$bigcap_{ninmathbb{N}} I^n = (0)$ if and only if no zero divisor of $R$ is of the form $1-z$ with $zin I$.
$begingroup$
Full problem, suppose that $R$ is a commutative Noetherian ring and $I$ is an ideal of $R$. We wish to prove that
$$bigcap_{n=1}^{infty} I^n=(0)$$ if and only if no zerodivisor of $R$ is of the form $1-z$ with $zin I$.
First I'll suppose that the intersection is $(0)$. Let $zin I$ and let $0neq rin R$ such that $r(1-z)=0$. Then $r=rz$ and so $rin I$. Is this useful here? I'm not sure how to use the Noetherian condition of $R$ since the chain of $I^n$ is descending, not ascending.
Any help would be much appreciated! I'm studying for a qual and need all the help I can get.
abstract-algebra ring-theory commutative-algebra ideals noetherian
$endgroup$
add a comment |
$begingroup$
Full problem, suppose that $R$ is a commutative Noetherian ring and $I$ is an ideal of $R$. We wish to prove that
$$bigcap_{n=1}^{infty} I^n=(0)$$ if and only if no zerodivisor of $R$ is of the form $1-z$ with $zin I$.
First I'll suppose that the intersection is $(0)$. Let $zin I$ and let $0neq rin R$ such that $r(1-z)=0$. Then $r=rz$ and so $rin I$. Is this useful here? I'm not sure how to use the Noetherian condition of $R$ since the chain of $I^n$ is descending, not ascending.
Any help would be much appreciated! I'm studying for a qual and need all the help I can get.
abstract-algebra ring-theory commutative-algebra ideals noetherian
$endgroup$
$begingroup$
Maybe take a look at Krulls intersection theorem?
$endgroup$
– red_trumpet
Dec 7 '18 at 18:38
$begingroup$
That assumes that R is local, though @red_trumpet
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
Right, I missed that :(
$endgroup$
– red_trumpet
Dec 7 '18 at 19:37
add a comment |
$begingroup$
Full problem, suppose that $R$ is a commutative Noetherian ring and $I$ is an ideal of $R$. We wish to prove that
$$bigcap_{n=1}^{infty} I^n=(0)$$ if and only if no zerodivisor of $R$ is of the form $1-z$ with $zin I$.
First I'll suppose that the intersection is $(0)$. Let $zin I$ and let $0neq rin R$ such that $r(1-z)=0$. Then $r=rz$ and so $rin I$. Is this useful here? I'm not sure how to use the Noetherian condition of $R$ since the chain of $I^n$ is descending, not ascending.
Any help would be much appreciated! I'm studying for a qual and need all the help I can get.
abstract-algebra ring-theory commutative-algebra ideals noetherian
$endgroup$
Full problem, suppose that $R$ is a commutative Noetherian ring and $I$ is an ideal of $R$. We wish to prove that
$$bigcap_{n=1}^{infty} I^n=(0)$$ if and only if no zerodivisor of $R$ is of the form $1-z$ with $zin I$.
First I'll suppose that the intersection is $(0)$. Let $zin I$ and let $0neq rin R$ such that $r(1-z)=0$. Then $r=rz$ and so $rin I$. Is this useful here? I'm not sure how to use the Noetherian condition of $R$ since the chain of $I^n$ is descending, not ascending.
Any help would be much appreciated! I'm studying for a qual and need all the help I can get.
abstract-algebra ring-theory commutative-algebra ideals noetherian
abstract-algebra ring-theory commutative-algebra ideals noetherian
edited Dec 7 '18 at 18:28
user10354138
7,3772925
7,3772925
asked Dec 7 '18 at 18:23
GengarGengar
1949
1949
$begingroup$
Maybe take a look at Krulls intersection theorem?
$endgroup$
– red_trumpet
Dec 7 '18 at 18:38
$begingroup$
That assumes that R is local, though @red_trumpet
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
Right, I missed that :(
$endgroup$
– red_trumpet
Dec 7 '18 at 19:37
add a comment |
$begingroup$
Maybe take a look at Krulls intersection theorem?
$endgroup$
– red_trumpet
Dec 7 '18 at 18:38
$begingroup$
That assumes that R is local, though @red_trumpet
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
Right, I missed that :(
$endgroup$
– red_trumpet
Dec 7 '18 at 19:37
$begingroup$
Maybe take a look at Krulls intersection theorem?
$endgroup$
– red_trumpet
Dec 7 '18 at 18:38
$begingroup$
Maybe take a look at Krulls intersection theorem?
$endgroup$
– red_trumpet
Dec 7 '18 at 18:38
$begingroup$
That assumes that R is local, though @red_trumpet
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
That assumes that R is local, though @red_trumpet
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
Right, I missed that :(
$endgroup$
– red_trumpet
Dec 7 '18 at 19:37
$begingroup$
Right, I missed that :(
$endgroup$
– red_trumpet
Dec 7 '18 at 19:37
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Yes, the observation that $r=rz$ $implies$ $rin I$ is very useful, since we can use the equation again to get that $rin I^2$, and thus $rin I^3$, and so on. Hence $rin bigcap_{n=1}^infty I^n$, so $r=0$, contradiction. No need to use Noetherianness here. (It may be necessary for the converse, but I haven't thought that far, since it's not clear from your question if you're also asking about that.)
Edit
Worked out my thoughts on the converse. I was being dumb. It's Nakayama's lemma (the general, not local ring version).
Let $$I^infty = bigcap_{n=1}^infty I^n.$$
Observe that clearly $I(I^infty) = I^infty$. Then, since $R$ is Noetherian, $I^infty$ is finitely generated, so Nakayama's lemma (Statement 1) applies.
Thus there exists $rin R$ with $r-1in I$ such that $rI^infty =0$. But then $r-1=i$ for some $iin I$, and $r=1+i$. Then $r$ is not a zero divisor by assumption, hence the fact that $rI^infty = 0$ implies that $I^infty=0$ as desired.
$endgroup$
$begingroup$
Do you have any ideas on the other direction? Thank you so much!
$endgroup$
– Gengar
Dec 7 '18 at 18:41
$begingroup$
@Gengar, for the converse red_trumpet's suggestion to use Krull's intersection theorem looks viable. I'll flesh out the details in my answer.
$endgroup$
– jgon
Dec 7 '18 at 18:47
$begingroup$
I appreciate it so much @jgon
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
@Gengar actually I have to go, and I can't complete my thoughts rn, I'll post what I have.
$endgroup$
– jgon
Dec 7 '18 at 19:09
$begingroup$
@Gengar, got it, I was being silly.
$endgroup$
– jgon
Dec 8 '18 at 0:30
|
show 3 more comments
$begingroup$
If there exists some y $in$ I such that (1-y) is a zero divisor, then there exists an x $not=$ 0 such that x(1-y) = 0. Then x = xy $in$ I. Thus x $in$ I $cap$ I$^2$. It follows that x $in$ $displaystylebigcap_n$ I$^n$.
$endgroup$
$begingroup$
Yeah, jgon already said that
$endgroup$
– Gengar
Dec 7 '18 at 19:53
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
$begingroup$
Yes, the observation that $r=rz$ $implies$ $rin I$ is very useful, since we can use the equation again to get that $rin I^2$, and thus $rin I^3$, and so on. Hence $rin bigcap_{n=1}^infty I^n$, so $r=0$, contradiction. No need to use Noetherianness here. (It may be necessary for the converse, but I haven't thought that far, since it's not clear from your question if you're also asking about that.)
Edit
Worked out my thoughts on the converse. I was being dumb. It's Nakayama's lemma (the general, not local ring version).
Let $$I^infty = bigcap_{n=1}^infty I^n.$$
Observe that clearly $I(I^infty) = I^infty$. Then, since $R$ is Noetherian, $I^infty$ is finitely generated, so Nakayama's lemma (Statement 1) applies.
Thus there exists $rin R$ with $r-1in I$ such that $rI^infty =0$. But then $r-1=i$ for some $iin I$, and $r=1+i$. Then $r$ is not a zero divisor by assumption, hence the fact that $rI^infty = 0$ implies that $I^infty=0$ as desired.
$endgroup$
$begingroup$
Do you have any ideas on the other direction? Thank you so much!
$endgroup$
– Gengar
Dec 7 '18 at 18:41
$begingroup$
@Gengar, for the converse red_trumpet's suggestion to use Krull's intersection theorem looks viable. I'll flesh out the details in my answer.
$endgroup$
– jgon
Dec 7 '18 at 18:47
$begingroup$
I appreciate it so much @jgon
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
@Gengar actually I have to go, and I can't complete my thoughts rn, I'll post what I have.
$endgroup$
– jgon
Dec 7 '18 at 19:09
$begingroup$
@Gengar, got it, I was being silly.
$endgroup$
– jgon
Dec 8 '18 at 0:30
|
show 3 more comments
$begingroup$
Yes, the observation that $r=rz$ $implies$ $rin I$ is very useful, since we can use the equation again to get that $rin I^2$, and thus $rin I^3$, and so on. Hence $rin bigcap_{n=1}^infty I^n$, so $r=0$, contradiction. No need to use Noetherianness here. (It may be necessary for the converse, but I haven't thought that far, since it's not clear from your question if you're also asking about that.)
Edit
Worked out my thoughts on the converse. I was being dumb. It's Nakayama's lemma (the general, not local ring version).
Let $$I^infty = bigcap_{n=1}^infty I^n.$$
Observe that clearly $I(I^infty) = I^infty$. Then, since $R$ is Noetherian, $I^infty$ is finitely generated, so Nakayama's lemma (Statement 1) applies.
Thus there exists $rin R$ with $r-1in I$ such that $rI^infty =0$. But then $r-1=i$ for some $iin I$, and $r=1+i$. Then $r$ is not a zero divisor by assumption, hence the fact that $rI^infty = 0$ implies that $I^infty=0$ as desired.
$endgroup$
$begingroup$
Do you have any ideas on the other direction? Thank you so much!
$endgroup$
– Gengar
Dec 7 '18 at 18:41
$begingroup$
@Gengar, for the converse red_trumpet's suggestion to use Krull's intersection theorem looks viable. I'll flesh out the details in my answer.
$endgroup$
– jgon
Dec 7 '18 at 18:47
$begingroup$
I appreciate it so much @jgon
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
@Gengar actually I have to go, and I can't complete my thoughts rn, I'll post what I have.
$endgroup$
– jgon
Dec 7 '18 at 19:09
$begingroup$
@Gengar, got it, I was being silly.
$endgroup$
– jgon
Dec 8 '18 at 0:30
|
show 3 more comments
$begingroup$
Yes, the observation that $r=rz$ $implies$ $rin I$ is very useful, since we can use the equation again to get that $rin I^2$, and thus $rin I^3$, and so on. Hence $rin bigcap_{n=1}^infty I^n$, so $r=0$, contradiction. No need to use Noetherianness here. (It may be necessary for the converse, but I haven't thought that far, since it's not clear from your question if you're also asking about that.)
Edit
Worked out my thoughts on the converse. I was being dumb. It's Nakayama's lemma (the general, not local ring version).
Let $$I^infty = bigcap_{n=1}^infty I^n.$$
Observe that clearly $I(I^infty) = I^infty$. Then, since $R$ is Noetherian, $I^infty$ is finitely generated, so Nakayama's lemma (Statement 1) applies.
Thus there exists $rin R$ with $r-1in I$ such that $rI^infty =0$. But then $r-1=i$ for some $iin I$, and $r=1+i$. Then $r$ is not a zero divisor by assumption, hence the fact that $rI^infty = 0$ implies that $I^infty=0$ as desired.
$endgroup$
Yes, the observation that $r=rz$ $implies$ $rin I$ is very useful, since we can use the equation again to get that $rin I^2$, and thus $rin I^3$, and so on. Hence $rin bigcap_{n=1}^infty I^n$, so $r=0$, contradiction. No need to use Noetherianness here. (It may be necessary for the converse, but I haven't thought that far, since it's not clear from your question if you're also asking about that.)
Edit
Worked out my thoughts on the converse. I was being dumb. It's Nakayama's lemma (the general, not local ring version).
Let $$I^infty = bigcap_{n=1}^infty I^n.$$
Observe that clearly $I(I^infty) = I^infty$. Then, since $R$ is Noetherian, $I^infty$ is finitely generated, so Nakayama's lemma (Statement 1) applies.
Thus there exists $rin R$ with $r-1in I$ such that $rI^infty =0$. But then $r-1=i$ for some $iin I$, and $r=1+i$. Then $r$ is not a zero divisor by assumption, hence the fact that $rI^infty = 0$ implies that $I^infty=0$ as desired.
edited Dec 8 '18 at 0:30
answered Dec 7 '18 at 18:35
jgonjgon
13.6k22041
13.6k22041
$begingroup$
Do you have any ideas on the other direction? Thank you so much!
$endgroup$
– Gengar
Dec 7 '18 at 18:41
$begingroup$
@Gengar, for the converse red_trumpet's suggestion to use Krull's intersection theorem looks viable. I'll flesh out the details in my answer.
$endgroup$
– jgon
Dec 7 '18 at 18:47
$begingroup$
I appreciate it so much @jgon
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
@Gengar actually I have to go, and I can't complete my thoughts rn, I'll post what I have.
$endgroup$
– jgon
Dec 7 '18 at 19:09
$begingroup$
@Gengar, got it, I was being silly.
$endgroup$
– jgon
Dec 8 '18 at 0:30
|
show 3 more comments
$begingroup$
Do you have any ideas on the other direction? Thank you so much!
$endgroup$
– Gengar
Dec 7 '18 at 18:41
$begingroup$
@Gengar, for the converse red_trumpet's suggestion to use Krull's intersection theorem looks viable. I'll flesh out the details in my answer.
$endgroup$
– jgon
Dec 7 '18 at 18:47
$begingroup$
I appreciate it so much @jgon
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
@Gengar actually I have to go, and I can't complete my thoughts rn, I'll post what I have.
$endgroup$
– jgon
Dec 7 '18 at 19:09
$begingroup$
@Gengar, got it, I was being silly.
$endgroup$
– jgon
Dec 8 '18 at 0:30
$begingroup$
Do you have any ideas on the other direction? Thank you so much!
$endgroup$
– Gengar
Dec 7 '18 at 18:41
$begingroup$
Do you have any ideas on the other direction? Thank you so much!
$endgroup$
– Gengar
Dec 7 '18 at 18:41
$begingroup$
@Gengar, for the converse red_trumpet's suggestion to use Krull's intersection theorem looks viable. I'll flesh out the details in my answer.
$endgroup$
– jgon
Dec 7 '18 at 18:47
$begingroup$
@Gengar, for the converse red_trumpet's suggestion to use Krull's intersection theorem looks viable. I'll flesh out the details in my answer.
$endgroup$
– jgon
Dec 7 '18 at 18:47
$begingroup$
I appreciate it so much @jgon
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
I appreciate it so much @jgon
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
@Gengar actually I have to go, and I can't complete my thoughts rn, I'll post what I have.
$endgroup$
– jgon
Dec 7 '18 at 19:09
$begingroup$
@Gengar actually I have to go, and I can't complete my thoughts rn, I'll post what I have.
$endgroup$
– jgon
Dec 7 '18 at 19:09
$begingroup$
@Gengar, got it, I was being silly.
$endgroup$
– jgon
Dec 8 '18 at 0:30
$begingroup$
@Gengar, got it, I was being silly.
$endgroup$
– jgon
Dec 8 '18 at 0:30
|
show 3 more comments
$begingroup$
If there exists some y $in$ I such that (1-y) is a zero divisor, then there exists an x $not=$ 0 such that x(1-y) = 0. Then x = xy $in$ I. Thus x $in$ I $cap$ I$^2$. It follows that x $in$ $displaystylebigcap_n$ I$^n$.
$endgroup$
$begingroup$
Yeah, jgon already said that
$endgroup$
– Gengar
Dec 7 '18 at 19:53
add a comment |
$begingroup$
If there exists some y $in$ I such that (1-y) is a zero divisor, then there exists an x $not=$ 0 such that x(1-y) = 0. Then x = xy $in$ I. Thus x $in$ I $cap$ I$^2$. It follows that x $in$ $displaystylebigcap_n$ I$^n$.
$endgroup$
$begingroup$
Yeah, jgon already said that
$endgroup$
– Gengar
Dec 7 '18 at 19:53
add a comment |
$begingroup$
If there exists some y $in$ I such that (1-y) is a zero divisor, then there exists an x $not=$ 0 such that x(1-y) = 0. Then x = xy $in$ I. Thus x $in$ I $cap$ I$^2$. It follows that x $in$ $displaystylebigcap_n$ I$^n$.
$endgroup$
If there exists some y $in$ I such that (1-y) is a zero divisor, then there exists an x $not=$ 0 such that x(1-y) = 0. Then x = xy $in$ I. Thus x $in$ I $cap$ I$^2$. It follows that x $in$ $displaystylebigcap_n$ I$^n$.
answered Dec 7 '18 at 19:43
Joel PereiraJoel Pereira
74519
74519
$begingroup$
Yeah, jgon already said that
$endgroup$
– Gengar
Dec 7 '18 at 19:53
add a comment |
$begingroup$
Yeah, jgon already said that
$endgroup$
– Gengar
Dec 7 '18 at 19:53
$begingroup$
Yeah, jgon already said that
$endgroup$
– Gengar
Dec 7 '18 at 19:53
$begingroup$
Yeah, jgon already said that
$endgroup$
– Gengar
Dec 7 '18 at 19:53
add a comment |
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$begingroup$
Maybe take a look at Krulls intersection theorem?
$endgroup$
– red_trumpet
Dec 7 '18 at 18:38
$begingroup$
That assumes that R is local, though @red_trumpet
$endgroup$
– Gengar
Dec 7 '18 at 18:47
$begingroup$
Right, I missed that :(
$endgroup$
– red_trumpet
Dec 7 '18 at 19:37