Is a scheme Noetherian if its topological space and its stalks are?
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Is a scheme being Noetherian equivalent to the underlying topological space being Noetherian and all its stalks being Noetherian?
ag.algebraic-geometry ac.commutative-algebra schemes counterexamples
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up vote
12
down vote
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Is a scheme being Noetherian equivalent to the underlying topological space being Noetherian and all its stalks being Noetherian?
ag.algebraic-geometry ac.commutative-algebra schemes counterexamples
1
Probably should be asked on MSE.
– Bernie
Dec 9 at 4:03
1
I think this is a good question for this site : it's an interesting question (and answer) and I at least have not seen the counter example to the seemingly plausible result before.
– Asvin
Dec 10 at 9:00
add a comment |
up vote
12
down vote
favorite
up vote
12
down vote
favorite
Is a scheme being Noetherian equivalent to the underlying topological space being Noetherian and all its stalks being Noetherian?
ag.algebraic-geometry ac.commutative-algebra schemes counterexamples
Is a scheme being Noetherian equivalent to the underlying topological space being Noetherian and all its stalks being Noetherian?
ag.algebraic-geometry ac.commutative-algebra schemes counterexamples
ag.algebraic-geometry ac.commutative-algebra schemes counterexamples
edited Dec 9 at 20:07
R. van Dobben de Bruyn
10.5k23263
10.5k23263
asked Dec 9 at 3:21
G.-S. Zhou
1487
1487
1
Probably should be asked on MSE.
– Bernie
Dec 9 at 4:03
1
I think this is a good question for this site : it's an interesting question (and answer) and I at least have not seen the counter example to the seemingly plausible result before.
– Asvin
Dec 10 at 9:00
add a comment |
1
Probably should be asked on MSE.
– Bernie
Dec 9 at 4:03
1
I think this is a good question for this site : it's an interesting question (and answer) and I at least have not seen the counter example to the seemingly plausible result before.
– Asvin
Dec 10 at 9:00
1
1
Probably should be asked on MSE.
– Bernie
Dec 9 at 4:03
Probably should be asked on MSE.
– Bernie
Dec 9 at 4:03
1
1
I think this is a good question for this site : it's an interesting question (and answer) and I at least have not seen the counter example to the seemingly plausible result before.
– Asvin
Dec 10 at 9:00
I think this is a good question for this site : it's an interesting question (and answer) and I at least have not seen the counter example to the seemingly plausible result before.
– Asvin
Dec 10 at 9:00
add a comment |
2 Answers
2
active
oldest
votes
up vote
24
down vote
accepted
This is false. The easiest counterexample I could come up with is the following "affine line with embedded points at every closed [rational] point":
Example. Let $k$ be an infinite field, let $R = k[x]$, and for each $alpha in k$ let $R_alpha = R[y_alpha]/((x-alpha)y_alpha,y_alpha^2)$. Then $R_alpha$ is an affine line with an embedded prime $mathfrak p_alpha = (x-alpha,y_alpha)$ at $x = alpha$, sticking out in the $y_alpha$-direction. Finally, let
$$R_infty = bigotimes_alpha in k R_alpha = operatorname*colim_substacklongrightarrow\I subseteq k\textfinite bigotimes_alpha in I R_alpha$$
be their tensor product over $R$ (not over $k$); that is
$$R_infty = frack[x]left[y_alpha_alpha in kright]sum_alpha in k((x-alpha)y_alpha, y_alpha^2).$$
This is not a Noetherian ring, because the radical $mathfrak r = (y_alpha_alpha in k)$ is not finitely generated. But $operatornameSpec R_infty$ agrees as a topological space with $operatornameSpec R_infty^operatornamered = mathbb A^1_k$, hence $|!operatornameSpec R_infty|$ is a Noetherian topological space.
On the other hand, the map $R to R_alpha$ is an isomorphism away from $alpha$, and similarly $R_alpha to R_infty$ induces isomorphisms on the stalks at $alpha$. Thus, the stalk $(R_infty)_mathfrak q_alpha = (R_alpha)_mathfrak p_alpha$ at $mathfrak q_alpha = mathfrak p_alpha R_infty + mathfrak r$ is Noetherian. Similarly, the stalk at the generic point $mathfrak r$ is just $R_(0) = k(x)$. Thus, we conclude that all the stalks of $R_infty$ are Noetherian. $square$
Remark. As requested in the comments, my motivation to come up with this example is the following: I was trying to prove by hand that the answer was positive. After an immediate reduction to the affine case, one needs to consider a chain $I_0 subseteq I_1 ldots$ of ideals. Because $|X|$ is Noetherian, we may assume they [eventually] define the same closed set $V$. Intuitively, to check that they agree as ideals, it should suffice to check it at each component of $V$. Then you can try to use the Noetherian rings $mathcal O_X,x$ for the generic points $x$ of $V$.
However this is not quite true, because of embedded points. The precise statement [Tags 0311 and 02M3] is that the inclusion $I_i-1 subseteq I_i$ is an equality if and only if the same holds for the localisation at every associated point of $I_i$. Now you run into trouble if the $I_i$ differ at an embedded point that is not seen by the topology of $V$. Moreover, if this embedded point varies as we move through the chain, there is a chance that the localisations $mathcal O_X,x$ at these embedded points are still Noetherian. Once you see this, coming up with the example is not so hard.
1
Can you give some motivation (when you are free) to think of this example..
– Praphulla Koushik
Dec 9 at 13:12
@PraphullaKoushik I edited my answer to include some motivation behind the construction.
– R. van Dobben de Bruyn
Dec 9 at 20:04
Thanks for the positive response.. This makes so much sense... Thanks..
– Praphulla Koushik
Dec 9 at 20:32
add a comment |
up vote
7
down vote
A further counterexample is Example 2.3 in W. Heinzer, J. Ohm, Locally noetherian commutative rings, Trans. Amer. Math. Soc. 158 (1971), 273-284.
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
24
down vote
accepted
This is false. The easiest counterexample I could come up with is the following "affine line with embedded points at every closed [rational] point":
Example. Let $k$ be an infinite field, let $R = k[x]$, and for each $alpha in k$ let $R_alpha = R[y_alpha]/((x-alpha)y_alpha,y_alpha^2)$. Then $R_alpha$ is an affine line with an embedded prime $mathfrak p_alpha = (x-alpha,y_alpha)$ at $x = alpha$, sticking out in the $y_alpha$-direction. Finally, let
$$R_infty = bigotimes_alpha in k R_alpha = operatorname*colim_substacklongrightarrow\I subseteq k\textfinite bigotimes_alpha in I R_alpha$$
be their tensor product over $R$ (not over $k$); that is
$$R_infty = frack[x]left[y_alpha_alpha in kright]sum_alpha in k((x-alpha)y_alpha, y_alpha^2).$$
This is not a Noetherian ring, because the radical $mathfrak r = (y_alpha_alpha in k)$ is not finitely generated. But $operatornameSpec R_infty$ agrees as a topological space with $operatornameSpec R_infty^operatornamered = mathbb A^1_k$, hence $|!operatornameSpec R_infty|$ is a Noetherian topological space.
On the other hand, the map $R to R_alpha$ is an isomorphism away from $alpha$, and similarly $R_alpha to R_infty$ induces isomorphisms on the stalks at $alpha$. Thus, the stalk $(R_infty)_mathfrak q_alpha = (R_alpha)_mathfrak p_alpha$ at $mathfrak q_alpha = mathfrak p_alpha R_infty + mathfrak r$ is Noetherian. Similarly, the stalk at the generic point $mathfrak r$ is just $R_(0) = k(x)$. Thus, we conclude that all the stalks of $R_infty$ are Noetherian. $square$
Remark. As requested in the comments, my motivation to come up with this example is the following: I was trying to prove by hand that the answer was positive. After an immediate reduction to the affine case, one needs to consider a chain $I_0 subseteq I_1 ldots$ of ideals. Because $|X|$ is Noetherian, we may assume they [eventually] define the same closed set $V$. Intuitively, to check that they agree as ideals, it should suffice to check it at each component of $V$. Then you can try to use the Noetherian rings $mathcal O_X,x$ for the generic points $x$ of $V$.
However this is not quite true, because of embedded points. The precise statement [Tags 0311 and 02M3] is that the inclusion $I_i-1 subseteq I_i$ is an equality if and only if the same holds for the localisation at every associated point of $I_i$. Now you run into trouble if the $I_i$ differ at an embedded point that is not seen by the topology of $V$. Moreover, if this embedded point varies as we move through the chain, there is a chance that the localisations $mathcal O_X,x$ at these embedded points are still Noetherian. Once you see this, coming up with the example is not so hard.
1
Can you give some motivation (when you are free) to think of this example..
– Praphulla Koushik
Dec 9 at 13:12
@PraphullaKoushik I edited my answer to include some motivation behind the construction.
– R. van Dobben de Bruyn
Dec 9 at 20:04
Thanks for the positive response.. This makes so much sense... Thanks..
– Praphulla Koushik
Dec 9 at 20:32
add a comment |
up vote
24
down vote
accepted
This is false. The easiest counterexample I could come up with is the following "affine line with embedded points at every closed [rational] point":
Example. Let $k$ be an infinite field, let $R = k[x]$, and for each $alpha in k$ let $R_alpha = R[y_alpha]/((x-alpha)y_alpha,y_alpha^2)$. Then $R_alpha$ is an affine line with an embedded prime $mathfrak p_alpha = (x-alpha,y_alpha)$ at $x = alpha$, sticking out in the $y_alpha$-direction. Finally, let
$$R_infty = bigotimes_alpha in k R_alpha = operatorname*colim_substacklongrightarrow\I subseteq k\textfinite bigotimes_alpha in I R_alpha$$
be their tensor product over $R$ (not over $k$); that is
$$R_infty = frack[x]left[y_alpha_alpha in kright]sum_alpha in k((x-alpha)y_alpha, y_alpha^2).$$
This is not a Noetherian ring, because the radical $mathfrak r = (y_alpha_alpha in k)$ is not finitely generated. But $operatornameSpec R_infty$ agrees as a topological space with $operatornameSpec R_infty^operatornamered = mathbb A^1_k$, hence $|!operatornameSpec R_infty|$ is a Noetherian topological space.
On the other hand, the map $R to R_alpha$ is an isomorphism away from $alpha$, and similarly $R_alpha to R_infty$ induces isomorphisms on the stalks at $alpha$. Thus, the stalk $(R_infty)_mathfrak q_alpha = (R_alpha)_mathfrak p_alpha$ at $mathfrak q_alpha = mathfrak p_alpha R_infty + mathfrak r$ is Noetherian. Similarly, the stalk at the generic point $mathfrak r$ is just $R_(0) = k(x)$. Thus, we conclude that all the stalks of $R_infty$ are Noetherian. $square$
Remark. As requested in the comments, my motivation to come up with this example is the following: I was trying to prove by hand that the answer was positive. After an immediate reduction to the affine case, one needs to consider a chain $I_0 subseteq I_1 ldots$ of ideals. Because $|X|$ is Noetherian, we may assume they [eventually] define the same closed set $V$. Intuitively, to check that they agree as ideals, it should suffice to check it at each component of $V$. Then you can try to use the Noetherian rings $mathcal O_X,x$ for the generic points $x$ of $V$.
However this is not quite true, because of embedded points. The precise statement [Tags 0311 and 02M3] is that the inclusion $I_i-1 subseteq I_i$ is an equality if and only if the same holds for the localisation at every associated point of $I_i$. Now you run into trouble if the $I_i$ differ at an embedded point that is not seen by the topology of $V$. Moreover, if this embedded point varies as we move through the chain, there is a chance that the localisations $mathcal O_X,x$ at these embedded points are still Noetherian. Once you see this, coming up with the example is not so hard.
1
Can you give some motivation (when you are free) to think of this example..
– Praphulla Koushik
Dec 9 at 13:12
@PraphullaKoushik I edited my answer to include some motivation behind the construction.
– R. van Dobben de Bruyn
Dec 9 at 20:04
Thanks for the positive response.. This makes so much sense... Thanks..
– Praphulla Koushik
Dec 9 at 20:32
add a comment |
up vote
24
down vote
accepted
up vote
24
down vote
accepted
This is false. The easiest counterexample I could come up with is the following "affine line with embedded points at every closed [rational] point":
Example. Let $k$ be an infinite field, let $R = k[x]$, and for each $alpha in k$ let $R_alpha = R[y_alpha]/((x-alpha)y_alpha,y_alpha^2)$. Then $R_alpha$ is an affine line with an embedded prime $mathfrak p_alpha = (x-alpha,y_alpha)$ at $x = alpha$, sticking out in the $y_alpha$-direction. Finally, let
$$R_infty = bigotimes_alpha in k R_alpha = operatorname*colim_substacklongrightarrow\I subseteq k\textfinite bigotimes_alpha in I R_alpha$$
be their tensor product over $R$ (not over $k$); that is
$$R_infty = frack[x]left[y_alpha_alpha in kright]sum_alpha in k((x-alpha)y_alpha, y_alpha^2).$$
This is not a Noetherian ring, because the radical $mathfrak r = (y_alpha_alpha in k)$ is not finitely generated. But $operatornameSpec R_infty$ agrees as a topological space with $operatornameSpec R_infty^operatornamered = mathbb A^1_k$, hence $|!operatornameSpec R_infty|$ is a Noetherian topological space.
On the other hand, the map $R to R_alpha$ is an isomorphism away from $alpha$, and similarly $R_alpha to R_infty$ induces isomorphisms on the stalks at $alpha$. Thus, the stalk $(R_infty)_mathfrak q_alpha = (R_alpha)_mathfrak p_alpha$ at $mathfrak q_alpha = mathfrak p_alpha R_infty + mathfrak r$ is Noetherian. Similarly, the stalk at the generic point $mathfrak r$ is just $R_(0) = k(x)$. Thus, we conclude that all the stalks of $R_infty$ are Noetherian. $square$
Remark. As requested in the comments, my motivation to come up with this example is the following: I was trying to prove by hand that the answer was positive. After an immediate reduction to the affine case, one needs to consider a chain $I_0 subseteq I_1 ldots$ of ideals. Because $|X|$ is Noetherian, we may assume they [eventually] define the same closed set $V$. Intuitively, to check that they agree as ideals, it should suffice to check it at each component of $V$. Then you can try to use the Noetherian rings $mathcal O_X,x$ for the generic points $x$ of $V$.
However this is not quite true, because of embedded points. The precise statement [Tags 0311 and 02M3] is that the inclusion $I_i-1 subseteq I_i$ is an equality if and only if the same holds for the localisation at every associated point of $I_i$. Now you run into trouble if the $I_i$ differ at an embedded point that is not seen by the topology of $V$. Moreover, if this embedded point varies as we move through the chain, there is a chance that the localisations $mathcal O_X,x$ at these embedded points are still Noetherian. Once you see this, coming up with the example is not so hard.
This is false. The easiest counterexample I could come up with is the following "affine line with embedded points at every closed [rational] point":
Example. Let $k$ be an infinite field, let $R = k[x]$, and for each $alpha in k$ let $R_alpha = R[y_alpha]/((x-alpha)y_alpha,y_alpha^2)$. Then $R_alpha$ is an affine line with an embedded prime $mathfrak p_alpha = (x-alpha,y_alpha)$ at $x = alpha$, sticking out in the $y_alpha$-direction. Finally, let
$$R_infty = bigotimes_alpha in k R_alpha = operatorname*colim_substacklongrightarrow\I subseteq k\textfinite bigotimes_alpha in I R_alpha$$
be their tensor product over $R$ (not over $k$); that is
$$R_infty = frack[x]left[y_alpha_alpha in kright]sum_alpha in k((x-alpha)y_alpha, y_alpha^2).$$
This is not a Noetherian ring, because the radical $mathfrak r = (y_alpha_alpha in k)$ is not finitely generated. But $operatornameSpec R_infty$ agrees as a topological space with $operatornameSpec R_infty^operatornamered = mathbb A^1_k$, hence $|!operatornameSpec R_infty|$ is a Noetherian topological space.
On the other hand, the map $R to R_alpha$ is an isomorphism away from $alpha$, and similarly $R_alpha to R_infty$ induces isomorphisms on the stalks at $alpha$. Thus, the stalk $(R_infty)_mathfrak q_alpha = (R_alpha)_mathfrak p_alpha$ at $mathfrak q_alpha = mathfrak p_alpha R_infty + mathfrak r$ is Noetherian. Similarly, the stalk at the generic point $mathfrak r$ is just $R_(0) = k(x)$. Thus, we conclude that all the stalks of $R_infty$ are Noetherian. $square$
Remark. As requested in the comments, my motivation to come up with this example is the following: I was trying to prove by hand that the answer was positive. After an immediate reduction to the affine case, one needs to consider a chain $I_0 subseteq I_1 ldots$ of ideals. Because $|X|$ is Noetherian, we may assume they [eventually] define the same closed set $V$. Intuitively, to check that they agree as ideals, it should suffice to check it at each component of $V$. Then you can try to use the Noetherian rings $mathcal O_X,x$ for the generic points $x$ of $V$.
However this is not quite true, because of embedded points. The precise statement [Tags 0311 and 02M3] is that the inclusion $I_i-1 subseteq I_i$ is an equality if and only if the same holds for the localisation at every associated point of $I_i$. Now you run into trouble if the $I_i$ differ at an embedded point that is not seen by the topology of $V$. Moreover, if this embedded point varies as we move through the chain, there is a chance that the localisations $mathcal O_X,x$ at these embedded points are still Noetherian. Once you see this, coming up with the example is not so hard.
edited Dec 9 at 19:57
answered Dec 9 at 5:05
R. van Dobben de Bruyn
10.5k23263
10.5k23263
1
Can you give some motivation (when you are free) to think of this example..
– Praphulla Koushik
Dec 9 at 13:12
@PraphullaKoushik I edited my answer to include some motivation behind the construction.
– R. van Dobben de Bruyn
Dec 9 at 20:04
Thanks for the positive response.. This makes so much sense... Thanks..
– Praphulla Koushik
Dec 9 at 20:32
add a comment |
1
Can you give some motivation (when you are free) to think of this example..
– Praphulla Koushik
Dec 9 at 13:12
@PraphullaKoushik I edited my answer to include some motivation behind the construction.
– R. van Dobben de Bruyn
Dec 9 at 20:04
Thanks for the positive response.. This makes so much sense... Thanks..
– Praphulla Koushik
Dec 9 at 20:32
1
1
Can you give some motivation (when you are free) to think of this example..
– Praphulla Koushik
Dec 9 at 13:12
Can you give some motivation (when you are free) to think of this example..
– Praphulla Koushik
Dec 9 at 13:12
@PraphullaKoushik I edited my answer to include some motivation behind the construction.
– R. van Dobben de Bruyn
Dec 9 at 20:04
@PraphullaKoushik I edited my answer to include some motivation behind the construction.
– R. van Dobben de Bruyn
Dec 9 at 20:04
Thanks for the positive response.. This makes so much sense... Thanks..
– Praphulla Koushik
Dec 9 at 20:32
Thanks for the positive response.. This makes so much sense... Thanks..
– Praphulla Koushik
Dec 9 at 20:32
add a comment |
up vote
7
down vote
A further counterexample is Example 2.3 in W. Heinzer, J. Ohm, Locally noetherian commutative rings, Trans. Amer. Math. Soc. 158 (1971), 273-284.
add a comment |
up vote
7
down vote
A further counterexample is Example 2.3 in W. Heinzer, J. Ohm, Locally noetherian commutative rings, Trans. Amer. Math. Soc. 158 (1971), 273-284.
add a comment |
up vote
7
down vote
up vote
7
down vote
A further counterexample is Example 2.3 in W. Heinzer, J. Ohm, Locally noetherian commutative rings, Trans. Amer. Math. Soc. 158 (1971), 273-284.
A further counterexample is Example 2.3 in W. Heinzer, J. Ohm, Locally noetherian commutative rings, Trans. Amer. Math. Soc. 158 (1971), 273-284.
answered Dec 9 at 19:01
Fred Rohrer
4,36111634
4,36111634
add a comment |
add a comment |
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1
Probably should be asked on MSE.
– Bernie
Dec 9 at 4:03
1
I think this is a good question for this site : it's an interesting question (and answer) and I at least have not seen the counter example to the seemingly plausible result before.
– Asvin
Dec 10 at 9:00