Does the category of artinian rings admit finite limits?












6












$begingroup$


Let $mathsf{Artin}$ be the category of artinian rings, viewed as a full subcategory of the category $mathsf{CRing}$ of rings. (Here "ring" means "commutative ring with one".)




Question 1. Does $mathsf{Artin}$ admit finite limits?




As $mathsf{Artin}$ has finite products, Question 1 is equivalent to




Question 2. Does $mathsf{Artin}$ admit equalizers?




A closely related question is




Question 3. Let $Ato B$ be the equalizer in $mathsf{CRing}$ of two morphisms from $B$ to $C$. Assume that $B$ and $C$ are artinian. Does this imply that $A$ is artinian?




Yes to Question 3 would imply yes to Questions 1 and 2. [Edit: I made a mistake when I claimed implicitly that yes to Question 3 would immediately imply yes to Questions 1 and 2. There was a subtlety I missed, but Jeremy's comment below his answer settles also Question 1 and 2. (I also missed that, sigh...)]



This answer of MooS implies that the category of noetherian rings does not admit finite limits.










share|cite|improve this question











$endgroup$

















    6












    $begingroup$


    Let $mathsf{Artin}$ be the category of artinian rings, viewed as a full subcategory of the category $mathsf{CRing}$ of rings. (Here "ring" means "commutative ring with one".)




    Question 1. Does $mathsf{Artin}$ admit finite limits?




    As $mathsf{Artin}$ has finite products, Question 1 is equivalent to




    Question 2. Does $mathsf{Artin}$ admit equalizers?




    A closely related question is




    Question 3. Let $Ato B$ be the equalizer in $mathsf{CRing}$ of two morphisms from $B$ to $C$. Assume that $B$ and $C$ are artinian. Does this imply that $A$ is artinian?




    Yes to Question 3 would imply yes to Questions 1 and 2. [Edit: I made a mistake when I claimed implicitly that yes to Question 3 would immediately imply yes to Questions 1 and 2. There was a subtlety I missed, but Jeremy's comment below his answer settles also Question 1 and 2. (I also missed that, sigh...)]



    This answer of MooS implies that the category of noetherian rings does not admit finite limits.










    share|cite|improve this question











    $endgroup$















      6












      6








      6





      $begingroup$


      Let $mathsf{Artin}$ be the category of artinian rings, viewed as a full subcategory of the category $mathsf{CRing}$ of rings. (Here "ring" means "commutative ring with one".)




      Question 1. Does $mathsf{Artin}$ admit finite limits?




      As $mathsf{Artin}$ has finite products, Question 1 is equivalent to




      Question 2. Does $mathsf{Artin}$ admit equalizers?




      A closely related question is




      Question 3. Let $Ato B$ be the equalizer in $mathsf{CRing}$ of two morphisms from $B$ to $C$. Assume that $B$ and $C$ are artinian. Does this imply that $A$ is artinian?




      Yes to Question 3 would imply yes to Questions 1 and 2. [Edit: I made a mistake when I claimed implicitly that yes to Question 3 would immediately imply yes to Questions 1 and 2. There was a subtlety I missed, but Jeremy's comment below his answer settles also Question 1 and 2. (I also missed that, sigh...)]



      This answer of MooS implies that the category of noetherian rings does not admit finite limits.










      share|cite|improve this question











      $endgroup$




      Let $mathsf{Artin}$ be the category of artinian rings, viewed as a full subcategory of the category $mathsf{CRing}$ of rings. (Here "ring" means "commutative ring with one".)




      Question 1. Does $mathsf{Artin}$ admit finite limits?




      As $mathsf{Artin}$ has finite products, Question 1 is equivalent to




      Question 2. Does $mathsf{Artin}$ admit equalizers?




      A closely related question is




      Question 3. Let $Ato B$ be the equalizer in $mathsf{CRing}$ of two morphisms from $B$ to $C$. Assume that $B$ and $C$ are artinian. Does this imply that $A$ is artinian?




      Yes to Question 3 would imply yes to Questions 1 and 2. [Edit: I made a mistake when I claimed implicitly that yes to Question 3 would immediately imply yes to Questions 1 and 2. There was a subtlety I missed, but Jeremy's comment below his answer settles also Question 1 and 2. (I also missed that, sigh...)]



      This answer of MooS implies that the category of noetherian rings does not admit finite limits.







      commutative-algebra category-theory limits-colimits artinian






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      share|cite|improve this question













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      share|cite|improve this question








      edited Feb 1 at 14:21







      Pierre-Yves Gaillard

















      asked Jan 31 at 13:21









      Pierre-Yves GaillardPierre-Yves Gaillard

      13.5k23184




      13.5k23184






















          1 Answer
          1






          active

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          3












          $begingroup$

          There's an example in



          Gilmer, Robert; Heinzer, William, Artinian subrings of a commutative ring, Trans. Am. Math. Soc. 336, No. 1, 295-310 (1993). ZBL0778.13012.



          of two artinian subrings of a commutative artinian ring whose intersection is not artinian. Since the intersection is the pullback of the inclusion maps, this answers the question.



          Let $R=mathbb{C}(x)[y]/(y^2)$. Then $R$ is a two-dimensional algebra over $mathbb{C}(x)$, and is therefore artinian.



          Let $R_1=mathbb{C}(x^2)+ymathbb{C}(x)subset R$. Then $R_1$ is a three-dimensional algebra over $mathbb{C}(x^2)$, and is therefore artinian.



          Let $R_2=mathbb{C}(x^2+x)+ymathbb{C}(x)subset R$. Then $R_2$ is a three-dimensional algebra over $mathbb{C}(x^2+x)$, and is therefore artinian.



          But $mathbb{C}(x^2)capmathbb{C}(x^2+x)=mathbb{C}$, so $R_1cap R_2=mathbb{C}+ymathbb{C}(x)$, which is not artinian, or even noetherian, since $ymathbb{C}(x)$ is not finitely generated as an ideal.






          share|cite|improve this answer









          $endgroup$









          • 1




            $begingroup$
            Thanks! I realized that I made a mistake in the statement of the question when I wrote "Yes to Question 3 would imply yes to Questions 1 and 2", because we must exclude the possibility that the pullback does exist in Artin but does not coincide with the pullback in CRing. I'm planning to remove Questions 1 and 2 (and change the title), but before doing so I wanted to ask you if you see a way of showing that the pullback does not exist in Artin (or of showing that it exists but differs from the other). - I just asked a related question: math.stackexchange.com/q/3096154/660
            $endgroup$
            – Pierre-Yves Gaillard
            Feb 1 at 12:13






          • 1




            $begingroup$
            @Pierre-YvesGaillard Good point. No, in the example given, there is no pullback $A$ in Artin. If there were, there would be a map $alpha:Ato R_1cap R_2$ through which every map from an artinian ring factors. But every element of $R_1cap R_2$ is in a fin.dim. and hence artinian $mathbb{C}$-subalgebra, so $alpha$ would have to be surjective. So $R_1cap R_2$ would be a finitely generated $A$-module, and so would have to be artinian.
            $endgroup$
            – Jeremy Rickard
            Feb 1 at 13:47












          • $begingroup$
            You're perfectly right! I should have thought of this... I edited the question.
            $endgroup$
            – Pierre-Yves Gaillard
            Feb 1 at 14:23












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          1 Answer
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          $begingroup$

          There's an example in



          Gilmer, Robert; Heinzer, William, Artinian subrings of a commutative ring, Trans. Am. Math. Soc. 336, No. 1, 295-310 (1993). ZBL0778.13012.



          of two artinian subrings of a commutative artinian ring whose intersection is not artinian. Since the intersection is the pullback of the inclusion maps, this answers the question.



          Let $R=mathbb{C}(x)[y]/(y^2)$. Then $R$ is a two-dimensional algebra over $mathbb{C}(x)$, and is therefore artinian.



          Let $R_1=mathbb{C}(x^2)+ymathbb{C}(x)subset R$. Then $R_1$ is a three-dimensional algebra over $mathbb{C}(x^2)$, and is therefore artinian.



          Let $R_2=mathbb{C}(x^2+x)+ymathbb{C}(x)subset R$. Then $R_2$ is a three-dimensional algebra over $mathbb{C}(x^2+x)$, and is therefore artinian.



          But $mathbb{C}(x^2)capmathbb{C}(x^2+x)=mathbb{C}$, so $R_1cap R_2=mathbb{C}+ymathbb{C}(x)$, which is not artinian, or even noetherian, since $ymathbb{C}(x)$ is not finitely generated as an ideal.






          share|cite|improve this answer









          $endgroup$









          • 1




            $begingroup$
            Thanks! I realized that I made a mistake in the statement of the question when I wrote "Yes to Question 3 would imply yes to Questions 1 and 2", because we must exclude the possibility that the pullback does exist in Artin but does not coincide with the pullback in CRing. I'm planning to remove Questions 1 and 2 (and change the title), but before doing so I wanted to ask you if you see a way of showing that the pullback does not exist in Artin (or of showing that it exists but differs from the other). - I just asked a related question: math.stackexchange.com/q/3096154/660
            $endgroup$
            – Pierre-Yves Gaillard
            Feb 1 at 12:13






          • 1




            $begingroup$
            @Pierre-YvesGaillard Good point. No, in the example given, there is no pullback $A$ in Artin. If there were, there would be a map $alpha:Ato R_1cap R_2$ through which every map from an artinian ring factors. But every element of $R_1cap R_2$ is in a fin.dim. and hence artinian $mathbb{C}$-subalgebra, so $alpha$ would have to be surjective. So $R_1cap R_2$ would be a finitely generated $A$-module, and so would have to be artinian.
            $endgroup$
            – Jeremy Rickard
            Feb 1 at 13:47












          • $begingroup$
            You're perfectly right! I should have thought of this... I edited the question.
            $endgroup$
            – Pierre-Yves Gaillard
            Feb 1 at 14:23
















          3












          $begingroup$

          There's an example in



          Gilmer, Robert; Heinzer, William, Artinian subrings of a commutative ring, Trans. Am. Math. Soc. 336, No. 1, 295-310 (1993). ZBL0778.13012.



          of two artinian subrings of a commutative artinian ring whose intersection is not artinian. Since the intersection is the pullback of the inclusion maps, this answers the question.



          Let $R=mathbb{C}(x)[y]/(y^2)$. Then $R$ is a two-dimensional algebra over $mathbb{C}(x)$, and is therefore artinian.



          Let $R_1=mathbb{C}(x^2)+ymathbb{C}(x)subset R$. Then $R_1$ is a three-dimensional algebra over $mathbb{C}(x^2)$, and is therefore artinian.



          Let $R_2=mathbb{C}(x^2+x)+ymathbb{C}(x)subset R$. Then $R_2$ is a three-dimensional algebra over $mathbb{C}(x^2+x)$, and is therefore artinian.



          But $mathbb{C}(x^2)capmathbb{C}(x^2+x)=mathbb{C}$, so $R_1cap R_2=mathbb{C}+ymathbb{C}(x)$, which is not artinian, or even noetherian, since $ymathbb{C}(x)$ is not finitely generated as an ideal.






          share|cite|improve this answer









          $endgroup$









          • 1




            $begingroup$
            Thanks! I realized that I made a mistake in the statement of the question when I wrote "Yes to Question 3 would imply yes to Questions 1 and 2", because we must exclude the possibility that the pullback does exist in Artin but does not coincide with the pullback in CRing. I'm planning to remove Questions 1 and 2 (and change the title), but before doing so I wanted to ask you if you see a way of showing that the pullback does not exist in Artin (or of showing that it exists but differs from the other). - I just asked a related question: math.stackexchange.com/q/3096154/660
            $endgroup$
            – Pierre-Yves Gaillard
            Feb 1 at 12:13






          • 1




            $begingroup$
            @Pierre-YvesGaillard Good point. No, in the example given, there is no pullback $A$ in Artin. If there were, there would be a map $alpha:Ato R_1cap R_2$ through which every map from an artinian ring factors. But every element of $R_1cap R_2$ is in a fin.dim. and hence artinian $mathbb{C}$-subalgebra, so $alpha$ would have to be surjective. So $R_1cap R_2$ would be a finitely generated $A$-module, and so would have to be artinian.
            $endgroup$
            – Jeremy Rickard
            Feb 1 at 13:47












          • $begingroup$
            You're perfectly right! I should have thought of this... I edited the question.
            $endgroup$
            – Pierre-Yves Gaillard
            Feb 1 at 14:23














          3












          3








          3





          $begingroup$

          There's an example in



          Gilmer, Robert; Heinzer, William, Artinian subrings of a commutative ring, Trans. Am. Math. Soc. 336, No. 1, 295-310 (1993). ZBL0778.13012.



          of two artinian subrings of a commutative artinian ring whose intersection is not artinian. Since the intersection is the pullback of the inclusion maps, this answers the question.



          Let $R=mathbb{C}(x)[y]/(y^2)$. Then $R$ is a two-dimensional algebra over $mathbb{C}(x)$, and is therefore artinian.



          Let $R_1=mathbb{C}(x^2)+ymathbb{C}(x)subset R$. Then $R_1$ is a three-dimensional algebra over $mathbb{C}(x^2)$, and is therefore artinian.



          Let $R_2=mathbb{C}(x^2+x)+ymathbb{C}(x)subset R$. Then $R_2$ is a three-dimensional algebra over $mathbb{C}(x^2+x)$, and is therefore artinian.



          But $mathbb{C}(x^2)capmathbb{C}(x^2+x)=mathbb{C}$, so $R_1cap R_2=mathbb{C}+ymathbb{C}(x)$, which is not artinian, or even noetherian, since $ymathbb{C}(x)$ is not finitely generated as an ideal.






          share|cite|improve this answer









          $endgroup$



          There's an example in



          Gilmer, Robert; Heinzer, William, Artinian subrings of a commutative ring, Trans. Am. Math. Soc. 336, No. 1, 295-310 (1993). ZBL0778.13012.



          of two artinian subrings of a commutative artinian ring whose intersection is not artinian. Since the intersection is the pullback of the inclusion maps, this answers the question.



          Let $R=mathbb{C}(x)[y]/(y^2)$. Then $R$ is a two-dimensional algebra over $mathbb{C}(x)$, and is therefore artinian.



          Let $R_1=mathbb{C}(x^2)+ymathbb{C}(x)subset R$. Then $R_1$ is a three-dimensional algebra over $mathbb{C}(x^2)$, and is therefore artinian.



          Let $R_2=mathbb{C}(x^2+x)+ymathbb{C}(x)subset R$. Then $R_2$ is a three-dimensional algebra over $mathbb{C}(x^2+x)$, and is therefore artinian.



          But $mathbb{C}(x^2)capmathbb{C}(x^2+x)=mathbb{C}$, so $R_1cap R_2=mathbb{C}+ymathbb{C}(x)$, which is not artinian, or even noetherian, since $ymathbb{C}(x)$ is not finitely generated as an ideal.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Feb 1 at 10:35









          Jeremy RickardJeremy Rickard

          16.9k11746




          16.9k11746








          • 1




            $begingroup$
            Thanks! I realized that I made a mistake in the statement of the question when I wrote "Yes to Question 3 would imply yes to Questions 1 and 2", because we must exclude the possibility that the pullback does exist in Artin but does not coincide with the pullback in CRing. I'm planning to remove Questions 1 and 2 (and change the title), but before doing so I wanted to ask you if you see a way of showing that the pullback does not exist in Artin (or of showing that it exists but differs from the other). - I just asked a related question: math.stackexchange.com/q/3096154/660
            $endgroup$
            – Pierre-Yves Gaillard
            Feb 1 at 12:13






          • 1




            $begingroup$
            @Pierre-YvesGaillard Good point. No, in the example given, there is no pullback $A$ in Artin. If there were, there would be a map $alpha:Ato R_1cap R_2$ through which every map from an artinian ring factors. But every element of $R_1cap R_2$ is in a fin.dim. and hence artinian $mathbb{C}$-subalgebra, so $alpha$ would have to be surjective. So $R_1cap R_2$ would be a finitely generated $A$-module, and so would have to be artinian.
            $endgroup$
            – Jeremy Rickard
            Feb 1 at 13:47












          • $begingroup$
            You're perfectly right! I should have thought of this... I edited the question.
            $endgroup$
            – Pierre-Yves Gaillard
            Feb 1 at 14:23














          • 1




            $begingroup$
            Thanks! I realized that I made a mistake in the statement of the question when I wrote "Yes to Question 3 would imply yes to Questions 1 and 2", because we must exclude the possibility that the pullback does exist in Artin but does not coincide with the pullback in CRing. I'm planning to remove Questions 1 and 2 (and change the title), but before doing so I wanted to ask you if you see a way of showing that the pullback does not exist in Artin (or of showing that it exists but differs from the other). - I just asked a related question: math.stackexchange.com/q/3096154/660
            $endgroup$
            – Pierre-Yves Gaillard
            Feb 1 at 12:13






          • 1




            $begingroup$
            @Pierre-YvesGaillard Good point. No, in the example given, there is no pullback $A$ in Artin. If there were, there would be a map $alpha:Ato R_1cap R_2$ through which every map from an artinian ring factors. But every element of $R_1cap R_2$ is in a fin.dim. and hence artinian $mathbb{C}$-subalgebra, so $alpha$ would have to be surjective. So $R_1cap R_2$ would be a finitely generated $A$-module, and so would have to be artinian.
            $endgroup$
            – Jeremy Rickard
            Feb 1 at 13:47












          • $begingroup$
            You're perfectly right! I should have thought of this... I edited the question.
            $endgroup$
            – Pierre-Yves Gaillard
            Feb 1 at 14:23








          1




          1




          $begingroup$
          Thanks! I realized that I made a mistake in the statement of the question when I wrote "Yes to Question 3 would imply yes to Questions 1 and 2", because we must exclude the possibility that the pullback does exist in Artin but does not coincide with the pullback in CRing. I'm planning to remove Questions 1 and 2 (and change the title), but before doing so I wanted to ask you if you see a way of showing that the pullback does not exist in Artin (or of showing that it exists but differs from the other). - I just asked a related question: math.stackexchange.com/q/3096154/660
          $endgroup$
          – Pierre-Yves Gaillard
          Feb 1 at 12:13




          $begingroup$
          Thanks! I realized that I made a mistake in the statement of the question when I wrote "Yes to Question 3 would imply yes to Questions 1 and 2", because we must exclude the possibility that the pullback does exist in Artin but does not coincide with the pullback in CRing. I'm planning to remove Questions 1 and 2 (and change the title), but before doing so I wanted to ask you if you see a way of showing that the pullback does not exist in Artin (or of showing that it exists but differs from the other). - I just asked a related question: math.stackexchange.com/q/3096154/660
          $endgroup$
          – Pierre-Yves Gaillard
          Feb 1 at 12:13




          1




          1




          $begingroup$
          @Pierre-YvesGaillard Good point. No, in the example given, there is no pullback $A$ in Artin. If there were, there would be a map $alpha:Ato R_1cap R_2$ through which every map from an artinian ring factors. But every element of $R_1cap R_2$ is in a fin.dim. and hence artinian $mathbb{C}$-subalgebra, so $alpha$ would have to be surjective. So $R_1cap R_2$ would be a finitely generated $A$-module, and so would have to be artinian.
          $endgroup$
          – Jeremy Rickard
          Feb 1 at 13:47






          $begingroup$
          @Pierre-YvesGaillard Good point. No, in the example given, there is no pullback $A$ in Artin. If there were, there would be a map $alpha:Ato R_1cap R_2$ through which every map from an artinian ring factors. But every element of $R_1cap R_2$ is in a fin.dim. and hence artinian $mathbb{C}$-subalgebra, so $alpha$ would have to be surjective. So $R_1cap R_2$ would be a finitely generated $A$-module, and so would have to be artinian.
          $endgroup$
          – Jeremy Rickard
          Feb 1 at 13:47














          $begingroup$
          You're perfectly right! I should have thought of this... I edited the question.
          $endgroup$
          – Pierre-Yves Gaillard
          Feb 1 at 14:23




          $begingroup$
          You're perfectly right! I should have thought of this... I edited the question.
          $endgroup$
          – Pierre-Yves Gaillard
          Feb 1 at 14:23


















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