking about a nonzero element of $R$ or do you mean something else (all associated primes for all $R$ modules?)
$endgroup$
– rschwieb
Jan 11 at 17:44
$begingroup$
About non zero elements of $R $.
$endgroup$
– Deroty
Jan 11 at 18:31
add a comment |
$begingroup$
Let $R $ be a commutative ring with identity. Recall that a prime ideal is called associated prime ideal whenever it is the annihilator of a nonzero element. Also a ring is called reduced whenever has no nonzero nilpotent.
Why is zero the intersection of all associated prime ideals of a reduced ring?
abstract-algebra algebraic-geometry ring-theory commutative-algebra
$endgroup$
Let $R $ be a commutative ring with identity. Recall that a prime ideal is called associated prime ideal whenever it is the annihilator of a nonzero element. Also a ring is called reduced whenever has no nonzero nilpotent.
Why is zero the intersection of all associated prime ideals of a reduced ring?
abstract-algebra algebraic-geometry ring-theory commutative-algebra
abstract-algebra algebraic-geometry ring-theory commutative-algebra
edited Jan 13 at 8:26
user26857
39.5k124284
39.5k124284
asked Jan 11 at 14:15
DerotyDeroty
31919
31919
$begingroup$
a nonzero element.... of $R$? Associated primes are usually defined with respect to a given module $M$. Are we supposed to assume $M=R$ or something?
$endgroup$
– rschwieb
Jan 11 at 17:04
$begingroup$
Yes. A prime ideal is a associated prime ideal of an $R $-module, if there exists a non zero element of that module such that the prime ideal is its annihilator.
$endgroup$
– Deroty
Jan 11 at 17:35
$begingroup$
I already knew everything after "yes", and it is not relevant to my question. Does your "yes" mean you are confirming that you are talking about a nonzero element of $R$ or do you mean something else (all associated primes for all $R$ modules?)
$endgroup$
– rschwieb
Jan 11 at 17:44
$begingroup$
About non zero elements of $R $.
$endgroup$
– Deroty
Jan 11 at 18:31
add a comment |
$begingroup$
a nonzero element.... of $R$? Associated primes are usually defined with respect to a given module $M$. Are we supposed to assume $M=R$ or something?
$endgroup$
– rschwieb
Jan 11 at 17:04
$begingroup$
Yes. A prime ideal is a associated prime ideal of an $R $-module, if there exists a non zero element of that module such that the prime ideal is its annihilator.
$endgroup$
– Deroty
Jan 11 at 17:35
$begingroup$
I already knew everything after "yes", and it is not relevant to my question. Does your "yes" mean you are confirming that you are talking about a nonzero element of $R$ or do you mean something else (all associated primes for all $R$ modules?)
$endgroup$
– rschwieb
Jan 11 at 17:44
$begingroup$
About non zero elements of $R $.
$endgroup$
– Deroty
Jan 11 at 18:31
$begingroup$
a nonzero element.... of $R$? Associated primes are usually defined with respect to a given module $M$. Are we supposed to assume $M=R$ or something?
$endgroup$
– rschwieb
Jan 11 at 17:04
$begingroup$
a nonzero element.... of $R$? Associated primes are usually defined with respect to a given module $M$. Are we supposed to assume $M=R$ or something?
$endgroup$
– rschwieb
Jan 11 at 17:04
$begingroup$
Yes. A prime ideal is a associated prime ideal of an $R $-module, if there exists a non zero element of that module such that the prime ideal is its annihilator.
$endgroup$
– Deroty
Jan 11 at 17:35
$begingroup$
Yes. A prime ideal is a associated prime ideal of an $R $-module, if there exists a non zero element of that module such that the prime ideal is its annihilator.
$endgroup$
– Deroty
Jan 11 at 17:35
$begingroup$
I already knew everything after "yes", and it is not relevant to my question. Does your "yes" mean you are confirming that you are talking about a nonzero element of $R$ or do you mean something else (all associated primes for all $R$ modules?)
$endgroup$
– rschwieb
Jan 11 at 17:44
$begingroup$
I already knew everything after "yes", and it is not relevant to my question. Does your "yes" mean you are confirming that you are talking about a nonzero element of $R$ or do you mean something else (all associated primes for all $R$ modules?)
$endgroup$
– rschwieb
Jan 11 at 17:44
$begingroup$
About non zero elements of $R $.
$endgroup$
– Deroty
Jan 11 at 18:31
$begingroup$
About non zero elements of $R $.
$endgroup$
– Deroty
Jan 11 at 18:31
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Let $R$ be a commutative ring. We denote by ${rm Ass}(R)$ the set of associated primes of $R$ and by ${rm Ass}^f(R)$ the set of weakly associated primes of $R$.
(1) By Bourbaki, AC.IV.1 Exercice 17 b), we have $bigcap{rm Ass}^f(R)={rm Nil}(R)$. Thus, if $R$ is reduced, then $bigcap{rm Ass}^f(R)=0$.
(2) If $R$ is noetherian, then by Bourbaki, AC.IV.1 Exercice 17 g) we have ${rm Ass}^f(R)={rm Ass}(R)$, and therefore $bigcap{rm Ass}(R)={rm Nil}(R)$ by (1). Thus, if $R$ is reduced and noetherian, then $bigcap{rm Ass}(R)=0$.
(3) There exists a non-noetherian reduced ring $R$ such that $bigcap{rm Ass}(R)=0$. For this it suffices to exhibit a non-noetherian reduced ring $R$ such that ${rm Ass}^f(R)={rm Ass}(R)$. Such an example (with $R$ even a domain) is given in P.-J. Cahen, Ascending chain conditions and associated primes, Commutative ring theory (Fès, 1992), 41-46, Lecture Notes in Pure and Appl. Math. 153, Dekker, New York, 1994.
(4) There exists a reduced ring $R$ such that $bigcap{rm Ass}(R)neq 0$. For this, any nonzero reduced ring $R$ with ${rm Ass}(R)=emptyset$ will do.
(Thanks to user25867 for pointing out how to improve statement (1).)
$endgroup$
1
$begingroup$
I think the nilradical equals the intersection of all weakly associated primes for any commutative ring, no need to assume that R is reduced. And this is the way one should read Bernard's answer.
$endgroup$
– user26857
Jan 18 at 11:21
1
$begingroup$
@user26857: Thanks for pointing this out!
$endgroup$
– Fred Rohrer
Jan 18 at 12:05
add a comment |
$begingroup$
The intersection of all associated prime ideals is the nilradical of $R$, i.e. the set of nilpotent elements. If the ring is reduced, this set is ${0}$.
$endgroup$
2
$begingroup$
I've seen that the nilradical is this intersection for Noetherian rings but does it hold in general?
$endgroup$
– rschwieb
Jan 11 at 14:28
2
$begingroup$
I believe this is true with correct notion of associated primes (weakly associated prime ideals – which are the same if the ring is noetherian).
$endgroup$
– Bernard
Jan 11 at 14:33
$begingroup$
The intersection of all minimal prime ideals is the nilradical of $R $. I want to know why it is true for associated primes in reduced rings, with the mentioned definition?
$endgroup$
– Deroty
Jan 11 at 14:33
2
$begingroup$
That's because one shows $operatorname{Spec} A$ and $operatorname{Ass} A$ have the same minimal elements.
$endgroup$
– Bernard
Jan 12 at 11:13
1
$begingroup$
As far as I remember, you can replace associated with weakly associated.
$endgroup$
– Bernard
Jan 12 at 12:50
|
show 7 more comments
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Let $R$ be a commutative ring. We denote by ${rm Ass}(R)$ the set of associated primes of $R$ and by ${rm Ass}^f(R)$ the set of weakly associated primes of $R$.
(1) By Bourbaki, AC.IV.1 Exercice 17 b), we have $bigcap{rm Ass}^f(R)={rm Nil}(R)$. Thus, if $R$ is reduced, then $bigcap{rm Ass}^f(R)=0$.
(2) If $R$ is noetherian, then by Bourbaki, AC.IV.1 Exercice 17 g) we have ${rm Ass}^f(R)={rm Ass}(R)$, and therefore $bigcap{rm Ass}(R)={rm Nil}(R)$ by (1). Thus, if $R$ is reduced and noetherian, then $bigcap{rm Ass}(R)=0$.
(3) There exists a non-noetherian reduced ring $R$ such that $bigcap{rm Ass}(R)=0$. For this it suffices to exhibit a non-noetherian reduced ring $R$ such that ${rm Ass}^f(R)={rm Ass}(R)$. Such an example (with $R$ even a domain) is given in P.-J. Cahen, Ascending chain conditions and associated primes, Commutative ring theory (Fès, 1992), 41-46, Lecture Notes in Pure and Appl. Math. 153, Dekker, New York, 1994.
(4) There exists a reduced ring $R$ such that $bigcap{rm Ass}(R)neq 0$. For this, any nonzero reduced ring $R$ with ${rm Ass}(R)=emptyset$ will do.
(Thanks to user25867 for pointing out how to improve statement (1).)
$endgroup$
1
$begingroup$
I think the nilradical equals the intersection of all weakly associated primes for any commutative ring, no need to assume that R is reduced. And this is the way one should read Bernard's answer.
$endgroup$
– user26857
Jan 18 at 11:21
1
$begingroup$
@user26857: Thanks for pointing this out!
$endgroup$
– Fred Rohrer
Jan 18 at 12:05
add a comment |
$begingroup$
Let $R$ be a commutative ring. We denote by ${rm Ass}(R)$ the set of associated primes of $R$ and by ${rm Ass}^f(R)$ the set of weakly associated primes of $R$.
(1) By Bourbaki, AC.IV.1 Exercice 17 b), we have $bigcap{rm Ass}^f(R)={rm Nil}(R)$. Thus, if $R$ is reduced, then $bigcap{rm Ass}^f(R)=0$.
(2) If $R$ is noetherian, then by Bourbaki, AC.IV.1 Exercice 17 g) we have ${rm Ass}^f(R)={rm Ass}(R)$, and therefore $bigcap{rm Ass}(R)={rm Nil}(R)$ by (1). Thus, if $R$ is reduced and noetherian, then $bigcap{rm Ass}(R)=0$.
(3) There exists a non-noetherian reduced ring $R$ such that $bigcap{rm Ass}(R)=0$. For this it suffices to exhibit a non-noetherian reduced ring $R$ such that ${rm Ass}^f(R)={rm Ass}(R)$. Such an example (with $R$ even a domain) is given in P.-J. Cahen, Ascending chain conditions and associated primes, Commutative ring theory (Fès, 1992), 41-46, Lecture Notes in Pure and Appl. Math. 153, Dekker, New York, 1994.
(4) There exists a reduced ring $R$ such that $bigcap{rm Ass}(R)neq 0$. For this, any nonzero reduced ring $R$ with ${rm Ass}(R)=emptyset$ will do.
(Thanks to user25867 for pointing out how to improve statement (1).)
$endgroup$
1
$begingroup$
I think the nilradical equals the intersection of all weakly associated primes for any commutative ring, no need to assume that R is reduced. And this is the way one should read Bernard's answer.
$endgroup$
– user26857
Jan 18 at 11:21
1
$begingroup$
@user26857: Thanks for pointing this out!
$endgroup$
– Fred Rohrer
Jan 18 at 12:05
add a comment |
$begingroup$
Let $R$ be a commutative ring. We denote by ${rm Ass}(R)$ the set of associated primes of $R$ and by ${rm Ass}^f(R)$ the set of weakly associated primes of $R$.
(1) By Bourbaki, AC.IV.1 Exercice 17 b), we have $bigcap{rm Ass}^f(R)={rm Nil}(R)$. Thus, if $R$ is reduced, then $bigcap{rm Ass}^f(R)=0$.
(2) If $R$ is noetherian, then by Bourbaki, AC.IV.1 Exercice 17 g) we have ${rm Ass}^f(R)={rm Ass}(R)$, and therefore $bigcap{rm Ass}(R)={rm Nil}(R)$ by (1). Thus, if $R$ is reduced and noetherian, then $bigcap{rm Ass}(R)=0$.
(3) There exists a non-noetherian reduced ring $R$ such that $bigcap{rm Ass}(R)=0$. For this it suffices to exhibit a non-noetherian reduced ring $R$ such that ${rm Ass}^f(R)={rm Ass}(R)$. Such an example (with $R$ even a domain) is given in P.-J. Cahen, Ascending chain conditions and associated primes, Commutative ring theory (Fès, 1992), 41-46, Lecture Notes in Pure and Appl. Math. 153, Dekker, New York, 1994.
(4) There exists a reduced ring $R$ such that $bigcap{rm Ass}(R)neq 0$. For this, any nonzero reduced ring $R$ with ${rm Ass}(R)=emptyset$ will do.
(Thanks to user25867 for pointing out how to improve statement (1).)
$endgroup$
Let $R$ be a commutative ring. We denote by ${rm Ass}(R)$ the set of associated primes of $R$ and by ${rm Ass}^f(R)$ the set of weakly associated primes of $R$.
(1) By Bourbaki, AC.IV.1 Exercice 17 b), we have $bigcap{rm Ass}^f(R)={rm Nil}(R)$. Thus, if $R$ is reduced, then $bigcap{rm Ass}^f(R)=0$.
(2) If $R$ is noetherian, then by Bourbaki, AC.IV.1 Exercice 17 g) we have ${rm Ass}^f(R)={rm Ass}(R)$, and therefore $bigcap{rm Ass}(R)={rm Nil}(R)$ by (1). Thus, if $R$ is reduced and noetherian, then $bigcap{rm Ass}(R)=0$.
(3) There exists a non-noetherian reduced ring $R$ such that $bigcap{rm Ass}(R)=0$. For this it suffices to exhibit a non-noetherian reduced ring $R$ such that ${rm Ass}^f(R)={rm Ass}(R)$. Such an example (with $R$ even a domain) is given in P.-J. Cahen, Ascending chain conditions and associated primes, Commutative ring theory (Fès, 1992), 41-46, Lecture Notes in Pure and Appl. Math. 153, Dekker, New York, 1994.
(4) There exists a reduced ring $R$ such that $bigcap{rm Ass}(R)neq 0$. For this, any nonzero reduced ring $R$ with ${rm Ass}(R)=emptyset$ will do.
(Thanks to user25867 for pointing out how to improve statement (1).)
edited Jan 18 at 12:05
answered Jan 16 at 13:52
Fred RohrerFred Rohrer
386110
386110
1
$begingroup$
I think the nilradical equals the intersection of all weakly associated primes for any commutative ring, no need to assume that R is reduced. And this is the way one should read Bernard's answer.
$endgroup$
– user26857
Jan 18 at 11:21
1
$begingroup$
@user26857: Thanks for pointing this out!
$endgroup$
– Fred Rohrer
Jan 18 at 12:05
add a comment |
1
$begingroup$
I think the nilradical equals the intersection of all weakly associated primes for any commutative ring, no need to assume that R is reduced. And this is the way one should read Bernard's answer.
$endgroup$
– user26857
Jan 18 at 11:21
1
$begingroup$
@user26857: Thanks for pointing this out!
$endgroup$
– Fred Rohrer
Jan 18 at 12:05
1
1
$begingroup$
I think the nilradical equals the intersection of all weakly associated primes for any commutative ring, no need to assume that R is reduced. And this is the way one should read Bernard's answer.
$endgroup$
– user26857
Jan 18 at 11:21
$begingroup$
I think the nilradical equals the intersection of all weakly associated primes for any commutative ring, no need to assume that R is reduced. And this is the way one should read Bernard's answer.
$endgroup$
– user26857
Jan 18 at 11:21
1
1
$begingroup$
@user26857: Thanks for pointing this out!
$endgroup$
– Fred Rohrer
Jan 18 at 12:05
$begingroup$
@user26857: Thanks for pointing this out!
$endgroup$
– Fred Rohrer
Jan 18 at 12:05
add a comment |
$begingroup$
The intersection of all associated prime ideals is the nilradical of $R$, i.e. the set of nilpotent elements. If the ring is reduced, this set is ${0}$.
$endgroup$
2
$begingroup$
I've seen that the nilradical is this intersection for Noetherian rings but does it hold in general?
$endgroup$
– rschwieb
Jan 11 at 14:28
2
$begingroup$
I believe this is true with correct notion of associated primes (weakly associated prime ideals – which are the same if the ring is noetherian).
$endgroup$
– Bernard
Jan 11 at 14:33
$begingroup$
The intersection of all minimal prime ideals is the nilradical of $R $. I want to know why it is true for associated primes in reduced rings, with the mentioned definition?
$endgroup$
– Deroty
Jan 11 at 14:33
2
$begingroup$
That's because one shows $operatorname{Spec} A$ and $operatorname{Ass} A$ have the same minimal elements.
$endgroup$
– Bernard
Jan 12 at 11:13
1
$begingroup$
As far as I remember, you can replace associated with weakly associated.
$endgroup$
– Bernard
Jan 12 at 12:50
|
show 7 more comments
$begingroup$
The intersection of all associated prime ideals is the nilradical of $R$, i.e. the set of nilpotent elements. If the ring is reduced, this set is ${0}$.
$endgroup$
2
$begingroup$
I've seen that the nilradical is this intersection for Noetherian rings but does it hold in general?
$endgroup$
– rschwieb
Jan 11 at 14:28
2
$begingroup$
I believe this is true with correct notion of associated primes (weakly associated prime ideals – which are the same if the ring is noetherian).
$endgroup$
– Bernard
Jan 11 at 14:33
$begingroup$
The intersection of all minimal prime ideals is the nilradical of $R $. I want to know why it is true for associated primes in reduced rings, with the mentioned definition?
$endgroup$
– Deroty
Jan 11 at 14:33
2
$begingroup$
That's because one shows $operatorname{Spec} A$ and $operatorname{Ass} A$ have the same minimal elements.
$endgroup$
– Bernard
Jan 12 at 11:13
1
$begingroup$
As far as I remember, you can replace associated with weakly associated.
$endgroup$
– Bernard
Jan 12 at 12:50
|
show 7 more comments
$begingroup$
The intersection of all associated prime ideals is the nilradical of $R$, i.e. the set of nilpotent elements. If the ring is reduced, this set is ${0}$.
$endgroup$
The intersection of all associated prime ideals is the nilradical of $R$, i.e. the set of nilpotent elements. If the ring is reduced, this set is ${0}$.
edited Jan 11 at 21:51
user26857
39.5k124284
39.5k124284
answered Jan 11 at 14:26
BernardBernard
124k741117
124k741117
2
$begingroup$
I've seen that the nilradical is this intersection for Noetherian rings but does it hold in general?
$endgroup$
– rschwieb
Jan 11 at 14:28
2
$begingroup$
I believe this is true with correct notion of associated primes (weakly associated prime ideals – which are the same if the ring is noetherian).
$endgroup$
– Bernard
Jan 11 at 14:33
$begingroup$
The intersection of all minimal prime ideals is the nilradical of $R $. I want to know why it is true for associated primes in reduced rings, with the mentioned definition?
$endgroup$
– Deroty
Jan 11 at 14:33
2
$begingroup$
That's because one shows $operatorname{Spec} A$ and $operatorname{Ass} A$ have the same minimal elements.
$endgroup$
– Bernard
Jan 12 at 11:13
1
$begingroup$
As far as I remember, you can replace associated with weakly associated.
$endgroup$
– Bernard
Jan 12 at 12:50
|
show 7 more comments
2
$begingroup$
I've seen that the nilradical is this intersection for Noetherian rings but does it hold in general?
$endgroup$
– rschwieb
Jan 11 at 14:28
2
$begingroup$
I believe this is true with correct notion of associated primes (weakly associated prime ideals – which are the same if the ring is noetherian).
$endgroup$
– Bernard
Jan 11 at 14:33
$begingroup$
The intersection of all minimal prime ideals is the nilradical of $R $. I want to know why it is true for associated primes in reduced rings, with the mentioned definition?
$endgroup$
– Deroty
Jan 11 at 14:33
2
$begingroup$
That's because one shows $operatorname{Spec} A$ and $operatorname{Ass} A$ have the same minimal elements.
$endgroup$
– Bernard
Jan 12 at 11:13
1
$begingroup$
As far as I remember, you can replace associated with weakly associated.
$endgroup$
– Bernard
Jan 12 at 12:50
2
2
$begingroup$
I've seen that the nilradical is this intersection for Noetherian rings but does it hold in general?
$endgroup$
– rschwieb
Jan 11 at 14:28
$begingroup$
I've seen that the nilradical is this intersection for Noetherian rings but does it hold in general?
$endgroup$
– rschwieb
Jan 11 at 14:28
2
2
$begingroup$
I believe this is true with correct notion of associated primes (weakly associated prime ideals – which are the same if the ring is noetherian).
$endgroup$
– Bernard
Jan 11 at 14:33
$begingroup$
I believe this is true with correct notion of associated primes (weakly associated prime ideals – which are the same if the ring is noetherian).
$endgroup$
– Bernard
Jan 11 at 14:33
$begingroup$
The intersection of all minimal prime ideals is the nilradical of $R $. I want to know why it is true for associated primes in reduced rings, with the mentioned definition?
$endgroup$
– Deroty
Jan 11 at 14:33
$begingroup$
The intersection of all minimal prime ideals is the nilradical of $R $. I want to know why it is true for associated primes in reduced rings, with the mentioned definition?
$endgroup$
– Deroty
Jan 11 at 14:33
2
2
$begingroup$
That's because one shows $operatorname{Spec} A$ and $operatorname{Ass} A$ have the same minimal elements.
$endgroup$
– Bernard
Jan 12 at 11:13
$begingroup$
That's because one shows $operatorname{Spec} A$ and $operatorname{Ass} A$ have the same minimal elements.
$endgroup$
– Bernard
Jan 12 at 11:13
1
1
$begingroup$
As far as I remember, you can replace associated with weakly associated.
$endgroup$
– Bernard
Jan 12 at 12:50
$begingroup$
As far as I remember, you can replace associated with weakly associated.
$endgroup$
– Bernard
Jan 12 at 12:50
|
show 7 more comments
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a nonzero element.... of $R$? Associated primes are usually defined with respect to a given module $M$. Are we supposed to assume $M=R$ or something?
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– rschwieb
Jan 11 at 17:04
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Yes. A prime ideal is a associated prime ideal of an $R $-module, if there exists a non zero element of that module such that the prime ideal is its annihilator.
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– Deroty
Jan 11 at 17:35
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I already knew everything after "yes", and it is not relevant to my question. Does your "yes" mean you are confirming that you are talking about a nonzero element of $R$ or do you mean something else (all associated primes for all $R$ modules?)
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– rschwieb
Jan 11 at 17:44
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About non zero elements of $R $.
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– Deroty
Jan 11 at 18:31