Another version of the Chinese Remainder Theorem

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Let $R$ be a ring, $I,J$ ideals such that $I+J=R$. The Chinese Remainder Theorem claims that if $Icap J=(0)$, then $Rsimeq R/Itimes R/J$.



I remember seeing a version of the theorem for the case of principal ideals $I=(p), J=(q)$ which assumes instead of the comaximality of $I$ and $J$ (and probably of the condition on the intersection) that $I$ and $J$) that $p$ and $q$ are coprime elements in $R$. (I'm not sure that my statement is precise.) This version is easier to use sometimes, e.g. it looks easier for me to prove that $x-1$ and $x^2+x+1$ are coprime than to prove that $(x-1)+(x^2+x+1)=(1)$ in $mathbb R[x]$.



So my questions are: whether my statement is precise, and how does it follow from the general Chinese remainder theorem?







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    Let $R$ be a ring, $I,J$ ideals such that $I+J=R$. The Chinese Remainder Theorem claims that if $Icap J=(0)$, then $Rsimeq R/Itimes R/J$.



    I remember seeing a version of the theorem for the case of principal ideals $I=(p), J=(q)$ which assumes instead of the comaximality of $I$ and $J$ (and probably of the condition on the intersection) that $I$ and $J$) that $p$ and $q$ are coprime elements in $R$. (I'm not sure that my statement is precise.) This version is easier to use sometimes, e.g. it looks easier for me to prove that $x-1$ and $x^2+x+1$ are coprime than to prove that $(x-1)+(x^2+x+1)=(1)$ in $mathbb R[x]$.



    So my questions are: whether my statement is precise, and how does it follow from the general Chinese remainder theorem?







    share|cite|improve this question





















      up vote
      0
      down vote

      favorite









      up vote
      0
      down vote

      favorite











      Let $R$ be a ring, $I,J$ ideals such that $I+J=R$. The Chinese Remainder Theorem claims that if $Icap J=(0)$, then $Rsimeq R/Itimes R/J$.



      I remember seeing a version of the theorem for the case of principal ideals $I=(p), J=(q)$ which assumes instead of the comaximality of $I$ and $J$ (and probably of the condition on the intersection) that $I$ and $J$) that $p$ and $q$ are coprime elements in $R$. (I'm not sure that my statement is precise.) This version is easier to use sometimes, e.g. it looks easier for me to prove that $x-1$ and $x^2+x+1$ are coprime than to prove that $(x-1)+(x^2+x+1)=(1)$ in $mathbb R[x]$.



      So my questions are: whether my statement is precise, and how does it follow from the general Chinese remainder theorem?







      share|cite|improve this question











      Let $R$ be a ring, $I,J$ ideals such that $I+J=R$. The Chinese Remainder Theorem claims that if $Icap J=(0)$, then $Rsimeq R/Itimes R/J$.



      I remember seeing a version of the theorem for the case of principal ideals $I=(p), J=(q)$ which assumes instead of the comaximality of $I$ and $J$ (and probably of the condition on the intersection) that $I$ and $J$) that $p$ and $q$ are coprime elements in $R$. (I'm not sure that my statement is precise.) This version is easier to use sometimes, e.g. it looks easier for me to prove that $x-1$ and $x^2+x+1$ are coprime than to prove that $(x-1)+(x^2+x+1)=(1)$ in $mathbb R[x]$.



      So my questions are: whether my statement is precise, and how does it follow from the general Chinese remainder theorem?









      share|cite|improve this question










      share|cite|improve this question




      share|cite|improve this question









      asked Jul 19 at 22:50









      user437309

      556212




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          2 Answers
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          active

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          1
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          The problem is what do you mean by the statement: "$x$ and $y$ are coprime elements in a ring $R$". Let's stick to integers for now. There are two different ways you can think about coprime integers. Two integers $x,yin mathbbZ$ are said to be coprime if one of the two equivalent conditions are satisfied.



          1. The greatest common divisor of $x,y$ is equal to one.

          2. There exists $a,bin mathbbZ$ such that $ax+by=1$ (Bezout's identity)

          The problem with definition (1) is that there are many rings out there with no such thing as a greatest common divisor. Say $mathbbC[x]/(x^2)$ is not even a domain. Or $mathbbZ[-sqrt3]$ which is a domain but the two elements $x=4=2times 2=(1+sqrt-3)(1-sqrt-3)$ and $y=(1+sqrt-3)times 2$ have no gcd. A commutative ring admitting a gcd for any pair of elements is called a GCD domain.



          Your example of $mathbbR[x]$ is a GCD domain (actually, more strongly, a principal ideal domain, PID). But even if a ring $R$ is a GCD domain, it does not mean $R$ admits a Bezout identity. An example would be $mathbbR[x,y]$ which is a GCD domain (actually a unique factorization domain, UFD) which does not admit a Bezout identity. A ring that admits a Bezout identity is called a Bezout domain. All Bezout domains are GCD domains. Alternatively, a Bezout domain can be defined as follows:




          An integral domain $R$ is called a Bezout domain if given any two elements $x,y$, the sum of ideals $(x)+(y)$ is again principal.




          So you can see that in a Bezout domain, $x,y$ are coprime if and only if the ideals $(x), (y)$ are coprime (Two ideals $I,Jsubset R$ are called coprime if $I+J=R$). However, one can define coprime ideals for all commutative rings, and coprime elements only for some rings. In a sense then, the way coprime ideals are defined is the natural generalization of how we expect "coprime" to be.



          Now the Chinese Reminder theorem says




          If the ideals $I,Jsubset R$ are comaximal, then $R/Icap Jsimeq R/Itimes R/J$.




          If we assume $R$ is a Bezout domain, and $x,yin R$ coprime, then $(x)cap (y)=(xy)$ (which is not zero y the way). And Chinese remainder theorem, in this case, becomes
          $$
          R/(xy)simeq R/(x)times R/(y)
          $$
          So sure, if in your ring of interest it makes sense to talk about coprime elements, you can go with your special version of the theorem. But if there is no such thing, well... I hope, this sheds some light on the matter.






          share|cite|improve this answer























          • I thought the property $I+J=R$ is called comaximality (not coprimeness)?
            – user437309
            Jul 19 at 23:34










          • Both terminologies are used interchangeably.
            – Hamed
            Jul 19 at 23:35

















          up vote
          0
          down vote













          The abstract Chinese remainder theorem that I know is this (stated for two ideals):




          Let $R$ be a (commutative) ring, $I, J$ two coprime ideals in $R$, i.e. satisfying$I+J=R$. Then $IJ=Icap J$, and we have an isomorphism
          beginalign
          R/Icap J&xrightarrow;sim;R/Itimes R/J\
          r+Icap J&longmapsto (r+ I, r+ J)
          endalign







          share|cite|improve this answer























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            2 Answers
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            2 Answers
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            active

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            The problem is what do you mean by the statement: "$x$ and $y$ are coprime elements in a ring $R$". Let's stick to integers for now. There are two different ways you can think about coprime integers. Two integers $x,yin mathbbZ$ are said to be coprime if one of the two equivalent conditions are satisfied.



            1. The greatest common divisor of $x,y$ is equal to one.

            2. There exists $a,bin mathbbZ$ such that $ax+by=1$ (Bezout's identity)

            The problem with definition (1) is that there are many rings out there with no such thing as a greatest common divisor. Say $mathbbC[x]/(x^2)$ is not even a domain. Or $mathbbZ[-sqrt3]$ which is a domain but the two elements $x=4=2times 2=(1+sqrt-3)(1-sqrt-3)$ and $y=(1+sqrt-3)times 2$ have no gcd. A commutative ring admitting a gcd for any pair of elements is called a GCD domain.



            Your example of $mathbbR[x]$ is a GCD domain (actually, more strongly, a principal ideal domain, PID). But even if a ring $R$ is a GCD domain, it does not mean $R$ admits a Bezout identity. An example would be $mathbbR[x,y]$ which is a GCD domain (actually a unique factorization domain, UFD) which does not admit a Bezout identity. A ring that admits a Bezout identity is called a Bezout domain. All Bezout domains are GCD domains. Alternatively, a Bezout domain can be defined as follows:




            An integral domain $R$ is called a Bezout domain if given any two elements $x,y$, the sum of ideals $(x)+(y)$ is again principal.




            So you can see that in a Bezout domain, $x,y$ are coprime if and only if the ideals $(x), (y)$ are coprime (Two ideals $I,Jsubset R$ are called coprime if $I+J=R$). However, one can define coprime ideals for all commutative rings, and coprime elements only for some rings. In a sense then, the way coprime ideals are defined is the natural generalization of how we expect "coprime" to be.



            Now the Chinese Reminder theorem says




            If the ideals $I,Jsubset R$ are comaximal, then $R/Icap Jsimeq R/Itimes R/J$.




            If we assume $R$ is a Bezout domain, and $x,yin R$ coprime, then $(x)cap (y)=(xy)$ (which is not zero y the way). And Chinese remainder theorem, in this case, becomes
            $$
            R/(xy)simeq R/(x)times R/(y)
            $$
            So sure, if in your ring of interest it makes sense to talk about coprime elements, you can go with your special version of the theorem. But if there is no such thing, well... I hope, this sheds some light on the matter.






            share|cite|improve this answer























            • I thought the property $I+J=R$ is called comaximality (not coprimeness)?
              – user437309
              Jul 19 at 23:34










            • Both terminologies are used interchangeably.
              – Hamed
              Jul 19 at 23:35














            up vote
            1
            down vote













            The problem is what do you mean by the statement: "$x$ and $y$ are coprime elements in a ring $R$". Let's stick to integers for now. There are two different ways you can think about coprime integers. Two integers $x,yin mathbbZ$ are said to be coprime if one of the two equivalent conditions are satisfied.



            1. The greatest common divisor of $x,y$ is equal to one.

            2. There exists $a,bin mathbbZ$ such that $ax+by=1$ (Bezout's identity)

            The problem with definition (1) is that there are many rings out there with no such thing as a greatest common divisor. Say $mathbbC[x]/(x^2)$ is not even a domain. Or $mathbbZ[-sqrt3]$ which is a domain but the two elements $x=4=2times 2=(1+sqrt-3)(1-sqrt-3)$ and $y=(1+sqrt-3)times 2$ have no gcd. A commutative ring admitting a gcd for any pair of elements is called a GCD domain.



            Your example of $mathbbR[x]$ is a GCD domain (actually, more strongly, a principal ideal domain, PID). But even if a ring $R$ is a GCD domain, it does not mean $R$ admits a Bezout identity. An example would be $mathbbR[x,y]$ which is a GCD domain (actually a unique factorization domain, UFD) which does not admit a Bezout identity. A ring that admits a Bezout identity is called a Bezout domain. All Bezout domains are GCD domains. Alternatively, a Bezout domain can be defined as follows:




            An integral domain $R$ is called a Bezout domain if given any two elements $x,y$, the sum of ideals $(x)+(y)$ is again principal.




            So you can see that in a Bezout domain, $x,y$ are coprime if and only if the ideals $(x), (y)$ are coprime (Two ideals $I,Jsubset R$ are called coprime if $I+J=R$). However, one can define coprime ideals for all commutative rings, and coprime elements only for some rings. In a sense then, the way coprime ideals are defined is the natural generalization of how we expect "coprime" to be.



            Now the Chinese Reminder theorem says




            If the ideals $I,Jsubset R$ are comaximal, then $R/Icap Jsimeq R/Itimes R/J$.




            If we assume $R$ is a Bezout domain, and $x,yin R$ coprime, then $(x)cap (y)=(xy)$ (which is not zero y the way). And Chinese remainder theorem, in this case, becomes
            $$
            R/(xy)simeq R/(x)times R/(y)
            $$
            So sure, if in your ring of interest it makes sense to talk about coprime elements, you can go with your special version of the theorem. But if there is no such thing, well... I hope, this sheds some light on the matter.






            share|cite|improve this answer























            • I thought the property $I+J=R$ is called comaximality (not coprimeness)?
              – user437309
              Jul 19 at 23:34










            • Both terminologies are used interchangeably.
              – Hamed
              Jul 19 at 23:35












            up vote
            1
            down vote










            up vote
            1
            down vote









            The problem is what do you mean by the statement: "$x$ and $y$ are coprime elements in a ring $R$". Let's stick to integers for now. There are two different ways you can think about coprime integers. Two integers $x,yin mathbbZ$ are said to be coprime if one of the two equivalent conditions are satisfied.



            1. The greatest common divisor of $x,y$ is equal to one.

            2. There exists $a,bin mathbbZ$ such that $ax+by=1$ (Bezout's identity)

            The problem with definition (1) is that there are many rings out there with no such thing as a greatest common divisor. Say $mathbbC[x]/(x^2)$ is not even a domain. Or $mathbbZ[-sqrt3]$ which is a domain but the two elements $x=4=2times 2=(1+sqrt-3)(1-sqrt-3)$ and $y=(1+sqrt-3)times 2$ have no gcd. A commutative ring admitting a gcd for any pair of elements is called a GCD domain.



            Your example of $mathbbR[x]$ is a GCD domain (actually, more strongly, a principal ideal domain, PID). But even if a ring $R$ is a GCD domain, it does not mean $R$ admits a Bezout identity. An example would be $mathbbR[x,y]$ which is a GCD domain (actually a unique factorization domain, UFD) which does not admit a Bezout identity. A ring that admits a Bezout identity is called a Bezout domain. All Bezout domains are GCD domains. Alternatively, a Bezout domain can be defined as follows:




            An integral domain $R$ is called a Bezout domain if given any two elements $x,y$, the sum of ideals $(x)+(y)$ is again principal.




            So you can see that in a Bezout domain, $x,y$ are coprime if and only if the ideals $(x), (y)$ are coprime (Two ideals $I,Jsubset R$ are called coprime if $I+J=R$). However, one can define coprime ideals for all commutative rings, and coprime elements only for some rings. In a sense then, the way coprime ideals are defined is the natural generalization of how we expect "coprime" to be.



            Now the Chinese Reminder theorem says




            If the ideals $I,Jsubset R$ are comaximal, then $R/Icap Jsimeq R/Itimes R/J$.




            If we assume $R$ is a Bezout domain, and $x,yin R$ coprime, then $(x)cap (y)=(xy)$ (which is not zero y the way). And Chinese remainder theorem, in this case, becomes
            $$
            R/(xy)simeq R/(x)times R/(y)
            $$
            So sure, if in your ring of interest it makes sense to talk about coprime elements, you can go with your special version of the theorem. But if there is no such thing, well... I hope, this sheds some light on the matter.






            share|cite|improve this answer















            The problem is what do you mean by the statement: "$x$ and $y$ are coprime elements in a ring $R$". Let's stick to integers for now. There are two different ways you can think about coprime integers. Two integers $x,yin mathbbZ$ are said to be coprime if one of the two equivalent conditions are satisfied.



            1. The greatest common divisor of $x,y$ is equal to one.

            2. There exists $a,bin mathbbZ$ such that $ax+by=1$ (Bezout's identity)

            The problem with definition (1) is that there are many rings out there with no such thing as a greatest common divisor. Say $mathbbC[x]/(x^2)$ is not even a domain. Or $mathbbZ[-sqrt3]$ which is a domain but the two elements $x=4=2times 2=(1+sqrt-3)(1-sqrt-3)$ and $y=(1+sqrt-3)times 2$ have no gcd. A commutative ring admitting a gcd for any pair of elements is called a GCD domain.



            Your example of $mathbbR[x]$ is a GCD domain (actually, more strongly, a principal ideal domain, PID). But even if a ring $R$ is a GCD domain, it does not mean $R$ admits a Bezout identity. An example would be $mathbbR[x,y]$ which is a GCD domain (actually a unique factorization domain, UFD) which does not admit a Bezout identity. A ring that admits a Bezout identity is called a Bezout domain. All Bezout domains are GCD domains. Alternatively, a Bezout domain can be defined as follows:




            An integral domain $R$ is called a Bezout domain if given any two elements $x,y$, the sum of ideals $(x)+(y)$ is again principal.




            So you can see that in a Bezout domain, $x,y$ are coprime if and only if the ideals $(x), (y)$ are coprime (Two ideals $I,Jsubset R$ are called coprime if $I+J=R$). However, one can define coprime ideals for all commutative rings, and coprime elements only for some rings. In a sense then, the way coprime ideals are defined is the natural generalization of how we expect "coprime" to be.



            Now the Chinese Reminder theorem says




            If the ideals $I,Jsubset R$ are comaximal, then $R/Icap Jsimeq R/Itimes R/J$.




            If we assume $R$ is a Bezout domain, and $x,yin R$ coprime, then $(x)cap (y)=(xy)$ (which is not zero y the way). And Chinese remainder theorem, in this case, becomes
            $$
            R/(xy)simeq R/(x)times R/(y)
            $$
            So sure, if in your ring of interest it makes sense to talk about coprime elements, you can go with your special version of the theorem. But if there is no such thing, well... I hope, this sheds some light on the matter.







            share|cite|improve this answer















            share|cite|improve this answer



            share|cite|improve this answer








            edited Jul 19 at 23:35


























            answered Jul 19 at 23:28









            Hamed

            4,391421




            4,391421











            • I thought the property $I+J=R$ is called comaximality (not coprimeness)?
              – user437309
              Jul 19 at 23:34










            • Both terminologies are used interchangeably.
              – Hamed
              Jul 19 at 23:35
















            • I thought the property $I+J=R$ is called comaximality (not coprimeness)?
              – user437309
              Jul 19 at 23:34










            • Both terminologies are used interchangeably.
              – Hamed
              Jul 19 at 23:35















            I thought the property $I+J=R$ is called comaximality (not coprimeness)?
            – user437309
            Jul 19 at 23:34




            I thought the property $I+J=R$ is called comaximality (not coprimeness)?
            – user437309
            Jul 19 at 23:34












            Both terminologies are used interchangeably.
            – Hamed
            Jul 19 at 23:35




            Both terminologies are used interchangeably.
            – Hamed
            Jul 19 at 23:35










            up vote
            0
            down vote













            The abstract Chinese remainder theorem that I know is this (stated for two ideals):




            Let $R$ be a (commutative) ring, $I, J$ two coprime ideals in $R$, i.e. satisfying$I+J=R$. Then $IJ=Icap J$, and we have an isomorphism
            beginalign
            R/Icap J&xrightarrow;sim;R/Itimes R/J\
            r+Icap J&longmapsto (r+ I, r+ J)
            endalign







            share|cite|improve this answer



























              up vote
              0
              down vote













              The abstract Chinese remainder theorem that I know is this (stated for two ideals):




              Let $R$ be a (commutative) ring, $I, J$ two coprime ideals in $R$, i.e. satisfying$I+J=R$. Then $IJ=Icap J$, and we have an isomorphism
              beginalign
              R/Icap J&xrightarrow;sim;R/Itimes R/J\
              r+Icap J&longmapsto (r+ I, r+ J)
              endalign







              share|cite|improve this answer

























                up vote
                0
                down vote










                up vote
                0
                down vote









                The abstract Chinese remainder theorem that I know is this (stated for two ideals):




                Let $R$ be a (commutative) ring, $I, J$ two coprime ideals in $R$, i.e. satisfying$I+J=R$. Then $IJ=Icap J$, and we have an isomorphism
                beginalign
                R/Icap J&xrightarrow;sim;R/Itimes R/J\
                r+Icap J&longmapsto (r+ I, r+ J)
                endalign







                share|cite|improve this answer















                The abstract Chinese remainder theorem that I know is this (stated for two ideals):




                Let $R$ be a (commutative) ring, $I, J$ two coprime ideals in $R$, i.e. satisfying$I+J=R$. Then $IJ=Icap J$, and we have an isomorphism
                beginalign
                R/Icap J&xrightarrow;sim;R/Itimes R/J\
                r+Icap J&longmapsto (r+ I, r+ J)
                endalign








                share|cite|improve this answer















                share|cite|improve this answer



                share|cite|improve this answer








                edited Jul 19 at 23:29









                Kaj Hansen

                26.9k43679




                26.9k43679











                answered Jul 19 at 23:26









                Bernard

                110k635103




                110k635103






















                     

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