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?










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    Probably should be asked on MSE.
    – Bernie
    Dec 9 at 4:03






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    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















up vote
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Is a scheme being Noetherian equivalent to the underlying topological space being Noetherian and all its stalks being Noetherian?










share|cite|improve this question



















  • 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













up vote
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Is a scheme being Noetherian equivalent to the underlying topological space being Noetherian and all its stalks being Noetherian?










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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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edited Dec 9 at 20:07









R. van Dobben de Bruyn

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asked Dec 9 at 3:21









G.-S. Zhou

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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













  • 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











2 Answers
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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.






share|cite|improve this answer


















  • 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

















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.






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    2 Answers
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    2 Answers
    2






    active

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    active

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    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.






    share|cite|improve this answer


















    • 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














    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.






    share|cite|improve this answer


















    • 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












    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.






    share|cite|improve this answer














    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.







    share|cite|improve this answer














    share|cite|improve this answer



    share|cite|improve this answer








    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












    • 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










    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.






    share|cite|improve this answer
























      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.






      share|cite|improve this answer






















        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.






        share|cite|improve this answer












        A further counterexample is Example 2.3 in W. Heinzer, J. Ohm, Locally noetherian commutative rings, Trans. Amer. Math. Soc. 158 (1971), 273-284.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Dec 9 at 19:01









        Fred Rohrer

        4,36111634




        4,36111634



























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