Exact additive functor preserves homology












0












$begingroup$



Let $R$ and $A$ be rings, $T:{}_{R}text{Mod}rightarrow {}_Atext{Mod}$ be an exact additive functor and $(C_bullet,d_bullet)$ a chain complex in ${}_{R}text{Mod}$. Prove that
$$H_n(TC_bullet,Td_bullet)cong TH_n(C_bullet,d_bullet) $$




I know this looks too easy, but I can't do an actual proof of this fact.



The thing is that is easy to prove that $Tker(d_n)cong ker(T d_n)$ and $T text{im}(d_{n+1})cong text{im}(Td_{n+1})$ but this isomorphims are to abstract so I can't prove that the latter isomorphism is a restriction of the former.



Also, I have the feeling that the additive hypothesis is not needed. Is that correct?










share|cite|improve this question











$endgroup$












  • $begingroup$
    I know there is already a question in the site about this. But the solution given there have some problems. For example the equality $F(partial_{n+1})(F(C_{n+1})))=F(partial_{n+1}(C_{n+1}))$ is not true without exactness an even with exactness it is just an isomorphism. So you need to show that this isomorphism relates well with respect to the other isomorphism of the kernel. Otherwise is not true the part $$frac{ker(F(partial_n))}{Im(F(partial_{n+1}))}stackrel{by (1)(2)}{cong}frac{F(ker(partial_n))}{F(Im(partial_{n+1}))}$$.
    $endgroup$
    – Walter Simon
    Oct 11 '17 at 0:10












  • $begingroup$
    you use additivity in the fact that $T$ maps chain complexes to chain complexes
    $endgroup$
    – M. Van
    Oct 11 '17 at 1:19






  • 1




    $begingroup$
    $T ker(d_n)$ is a kernel of $T d_n$. Your issue boils down to how to understand in what way homology is independent of which specific choices of kernels, images, and quotients you use in its construction.
    $endgroup$
    – Hurkyl
    Oct 11 '17 at 16:18






  • 1




    $begingroup$
    @WalterSimon we need $Td_i circ Td_{i-1}=0$, for this we need $T$ to map the $0$ map to the $0$ map and again for this we need additivity. But maybe it follows from exactness as well (?)
    $endgroup$
    – M. Van
    Oct 11 '17 at 18:35






  • 2




    $begingroup$
    @M.Van Yes this follows from exactness. Indeed, an exact functor preserve the zero object, and thus preserve zero maps (these are the maps which factors through the zero object). In fact, an exact functor also preserve direct sums and therefore it will necessarily be additive.
    $endgroup$
    – Roland
    Oct 11 '17 at 20:25
















0












$begingroup$



Let $R$ and $A$ be rings, $T:{}_{R}text{Mod}rightarrow {}_Atext{Mod}$ be an exact additive functor and $(C_bullet,d_bullet)$ a chain complex in ${}_{R}text{Mod}$. Prove that
$$H_n(TC_bullet,Td_bullet)cong TH_n(C_bullet,d_bullet) $$




I know this looks too easy, but I can't do an actual proof of this fact.



The thing is that is easy to prove that $Tker(d_n)cong ker(T d_n)$ and $T text{im}(d_{n+1})cong text{im}(Td_{n+1})$ but this isomorphims are to abstract so I can't prove that the latter isomorphism is a restriction of the former.



Also, I have the feeling that the additive hypothesis is not needed. Is that correct?










share|cite|improve this question











$endgroup$












  • $begingroup$
    I know there is already a question in the site about this. But the solution given there have some problems. For example the equality $F(partial_{n+1})(F(C_{n+1})))=F(partial_{n+1}(C_{n+1}))$ is not true without exactness an even with exactness it is just an isomorphism. So you need to show that this isomorphism relates well with respect to the other isomorphism of the kernel. Otherwise is not true the part $$frac{ker(F(partial_n))}{Im(F(partial_{n+1}))}stackrel{by (1)(2)}{cong}frac{F(ker(partial_n))}{F(Im(partial_{n+1}))}$$.
    $endgroup$
    – Walter Simon
    Oct 11 '17 at 0:10












  • $begingroup$
    you use additivity in the fact that $T$ maps chain complexes to chain complexes
    $endgroup$
    – M. Van
    Oct 11 '17 at 1:19






  • 1




    $begingroup$
    $T ker(d_n)$ is a kernel of $T d_n$. Your issue boils down to how to understand in what way homology is independent of which specific choices of kernels, images, and quotients you use in its construction.
    $endgroup$
    – Hurkyl
    Oct 11 '17 at 16:18






  • 1




    $begingroup$
    @WalterSimon we need $Td_i circ Td_{i-1}=0$, for this we need $T$ to map the $0$ map to the $0$ map and again for this we need additivity. But maybe it follows from exactness as well (?)
    $endgroup$
    – M. Van
    Oct 11 '17 at 18:35






  • 2




    $begingroup$
    @M.Van Yes this follows from exactness. Indeed, an exact functor preserve the zero object, and thus preserve zero maps (these are the maps which factors through the zero object). In fact, an exact functor also preserve direct sums and therefore it will necessarily be additive.
    $endgroup$
    – Roland
    Oct 11 '17 at 20:25














0












0








0





$begingroup$



Let $R$ and $A$ be rings, $T:{}_{R}text{Mod}rightarrow {}_Atext{Mod}$ be an exact additive functor and $(C_bullet,d_bullet)$ a chain complex in ${}_{R}text{Mod}$. Prove that
$$H_n(TC_bullet,Td_bullet)cong TH_n(C_bullet,d_bullet) $$




I know this looks too easy, but I can't do an actual proof of this fact.



The thing is that is easy to prove that $Tker(d_n)cong ker(T d_n)$ and $T text{im}(d_{n+1})cong text{im}(Td_{n+1})$ but this isomorphims are to abstract so I can't prove that the latter isomorphism is a restriction of the former.



Also, I have the feeling that the additive hypothesis is not needed. Is that correct?










share|cite|improve this question











$endgroup$





Let $R$ and $A$ be rings, $T:{}_{R}text{Mod}rightarrow {}_Atext{Mod}$ be an exact additive functor and $(C_bullet,d_bullet)$ a chain complex in ${}_{R}text{Mod}$. Prove that
$$H_n(TC_bullet,Td_bullet)cong TH_n(C_bullet,d_bullet) $$




I know this looks too easy, but I can't do an actual proof of this fact.



The thing is that is easy to prove that $Tker(d_n)cong ker(T d_n)$ and $T text{im}(d_{n+1})cong text{im}(Td_{n+1})$ but this isomorphims are to abstract so I can't prove that the latter isomorphism is a restriction of the former.



Also, I have the feeling that the additive hypothesis is not needed. Is that correct?







category-theory homology-cohomology homological-algebra abelian-categories






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Oct 11 '17 at 1:14







Walter Simon

















asked Oct 10 '17 at 23:59









Walter SimonWalter Simon

15410




15410












  • $begingroup$
    I know there is already a question in the site about this. But the solution given there have some problems. For example the equality $F(partial_{n+1})(F(C_{n+1})))=F(partial_{n+1}(C_{n+1}))$ is not true without exactness an even with exactness it is just an isomorphism. So you need to show that this isomorphism relates well with respect to the other isomorphism of the kernel. Otherwise is not true the part $$frac{ker(F(partial_n))}{Im(F(partial_{n+1}))}stackrel{by (1)(2)}{cong}frac{F(ker(partial_n))}{F(Im(partial_{n+1}))}$$.
    $endgroup$
    – Walter Simon
    Oct 11 '17 at 0:10












  • $begingroup$
    you use additivity in the fact that $T$ maps chain complexes to chain complexes
    $endgroup$
    – M. Van
    Oct 11 '17 at 1:19






  • 1




    $begingroup$
    $T ker(d_n)$ is a kernel of $T d_n$. Your issue boils down to how to understand in what way homology is independent of which specific choices of kernels, images, and quotients you use in its construction.
    $endgroup$
    – Hurkyl
    Oct 11 '17 at 16:18






  • 1




    $begingroup$
    @WalterSimon we need $Td_i circ Td_{i-1}=0$, for this we need $T$ to map the $0$ map to the $0$ map and again for this we need additivity. But maybe it follows from exactness as well (?)
    $endgroup$
    – M. Van
    Oct 11 '17 at 18:35






  • 2




    $begingroup$
    @M.Van Yes this follows from exactness. Indeed, an exact functor preserve the zero object, and thus preserve zero maps (these are the maps which factors through the zero object). In fact, an exact functor also preserve direct sums and therefore it will necessarily be additive.
    $endgroup$
    – Roland
    Oct 11 '17 at 20:25


















  • $begingroup$
    I know there is already a question in the site about this. But the solution given there have some problems. For example the equality $F(partial_{n+1})(F(C_{n+1})))=F(partial_{n+1}(C_{n+1}))$ is not true without exactness an even with exactness it is just an isomorphism. So you need to show that this isomorphism relates well with respect to the other isomorphism of the kernel. Otherwise is not true the part $$frac{ker(F(partial_n))}{Im(F(partial_{n+1}))}stackrel{by (1)(2)}{cong}frac{F(ker(partial_n))}{F(Im(partial_{n+1}))}$$.
    $endgroup$
    – Walter Simon
    Oct 11 '17 at 0:10












  • $begingroup$
    you use additivity in the fact that $T$ maps chain complexes to chain complexes
    $endgroup$
    – M. Van
    Oct 11 '17 at 1:19






  • 1




    $begingroup$
    $T ker(d_n)$ is a kernel of $T d_n$. Your issue boils down to how to understand in what way homology is independent of which specific choices of kernels, images, and quotients you use in its construction.
    $endgroup$
    – Hurkyl
    Oct 11 '17 at 16:18






  • 1




    $begingroup$
    @WalterSimon we need $Td_i circ Td_{i-1}=0$, for this we need $T$ to map the $0$ map to the $0$ map and again for this we need additivity. But maybe it follows from exactness as well (?)
    $endgroup$
    – M. Van
    Oct 11 '17 at 18:35






  • 2




    $begingroup$
    @M.Van Yes this follows from exactness. Indeed, an exact functor preserve the zero object, and thus preserve zero maps (these are the maps which factors through the zero object). In fact, an exact functor also preserve direct sums and therefore it will necessarily be additive.
    $endgroup$
    – Roland
    Oct 11 '17 at 20:25
















$begingroup$
I know there is already a question in the site about this. But the solution given there have some problems. For example the equality $F(partial_{n+1})(F(C_{n+1})))=F(partial_{n+1}(C_{n+1}))$ is not true without exactness an even with exactness it is just an isomorphism. So you need to show that this isomorphism relates well with respect to the other isomorphism of the kernel. Otherwise is not true the part $$frac{ker(F(partial_n))}{Im(F(partial_{n+1}))}stackrel{by (1)(2)}{cong}frac{F(ker(partial_n))}{F(Im(partial_{n+1}))}$$.
$endgroup$
– Walter Simon
Oct 11 '17 at 0:10






$begingroup$
I know there is already a question in the site about this. But the solution given there have some problems. For example the equality $F(partial_{n+1})(F(C_{n+1})))=F(partial_{n+1}(C_{n+1}))$ is not true without exactness an even with exactness it is just an isomorphism. So you need to show that this isomorphism relates well with respect to the other isomorphism of the kernel. Otherwise is not true the part $$frac{ker(F(partial_n))}{Im(F(partial_{n+1}))}stackrel{by (1)(2)}{cong}frac{F(ker(partial_n))}{F(Im(partial_{n+1}))}$$.
$endgroup$
– Walter Simon
Oct 11 '17 at 0:10














$begingroup$
you use additivity in the fact that $T$ maps chain complexes to chain complexes
$endgroup$
– M. Van
Oct 11 '17 at 1:19




$begingroup$
you use additivity in the fact that $T$ maps chain complexes to chain complexes
$endgroup$
– M. Van
Oct 11 '17 at 1:19




1




1




$begingroup$
$T ker(d_n)$ is a kernel of $T d_n$. Your issue boils down to how to understand in what way homology is independent of which specific choices of kernels, images, and quotients you use in its construction.
$endgroup$
– Hurkyl
Oct 11 '17 at 16:18




$begingroup$
$T ker(d_n)$ is a kernel of $T d_n$. Your issue boils down to how to understand in what way homology is independent of which specific choices of kernels, images, and quotients you use in its construction.
$endgroup$
– Hurkyl
Oct 11 '17 at 16:18




1




1




$begingroup$
@WalterSimon we need $Td_i circ Td_{i-1}=0$, for this we need $T$ to map the $0$ map to the $0$ map and again for this we need additivity. But maybe it follows from exactness as well (?)
$endgroup$
– M. Van
Oct 11 '17 at 18:35




$begingroup$
@WalterSimon we need $Td_i circ Td_{i-1}=0$, for this we need $T$ to map the $0$ map to the $0$ map and again for this we need additivity. But maybe it follows from exactness as well (?)
$endgroup$
– M. Van
Oct 11 '17 at 18:35




2




2




$begingroup$
@M.Van Yes this follows from exactness. Indeed, an exact functor preserve the zero object, and thus preserve zero maps (these are the maps which factors through the zero object). In fact, an exact functor also preserve direct sums and therefore it will necessarily be additive.
$endgroup$
– Roland
Oct 11 '17 at 20:25




$begingroup$
@M.Van Yes this follows from exactness. Indeed, an exact functor preserve the zero object, and thus preserve zero maps (these are the maps which factors through the zero object). In fact, an exact functor also preserve direct sums and therefore it will necessarily be additive.
$endgroup$
– Roland
Oct 11 '17 at 20:25










1 Answer
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$begingroup$

Thanks to the comments I got with a proof that convince me.



As $T$ is left exact is easy to show that $ker (Td_n)cong Tker (d_n)$ as subobjects of $C_n$. Similarly using both right and left exactness one can prove that $text{im}(Td_{n+1})cong T text{im}(d_{n+1})$ as subobjects of $C_n$.



Therefore we have the diagram



diagram



Where $B_n=text{im}(d_{n+1})$, $B_n^T=text{im}(Td_{n+1})$, and so on



Now using the commutative triangles (given by the isomorphism at the level of subobjects) we can prove that $$(TB_nrightarrow TZ_nrightarrow Z_n^Trightarrow C_n)=(TB_nrightarrow B_n^Trightarrow Z_n^Trightarrow C_n)$$



And then as $Z_n^Trightarrow C_n$ is a monomorphism the square is commutative. Then we can finish the proof constructing $TH_ncong H_n^T$ by a diagram chasing or using the universal property of the cokernel.






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    $begingroup$

    Thanks to the comments I got with a proof that convince me.



    As $T$ is left exact is easy to show that $ker (Td_n)cong Tker (d_n)$ as subobjects of $C_n$. Similarly using both right and left exactness one can prove that $text{im}(Td_{n+1})cong T text{im}(d_{n+1})$ as subobjects of $C_n$.



    Therefore we have the diagram



    diagram



    Where $B_n=text{im}(d_{n+1})$, $B_n^T=text{im}(Td_{n+1})$, and so on



    Now using the commutative triangles (given by the isomorphism at the level of subobjects) we can prove that $$(TB_nrightarrow TZ_nrightarrow Z_n^Trightarrow C_n)=(TB_nrightarrow B_n^Trightarrow Z_n^Trightarrow C_n)$$



    And then as $Z_n^Trightarrow C_n$ is a monomorphism the square is commutative. Then we can finish the proof constructing $TH_ncong H_n^T$ by a diagram chasing or using the universal property of the cokernel.






    share|cite|improve this answer











    $endgroup$


















      0












      $begingroup$

      Thanks to the comments I got with a proof that convince me.



      As $T$ is left exact is easy to show that $ker (Td_n)cong Tker (d_n)$ as subobjects of $C_n$. Similarly using both right and left exactness one can prove that $text{im}(Td_{n+1})cong T text{im}(d_{n+1})$ as subobjects of $C_n$.



      Therefore we have the diagram



      diagram



      Where $B_n=text{im}(d_{n+1})$, $B_n^T=text{im}(Td_{n+1})$, and so on



      Now using the commutative triangles (given by the isomorphism at the level of subobjects) we can prove that $$(TB_nrightarrow TZ_nrightarrow Z_n^Trightarrow C_n)=(TB_nrightarrow B_n^Trightarrow Z_n^Trightarrow C_n)$$



      And then as $Z_n^Trightarrow C_n$ is a monomorphism the square is commutative. Then we can finish the proof constructing $TH_ncong H_n^T$ by a diagram chasing or using the universal property of the cokernel.






      share|cite|improve this answer











      $endgroup$
















        0












        0








        0





        $begingroup$

        Thanks to the comments I got with a proof that convince me.



        As $T$ is left exact is easy to show that $ker (Td_n)cong Tker (d_n)$ as subobjects of $C_n$. Similarly using both right and left exactness one can prove that $text{im}(Td_{n+1})cong T text{im}(d_{n+1})$ as subobjects of $C_n$.



        Therefore we have the diagram



        diagram



        Where $B_n=text{im}(d_{n+1})$, $B_n^T=text{im}(Td_{n+1})$, and so on



        Now using the commutative triangles (given by the isomorphism at the level of subobjects) we can prove that $$(TB_nrightarrow TZ_nrightarrow Z_n^Trightarrow C_n)=(TB_nrightarrow B_n^Trightarrow Z_n^Trightarrow C_n)$$



        And then as $Z_n^Trightarrow C_n$ is a monomorphism the square is commutative. Then we can finish the proof constructing $TH_ncong H_n^T$ by a diagram chasing or using the universal property of the cokernel.






        share|cite|improve this answer











        $endgroup$



        Thanks to the comments I got with a proof that convince me.



        As $T$ is left exact is easy to show that $ker (Td_n)cong Tker (d_n)$ as subobjects of $C_n$. Similarly using both right and left exactness one can prove that $text{im}(Td_{n+1})cong T text{im}(d_{n+1})$ as subobjects of $C_n$.



        Therefore we have the diagram



        diagram



        Where $B_n=text{im}(d_{n+1})$, $B_n^T=text{im}(Td_{n+1})$, and so on



        Now using the commutative triangles (given by the isomorphism at the level of subobjects) we can prove that $$(TB_nrightarrow TZ_nrightarrow Z_n^Trightarrow C_n)=(TB_nrightarrow B_n^Trightarrow Z_n^Trightarrow C_n)$$



        And then as $Z_n^Trightarrow C_n$ is a monomorphism the square is commutative. Then we can finish the proof constructing $TH_ncong H_n^T$ by a diagram chasing or using the universal property of the cokernel.







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Dec 16 '18 at 10:58

























        answered Oct 11 '17 at 18:20









        Walter SimonWalter Simon

        15410




        15410






























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