Is a scheme Noetherian if its topological space and its stalks are?

The name of the pictureThe name of the pictureThe name of the pictureClash Royale CLAN TAG#URR8PPP











up vote
12
down vote

favorite
2












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
12
down vote

favorite
2












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
12
down vote

favorite
2









up vote
12
down vote

favorite
2






2





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










share|cite|improve this question















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






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








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













  • 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
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.






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.






share|cite|improve this answer




















    Your Answer





    StackExchange.ifUsing("editor", function ()
    return StackExchange.using("mathjaxEditing", function ()
    StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix)
    StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
    );
    );
    , "mathjax-editing");

    StackExchange.ready(function()
    var channelOptions =
    tags: "".split(" "),
    id: "504"
    ;
    initTagRenderer("".split(" "), "".split(" "), channelOptions);

    StackExchange.using("externalEditor", function()
    // Have to fire editor after snippets, if snippets enabled
    if (StackExchange.settings.snippets.snippetsEnabled)
    StackExchange.using("snippets", function()
    createEditor();
    );

    else
    createEditor();

    );

    function createEditor()
    StackExchange.prepareEditor(
    heartbeatType: 'answer',
    autoActivateHeartbeat: false,
    convertImagesToLinks: true,
    noModals: true,
    showLowRepImageUploadWarning: true,
    reputationToPostImages: 10,
    bindNavPrevention: true,
    postfix: "",
    imageUploader:
    brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
    contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
    allowUrls: true
    ,
    noCode: true, onDemand: true,
    discardSelector: ".discard-answer"
    ,immediatelyShowMarkdownHelp:true
    );



    );













    draft saved

    draft discarded


















    StackExchange.ready(
    function ()
    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmathoverflow.net%2fquestions%2f317222%2fis-a-scheme-noetherian-if-its-topological-space-and-its-stalks-are%23new-answer', 'question_page');

    );

    Post as a guest















    Required, but never shown

























    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.






    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



























            draft saved

            draft discarded
















































            Thanks for contributing an answer to MathOverflow!


            • Please be sure to answer the question. Provide details and share your research!

            But avoid


            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.

            Use MathJax to format equations. MathJax reference.


            To learn more, see our tips on writing great answers.





            Some of your past answers have not been well-received, and you're in danger of being blocked from answering.


            Please pay close attention to the following guidance:


            • Please be sure to answer the question. Provide details and share your research!

            But avoid


            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.

            To learn more, see our tips on writing great answers.




            draft saved


            draft discarded














            StackExchange.ready(
            function ()
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmathoverflow.net%2fquestions%2f317222%2fis-a-scheme-noetherian-if-its-topological-space-and-its-stalks-are%23new-answer', 'question_page');

            );

            Post as a guest















            Required, but never shown





















































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown

































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown






            Popular posts from this blog

            How to check contact read email or not when send email to Individual?

            Bahrain

            Postfix configuration issue with fips on centos 7; mailgun relay