How is it obvious that $times : C times C to C$ is right adjoint to the diagonal functor?












1












$begingroup$


This is from "Sheaves in Geometry & Logic".



$times : C times C to C$ is the cartesian product of two objects. So assume that finite products exist in $C$ the above is a functor.



To say that it is right adjoint to a functor $L$, is to say that for all $a times b in C times C$ and all $d in C$ we have a bijection of sets:



$$
text{Hom}_{C}(d, a times b) simeq text{Hom}_{Ctimes C}((d, d), (a,b))
$$

where we've used $(x,y)$ to distinguish elements in $C times C$ from those in $C$.



This is supposed to be obvious, but it isn't to me.



Further, we have to show that the above is natural in $d, (a,b)$. Is there some sort of composition of natural maps and functors we can find here?





Is it:



$text{Hom}_{Ctimes C}((d,d), (a,b)) = text{Hom}_C(d, a) times text{Hom}_C(d, b) simeq text{Hom}_C(d, atimes b)$.
?
But how do I show naturality in $(a,b)$?










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    1












    $begingroup$


    This is from "Sheaves in Geometry & Logic".



    $times : C times C to C$ is the cartesian product of two objects. So assume that finite products exist in $C$ the above is a functor.



    To say that it is right adjoint to a functor $L$, is to say that for all $a times b in C times C$ and all $d in C$ we have a bijection of sets:



    $$
    text{Hom}_{C}(d, a times b) simeq text{Hom}_{Ctimes C}((d, d), (a,b))
    $$

    where we've used $(x,y)$ to distinguish elements in $C times C$ from those in $C$.



    This is supposed to be obvious, but it isn't to me.



    Further, we have to show that the above is natural in $d, (a,b)$. Is there some sort of composition of natural maps and functors we can find here?





    Is it:



    $text{Hom}_{Ctimes C}((d,d), (a,b)) = text{Hom}_C(d, a) times text{Hom}_C(d, b) simeq text{Hom}_C(d, atimes b)$.
    ?
    But how do I show naturality in $(a,b)$?










    share|cite|improve this question











    $endgroup$















      1












      1








      1





      $begingroup$


      This is from "Sheaves in Geometry & Logic".



      $times : C times C to C$ is the cartesian product of two objects. So assume that finite products exist in $C$ the above is a functor.



      To say that it is right adjoint to a functor $L$, is to say that for all $a times b in C times C$ and all $d in C$ we have a bijection of sets:



      $$
      text{Hom}_{C}(d, a times b) simeq text{Hom}_{Ctimes C}((d, d), (a,b))
      $$

      where we've used $(x,y)$ to distinguish elements in $C times C$ from those in $C$.



      This is supposed to be obvious, but it isn't to me.



      Further, we have to show that the above is natural in $d, (a,b)$. Is there some sort of composition of natural maps and functors we can find here?





      Is it:



      $text{Hom}_{Ctimes C}((d,d), (a,b)) = text{Hom}_C(d, a) times text{Hom}_C(d, b) simeq text{Hom}_C(d, atimes b)$.
      ?
      But how do I show naturality in $(a,b)$?










      share|cite|improve this question











      $endgroup$




      This is from "Sheaves in Geometry & Logic".



      $times : C times C to C$ is the cartesian product of two objects. So assume that finite products exist in $C$ the above is a functor.



      To say that it is right adjoint to a functor $L$, is to say that for all $a times b in C times C$ and all $d in C$ we have a bijection of sets:



      $$
      text{Hom}_{C}(d, a times b) simeq text{Hom}_{Ctimes C}((d, d), (a,b))
      $$

      where we've used $(x,y)$ to distinguish elements in $C times C$ from those in $C$.



      This is supposed to be obvious, but it isn't to me.



      Further, we have to show that the above is natural in $d, (a,b)$. Is there some sort of composition of natural maps and functors we can find here?





      Is it:



      $text{Hom}_{Ctimes C}((d,d), (a,b)) = text{Hom}_C(d, a) times text{Hom}_C(d, b) simeq text{Hom}_C(d, atimes b)$.
      ?
      But how do I show naturality in $(a,b)$?







      category-theory products adjoint-functors functors natural-transformations






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Jan 2 at 22:11







      Roll up and smoke Adjoint

















      asked Jan 2 at 22:05









      Roll up and smoke AdjointRoll up and smoke Adjoint

      9,10052458




      9,10052458






















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          $begingroup$

          You are on the right track. The diagonal functor is defined by $ Delta: Cto Ctimes C$. On objects: $cmapsto ctimes c$. On arrows: $(f,f):(c, c)to (c', c')$.



          From this, we can construct a right adjoint simply by enforcing the rules:



          A right adjoint is a functor $G:Ctimes Cto C$ such that



          $text{Hom}_C(c,G(a,b))cong text{Hom}_{Ctimes C}(Delta c,(a,b)).$



          The left-hand side of this is clear. The right-hand side is a set of arrows of the form $(c,c)to (a,b)$ in the category $Ctimes C.$ But these arrows, by definition, are $pairs (f,g)$ where $f:cto a$ and $g:cto b$, so there is the obvious bijection $text{Hom}_{Ctimes C}(Delta c,(a,b))cong text{Hom}_{C}( c,a)times text{Hom}_{C}(c,b)$. This is natural as well, but that's not important here. It just gives us an idea of what $G should $ be.



          That is, $G=times$, the product functor. On objects: $(a,b)to atimes b$ and on arrows, the unique morphism induced by the UMP of the product.



          As long as $C$ has products and small hom-sets, this will work.



          The obvious unit is $eta_c=langle 1_c,1_crangle .$



          All that remains is to check that $GDelta fcirc eta_c=f$ for $f:cto atimes b.$



          I guess you can take it from here.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Congrats on reaching 10k rep
            $endgroup$
            – Roll up and smoke Adjoint
            Jan 2 at 23:05










          • $begingroup$
            Thank you!-----
            $endgroup$
            – Matematleta
            Jan 2 at 23:11











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          $begingroup$

          You are on the right track. The diagonal functor is defined by $ Delta: Cto Ctimes C$. On objects: $cmapsto ctimes c$. On arrows: $(f,f):(c, c)to (c', c')$.



          From this, we can construct a right adjoint simply by enforcing the rules:



          A right adjoint is a functor $G:Ctimes Cto C$ such that



          $text{Hom}_C(c,G(a,b))cong text{Hom}_{Ctimes C}(Delta c,(a,b)).$



          The left-hand side of this is clear. The right-hand side is a set of arrows of the form $(c,c)to (a,b)$ in the category $Ctimes C.$ But these arrows, by definition, are $pairs (f,g)$ where $f:cto a$ and $g:cto b$, so there is the obvious bijection $text{Hom}_{Ctimes C}(Delta c,(a,b))cong text{Hom}_{C}( c,a)times text{Hom}_{C}(c,b)$. This is natural as well, but that's not important here. It just gives us an idea of what $G should $ be.



          That is, $G=times$, the product functor. On objects: $(a,b)to atimes b$ and on arrows, the unique morphism induced by the UMP of the product.



          As long as $C$ has products and small hom-sets, this will work.



          The obvious unit is $eta_c=langle 1_c,1_crangle .$



          All that remains is to check that $GDelta fcirc eta_c=f$ for $f:cto atimes b.$



          I guess you can take it from here.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Congrats on reaching 10k rep
            $endgroup$
            – Roll up and smoke Adjoint
            Jan 2 at 23:05










          • $begingroup$
            Thank you!-----
            $endgroup$
            – Matematleta
            Jan 2 at 23:11
















          2












          $begingroup$

          You are on the right track. The diagonal functor is defined by $ Delta: Cto Ctimes C$. On objects: $cmapsto ctimes c$. On arrows: $(f,f):(c, c)to (c', c')$.



          From this, we can construct a right adjoint simply by enforcing the rules:



          A right adjoint is a functor $G:Ctimes Cto C$ such that



          $text{Hom}_C(c,G(a,b))cong text{Hom}_{Ctimes C}(Delta c,(a,b)).$



          The left-hand side of this is clear. The right-hand side is a set of arrows of the form $(c,c)to (a,b)$ in the category $Ctimes C.$ But these arrows, by definition, are $pairs (f,g)$ where $f:cto a$ and $g:cto b$, so there is the obvious bijection $text{Hom}_{Ctimes C}(Delta c,(a,b))cong text{Hom}_{C}( c,a)times text{Hom}_{C}(c,b)$. This is natural as well, but that's not important here. It just gives us an idea of what $G should $ be.



          That is, $G=times$, the product functor. On objects: $(a,b)to atimes b$ and on arrows, the unique morphism induced by the UMP of the product.



          As long as $C$ has products and small hom-sets, this will work.



          The obvious unit is $eta_c=langle 1_c,1_crangle .$



          All that remains is to check that $GDelta fcirc eta_c=f$ for $f:cto atimes b.$



          I guess you can take it from here.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Congrats on reaching 10k rep
            $endgroup$
            – Roll up and smoke Adjoint
            Jan 2 at 23:05










          • $begingroup$
            Thank you!-----
            $endgroup$
            – Matematleta
            Jan 2 at 23:11














          2












          2








          2





          $begingroup$

          You are on the right track. The diagonal functor is defined by $ Delta: Cto Ctimes C$. On objects: $cmapsto ctimes c$. On arrows: $(f,f):(c, c)to (c', c')$.



          From this, we can construct a right adjoint simply by enforcing the rules:



          A right adjoint is a functor $G:Ctimes Cto C$ such that



          $text{Hom}_C(c,G(a,b))cong text{Hom}_{Ctimes C}(Delta c,(a,b)).$



          The left-hand side of this is clear. The right-hand side is a set of arrows of the form $(c,c)to (a,b)$ in the category $Ctimes C.$ But these arrows, by definition, are $pairs (f,g)$ where $f:cto a$ and $g:cto b$, so there is the obvious bijection $text{Hom}_{Ctimes C}(Delta c,(a,b))cong text{Hom}_{C}( c,a)times text{Hom}_{C}(c,b)$. This is natural as well, but that's not important here. It just gives us an idea of what $G should $ be.



          That is, $G=times$, the product functor. On objects: $(a,b)to atimes b$ and on arrows, the unique morphism induced by the UMP of the product.



          As long as $C$ has products and small hom-sets, this will work.



          The obvious unit is $eta_c=langle 1_c,1_crangle .$



          All that remains is to check that $GDelta fcirc eta_c=f$ for $f:cto atimes b.$



          I guess you can take it from here.






          share|cite|improve this answer











          $endgroup$



          You are on the right track. The diagonal functor is defined by $ Delta: Cto Ctimes C$. On objects: $cmapsto ctimes c$. On arrows: $(f,f):(c, c)to (c', c')$.



          From this, we can construct a right adjoint simply by enforcing the rules:



          A right adjoint is a functor $G:Ctimes Cto C$ such that



          $text{Hom}_C(c,G(a,b))cong text{Hom}_{Ctimes C}(Delta c,(a,b)).$



          The left-hand side of this is clear. The right-hand side is a set of arrows of the form $(c,c)to (a,b)$ in the category $Ctimes C.$ But these arrows, by definition, are $pairs (f,g)$ where $f:cto a$ and $g:cto b$, so there is the obvious bijection $text{Hom}_{Ctimes C}(Delta c,(a,b))cong text{Hom}_{C}( c,a)times text{Hom}_{C}(c,b)$. This is natural as well, but that's not important here. It just gives us an idea of what $G should $ be.



          That is, $G=times$, the product functor. On objects: $(a,b)to atimes b$ and on arrows, the unique morphism induced by the UMP of the product.



          As long as $C$ has products and small hom-sets, this will work.



          The obvious unit is $eta_c=langle 1_c,1_crangle .$



          All that remains is to check that $GDelta fcirc eta_c=f$ for $f:cto atimes b.$



          I guess you can take it from here.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Jan 2 at 23:20

























          answered Jan 2 at 23:00









          MatematletaMatematleta

          10.2k2918




          10.2k2918












          • $begingroup$
            Congrats on reaching 10k rep
            $endgroup$
            – Roll up and smoke Adjoint
            Jan 2 at 23:05










          • $begingroup$
            Thank you!-----
            $endgroup$
            – Matematleta
            Jan 2 at 23:11


















          • $begingroup$
            Congrats on reaching 10k rep
            $endgroup$
            – Roll up and smoke Adjoint
            Jan 2 at 23:05










          • $begingroup$
            Thank you!-----
            $endgroup$
            – Matematleta
            Jan 2 at 23:11
















          $begingroup$
          Congrats on reaching 10k rep
          $endgroup$
          – Roll up and smoke Adjoint
          Jan 2 at 23:05




          $begingroup$
          Congrats on reaching 10k rep
          $endgroup$
          – Roll up and smoke Adjoint
          Jan 2 at 23:05












          $begingroup$
          Thank you!-----
          $endgroup$
          – Matematleta
          Jan 2 at 23:11




          $begingroup$
          Thank you!-----
          $endgroup$
          – Matematleta
          Jan 2 at 23:11


















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