Can not tell colimits from limits












2












$begingroup$


Proposition 71 here reads:




Let $X$ be a concentrated scheme and $F$ a quasi-coherent sheaf of modules. The
following are equivalent:
(a) The functor $mathrm{Hom}(F, −):Qco(X)rightarrow Ab$ preserves direct limits. In other words, $F$ is a
finitely presented object of $Qco(X)$.
(b) The functor $mathit{Hom}(F, −) : Qco(X)rightarrow Mod(X)$ preserves direct limits.




The question is: is there a typo? My understanding is that the covariant $mathrm{Hom}$ (not the internal one) preserves limits for any category. Should it be "colimits" instead of "limits" in the point (a)?










share|cite|improve this question







New contributor




gelfand_dominates is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







$endgroup$

















    2












    $begingroup$


    Proposition 71 here reads:




    Let $X$ be a concentrated scheme and $F$ a quasi-coherent sheaf of modules. The
    following are equivalent:
    (a) The functor $mathrm{Hom}(F, −):Qco(X)rightarrow Ab$ preserves direct limits. In other words, $F$ is a
    finitely presented object of $Qco(X)$.
    (b) The functor $mathit{Hom}(F, −) : Qco(X)rightarrow Mod(X)$ preserves direct limits.




    The question is: is there a typo? My understanding is that the covariant $mathrm{Hom}$ (not the internal one) preserves limits for any category. Should it be "colimits" instead of "limits" in the point (a)?










    share|cite|improve this question







    New contributor




    gelfand_dominates is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
    Check out our Code of Conduct.







    $endgroup$















      2












      2








      2





      $begingroup$


      Proposition 71 here reads:




      Let $X$ be a concentrated scheme and $F$ a quasi-coherent sheaf of modules. The
      following are equivalent:
      (a) The functor $mathrm{Hom}(F, −):Qco(X)rightarrow Ab$ preserves direct limits. In other words, $F$ is a
      finitely presented object of $Qco(X)$.
      (b) The functor $mathit{Hom}(F, −) : Qco(X)rightarrow Mod(X)$ preserves direct limits.




      The question is: is there a typo? My understanding is that the covariant $mathrm{Hom}$ (not the internal one) preserves limits for any category. Should it be "colimits" instead of "limits" in the point (a)?










      share|cite|improve this question







      New contributor




      gelfand_dominates is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.







      $endgroup$




      Proposition 71 here reads:




      Let $X$ be a concentrated scheme and $F$ a quasi-coherent sheaf of modules. The
      following are equivalent:
      (a) The functor $mathrm{Hom}(F, −):Qco(X)rightarrow Ab$ preserves direct limits. In other words, $F$ is a
      finitely presented object of $Qco(X)$.
      (b) The functor $mathit{Hom}(F, −) : Qco(X)rightarrow Mod(X)$ preserves direct limits.




      The question is: is there a typo? My understanding is that the covariant $mathrm{Hom}$ (not the internal one) preserves limits for any category. Should it be "colimits" instead of "limits" in the point (a)?







      homological-algebra sheaf-theory schemes






      share|cite|improve this question







      New contributor




      gelfand_dominates is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      share|cite|improve this question







      New contributor




      gelfand_dominates is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.









      share|cite|improve this question




      share|cite|improve this question






      New contributor




      gelfand_dominates is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.









      asked 45 mins ago









      gelfand_dominatesgelfand_dominates

      111




      111




      New contributor




      gelfand_dominates is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.





      New contributor





      gelfand_dominates is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






      gelfand_dominates is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






















          1 Answer
          1






          active

          oldest

          votes


















          3












          $begingroup$

          EDIT: The main body of my answer is there because I didn't understand the question, so let me answer it in this edit (I'll leave the main body of the answer there, just in case)



          No, there is no typo : direct limits are a special kind of colimits. The terminology for them was presumably coined before category theory defined the general notions of "limits" and "colimits". In particular, "direct limit" means "colimit over a directed system" and "inverse" or "projective limit" means "limit over a (directed system)$^{op}$"; thankfully, in general there isn't too much confusion about what one means in a given situation.



          The rest does not adress the question, because I had misunderstood it. I'll leave it there regardless, hoping it's not too much of a problem.



          No there is no typo : indeed the covariant hom always preserves limits, but when $F$ is "small enough" (here : finitely presented) it also preserves direct limits.



          Here's a sketch of proof where I replace sheaves by mere modules : let $F$ be an $R$-module; then $F$ is finitely presented iff $hom(F,-)$ preserves direct limits.



          Indeed if you have a directed system $(M_i)_{iin I}$ and a map $Fto varinjlim M_i$, the generators land in some $M_{i_0}$ (there's finitely many of them) and every relation is satisfied in some $M_{j_0}, j_0geq i_0$ because the relations are satisfied in the colimit and there's only finitely many of them too.



          For the converse you may want to take the directed system of finitely generated submodules of $F$ to get "finitely generated" and then you work in a similar fashion to get finitely presented (taking quotients of the free module on the generators by finitely generated submodules of the relations for instance)



          This works for modules and actually all algebraic structures so that "$hom(F,-)$ preserves direct limits" is a good generalization of "$F$ is finitely presented"






          share|cite|improve this answer











          $endgroup$









          • 1




            $begingroup$
            I think the OP maybe confused because the terminology kind of sucks. "Direct limits" are colimits, right?
            $endgroup$
            – Aknazar Kazhymurat
            36 mins ago










          • $begingroup$
            @AknazarKazhymurat yes, I'll add a word about that !
            $endgroup$
            – Max
            36 mins ago










          • $begingroup$
            @AknazarKazhymurat oh wait, sorry, I had completely misunderstood your point and the point of the OP !
            $endgroup$
            – Max
            35 mins ago












          Your Answer








          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
          });


          }
          });






          gelfand_dominates is a new contributor. Be nice, and check out our Code of Conduct.










          draft saved

          draft discarded


















          StackExchange.ready(
          function () {
          StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmathoverflow.net%2fquestions%2f330188%2fcan-not-tell-colimits-from-limits%23new-answer', 'question_page');
          }
          );

          Post as a guest















          Required, but never shown

























          1 Answer
          1






          active

          oldest

          votes








          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          3












          $begingroup$

          EDIT: The main body of my answer is there because I didn't understand the question, so let me answer it in this edit (I'll leave the main body of the answer there, just in case)



          No, there is no typo : direct limits are a special kind of colimits. The terminology for them was presumably coined before category theory defined the general notions of "limits" and "colimits". In particular, "direct limit" means "colimit over a directed system" and "inverse" or "projective limit" means "limit over a (directed system)$^{op}$"; thankfully, in general there isn't too much confusion about what one means in a given situation.



          The rest does not adress the question, because I had misunderstood it. I'll leave it there regardless, hoping it's not too much of a problem.



          No there is no typo : indeed the covariant hom always preserves limits, but when $F$ is "small enough" (here : finitely presented) it also preserves direct limits.



          Here's a sketch of proof where I replace sheaves by mere modules : let $F$ be an $R$-module; then $F$ is finitely presented iff $hom(F,-)$ preserves direct limits.



          Indeed if you have a directed system $(M_i)_{iin I}$ and a map $Fto varinjlim M_i$, the generators land in some $M_{i_0}$ (there's finitely many of them) and every relation is satisfied in some $M_{j_0}, j_0geq i_0$ because the relations are satisfied in the colimit and there's only finitely many of them too.



          For the converse you may want to take the directed system of finitely generated submodules of $F$ to get "finitely generated" and then you work in a similar fashion to get finitely presented (taking quotients of the free module on the generators by finitely generated submodules of the relations for instance)



          This works for modules and actually all algebraic structures so that "$hom(F,-)$ preserves direct limits" is a good generalization of "$F$ is finitely presented"






          share|cite|improve this answer











          $endgroup$









          • 1




            $begingroup$
            I think the OP maybe confused because the terminology kind of sucks. "Direct limits" are colimits, right?
            $endgroup$
            – Aknazar Kazhymurat
            36 mins ago










          • $begingroup$
            @AknazarKazhymurat yes, I'll add a word about that !
            $endgroup$
            – Max
            36 mins ago










          • $begingroup$
            @AknazarKazhymurat oh wait, sorry, I had completely misunderstood your point and the point of the OP !
            $endgroup$
            – Max
            35 mins ago
















          3












          $begingroup$

          EDIT: The main body of my answer is there because I didn't understand the question, so let me answer it in this edit (I'll leave the main body of the answer there, just in case)



          No, there is no typo : direct limits are a special kind of colimits. The terminology for them was presumably coined before category theory defined the general notions of "limits" and "colimits". In particular, "direct limit" means "colimit over a directed system" and "inverse" or "projective limit" means "limit over a (directed system)$^{op}$"; thankfully, in general there isn't too much confusion about what one means in a given situation.



          The rest does not adress the question, because I had misunderstood it. I'll leave it there regardless, hoping it's not too much of a problem.



          No there is no typo : indeed the covariant hom always preserves limits, but when $F$ is "small enough" (here : finitely presented) it also preserves direct limits.



          Here's a sketch of proof where I replace sheaves by mere modules : let $F$ be an $R$-module; then $F$ is finitely presented iff $hom(F,-)$ preserves direct limits.



          Indeed if you have a directed system $(M_i)_{iin I}$ and a map $Fto varinjlim M_i$, the generators land in some $M_{i_0}$ (there's finitely many of them) and every relation is satisfied in some $M_{j_0}, j_0geq i_0$ because the relations are satisfied in the colimit and there's only finitely many of them too.



          For the converse you may want to take the directed system of finitely generated submodules of $F$ to get "finitely generated" and then you work in a similar fashion to get finitely presented (taking quotients of the free module on the generators by finitely generated submodules of the relations for instance)



          This works for modules and actually all algebraic structures so that "$hom(F,-)$ preserves direct limits" is a good generalization of "$F$ is finitely presented"






          share|cite|improve this answer











          $endgroup$









          • 1




            $begingroup$
            I think the OP maybe confused because the terminology kind of sucks. "Direct limits" are colimits, right?
            $endgroup$
            – Aknazar Kazhymurat
            36 mins ago










          • $begingroup$
            @AknazarKazhymurat yes, I'll add a word about that !
            $endgroup$
            – Max
            36 mins ago










          • $begingroup$
            @AknazarKazhymurat oh wait, sorry, I had completely misunderstood your point and the point of the OP !
            $endgroup$
            – Max
            35 mins ago














          3












          3








          3





          $begingroup$

          EDIT: The main body of my answer is there because I didn't understand the question, so let me answer it in this edit (I'll leave the main body of the answer there, just in case)



          No, there is no typo : direct limits are a special kind of colimits. The terminology for them was presumably coined before category theory defined the general notions of "limits" and "colimits". In particular, "direct limit" means "colimit over a directed system" and "inverse" or "projective limit" means "limit over a (directed system)$^{op}$"; thankfully, in general there isn't too much confusion about what one means in a given situation.



          The rest does not adress the question, because I had misunderstood it. I'll leave it there regardless, hoping it's not too much of a problem.



          No there is no typo : indeed the covariant hom always preserves limits, but when $F$ is "small enough" (here : finitely presented) it also preserves direct limits.



          Here's a sketch of proof where I replace sheaves by mere modules : let $F$ be an $R$-module; then $F$ is finitely presented iff $hom(F,-)$ preserves direct limits.



          Indeed if you have a directed system $(M_i)_{iin I}$ and a map $Fto varinjlim M_i$, the generators land in some $M_{i_0}$ (there's finitely many of them) and every relation is satisfied in some $M_{j_0}, j_0geq i_0$ because the relations are satisfied in the colimit and there's only finitely many of them too.



          For the converse you may want to take the directed system of finitely generated submodules of $F$ to get "finitely generated" and then you work in a similar fashion to get finitely presented (taking quotients of the free module on the generators by finitely generated submodules of the relations for instance)



          This works for modules and actually all algebraic structures so that "$hom(F,-)$ preserves direct limits" is a good generalization of "$F$ is finitely presented"






          share|cite|improve this answer











          $endgroup$



          EDIT: The main body of my answer is there because I didn't understand the question, so let me answer it in this edit (I'll leave the main body of the answer there, just in case)



          No, there is no typo : direct limits are a special kind of colimits. The terminology for them was presumably coined before category theory defined the general notions of "limits" and "colimits". In particular, "direct limit" means "colimit over a directed system" and "inverse" or "projective limit" means "limit over a (directed system)$^{op}$"; thankfully, in general there isn't too much confusion about what one means in a given situation.



          The rest does not adress the question, because I had misunderstood it. I'll leave it there regardless, hoping it's not too much of a problem.



          No there is no typo : indeed the covariant hom always preserves limits, but when $F$ is "small enough" (here : finitely presented) it also preserves direct limits.



          Here's a sketch of proof where I replace sheaves by mere modules : let $F$ be an $R$-module; then $F$ is finitely presented iff $hom(F,-)$ preserves direct limits.



          Indeed if you have a directed system $(M_i)_{iin I}$ and a map $Fto varinjlim M_i$, the generators land in some $M_{i_0}$ (there's finitely many of them) and every relation is satisfied in some $M_{j_0}, j_0geq i_0$ because the relations are satisfied in the colimit and there's only finitely many of them too.



          For the converse you may want to take the directed system of finitely generated submodules of $F$ to get "finitely generated" and then you work in a similar fashion to get finitely presented (taking quotients of the free module on the generators by finitely generated submodules of the relations for instance)



          This works for modules and actually all algebraic structures so that "$hom(F,-)$ preserves direct limits" is a good generalization of "$F$ is finitely presented"







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited 31 mins ago

























          answered 38 mins ago









          MaxMax

          6791619




          6791619








          • 1




            $begingroup$
            I think the OP maybe confused because the terminology kind of sucks. "Direct limits" are colimits, right?
            $endgroup$
            – Aknazar Kazhymurat
            36 mins ago










          • $begingroup$
            @AknazarKazhymurat yes, I'll add a word about that !
            $endgroup$
            – Max
            36 mins ago










          • $begingroup$
            @AknazarKazhymurat oh wait, sorry, I had completely misunderstood your point and the point of the OP !
            $endgroup$
            – Max
            35 mins ago














          • 1




            $begingroup$
            I think the OP maybe confused because the terminology kind of sucks. "Direct limits" are colimits, right?
            $endgroup$
            – Aknazar Kazhymurat
            36 mins ago










          • $begingroup$
            @AknazarKazhymurat yes, I'll add a word about that !
            $endgroup$
            – Max
            36 mins ago










          • $begingroup$
            @AknazarKazhymurat oh wait, sorry, I had completely misunderstood your point and the point of the OP !
            $endgroup$
            – Max
            35 mins ago








          1




          1




          $begingroup$
          I think the OP maybe confused because the terminology kind of sucks. "Direct limits" are colimits, right?
          $endgroup$
          – Aknazar Kazhymurat
          36 mins ago




          $begingroup$
          I think the OP maybe confused because the terminology kind of sucks. "Direct limits" are colimits, right?
          $endgroup$
          – Aknazar Kazhymurat
          36 mins ago












          $begingroup$
          @AknazarKazhymurat yes, I'll add a word about that !
          $endgroup$
          – Max
          36 mins ago




          $begingroup$
          @AknazarKazhymurat yes, I'll add a word about that !
          $endgroup$
          – Max
          36 mins ago












          $begingroup$
          @AknazarKazhymurat oh wait, sorry, I had completely misunderstood your point and the point of the OP !
          $endgroup$
          – Max
          35 mins ago




          $begingroup$
          @AknazarKazhymurat oh wait, sorry, I had completely misunderstood your point and the point of the OP !
          $endgroup$
          – Max
          35 mins ago










          gelfand_dominates is a new contributor. Be nice, and check out our Code of Conduct.










          draft saved

          draft discarded


















          gelfand_dominates is a new contributor. Be nice, and check out our Code of Conduct.













          gelfand_dominates is a new contributor. Be nice, and check out our Code of Conduct.












          gelfand_dominates is a new contributor. Be nice, and check out our Code of Conduct.
















          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.




          draft saved


          draft discarded














          StackExchange.ready(
          function () {
          StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmathoverflow.net%2fquestions%2f330188%2fcan-not-tell-colimits-from-limits%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

          Hivernacle

          Fluorita

          Hulsita