$bigcap_{ninmathbb{N}} I^n = (0)$ if and only if no zero divisor of $R$ is of the form $1-z$ with $zin I$.












4












$begingroup$



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.










share|cite|improve this question











$endgroup$












  • $begingroup$
    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
















4












$begingroup$



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.










share|cite|improve this question











$endgroup$












  • $begingroup$
    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














4












4








4





$begingroup$



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.










share|cite|improve this question











$endgroup$





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






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








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


















  • $begingroup$
    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










2 Answers
2






active

oldest

votes


















3












$begingroup$

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.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    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



















-1












$begingroup$

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$^n$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Yeah, jgon already said that
    $endgroup$
    – Gengar
    Dec 7 '18 at 19:53











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2 Answers
2






active

oldest

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2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









3












$begingroup$

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.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    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
















3












$begingroup$

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.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    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














3












3








3





$begingroup$

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.






share|cite|improve this answer











$endgroup$



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.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








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


















  • $begingroup$
    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











-1












$begingroup$

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$^n$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Yeah, jgon already said that
    $endgroup$
    – Gengar
    Dec 7 '18 at 19:53
















-1












$begingroup$

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$^n$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Yeah, jgon already said that
    $endgroup$
    – Gengar
    Dec 7 '18 at 19:53














-1












-1








-1





$begingroup$

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$^n$.






share|cite|improve this answer









$endgroup$



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$^n$.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










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


















  • $begingroup$
    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


















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