Determine the maximal ideals of $mathbb R^2$ by noting $mathbb R^2 cong mathbb R[x]/(x^2-1)$












0












$begingroup$


This is a letter of an exericse in Artin Algebra and has been asked and answered here as well as by Brian Bi here and Takumi Murayama here. I have a different approach.



enter image description here



An earlier exercise is to prove $mathbb R^2 cong mathbb R[x]/(x^2-1)$. One way to do that is with the surjective homomorphism $varphi:mathbb R[x] to mathbb R^2$ defined by $varphi(x)=(1,-1)$. This solution is in the answer here and in the answer by Takumi Murayama.



enter image description here




  1. The maximal ideals in $mathbb R[x]/(x^2-1)$ include the principal ideals $([x-1+(x^2-1)])$ and $([x+1+(x^2-1)])$.


  2. The corresponding maximal ideals in $mathbb R^2$ are principal ideals generated by



$$overlinevarphi([x-1+(x^2-1)])=(overlinevarphi circ pi) (x-1) = varphi(x-1)=(0,-2)$$



and



$$overlinevarphi([x+1+(x^2-1)])=(overlinevarphi circ pi) (x+1) = varphi(x+1)=(2,0)$$



where $overline varphi$ is the isomorphism from $R[x]/(x^2-1) to mathbb R^2$.




  1. The principal ideal generated by $(0,pm 2)$ is the same as the one generated by $(0,1)$. Also, the one by $(pm 2,0)$ is the same as the one by $(1,0)$.


  2. We know that $((0,1))=0times mathbb R, ((1,0))=mathbb R times 0$.


  3. After proving that $([x-1+(x^2-1)])$ and $([x+1+(x^2-1)])$ are the only maximal ideals in the quotient ring, we can say $((0,1))=0times mathbb R, ((1,0))=mathbb R times 0$ are the only maximal ideals in $mathbb R^2$.



Is this correct?



To recap, I answered the maximal ideals of $mathbb R[x]/(x^2-1)$ instead of for $mathbb R^2$. Then I converted the ideals from $mathbb R[x]/(x^2-1)$ to $mathbb R^2$.










share|cite|improve this question











$endgroup$

















    0












    $begingroup$


    This is a letter of an exericse in Artin Algebra and has been asked and answered here as well as by Brian Bi here and Takumi Murayama here. I have a different approach.



    enter image description here



    An earlier exercise is to prove $mathbb R^2 cong mathbb R[x]/(x^2-1)$. One way to do that is with the surjective homomorphism $varphi:mathbb R[x] to mathbb R^2$ defined by $varphi(x)=(1,-1)$. This solution is in the answer here and in the answer by Takumi Murayama.



    enter image description here




    1. The maximal ideals in $mathbb R[x]/(x^2-1)$ include the principal ideals $([x-1+(x^2-1)])$ and $([x+1+(x^2-1)])$.


    2. The corresponding maximal ideals in $mathbb R^2$ are principal ideals generated by



    $$overlinevarphi([x-1+(x^2-1)])=(overlinevarphi circ pi) (x-1) = varphi(x-1)=(0,-2)$$



    and



    $$overlinevarphi([x+1+(x^2-1)])=(overlinevarphi circ pi) (x+1) = varphi(x+1)=(2,0)$$



    where $overline varphi$ is the isomorphism from $R[x]/(x^2-1) to mathbb R^2$.




    1. The principal ideal generated by $(0,pm 2)$ is the same as the one generated by $(0,1)$. Also, the one by $(pm 2,0)$ is the same as the one by $(1,0)$.


    2. We know that $((0,1))=0times mathbb R, ((1,0))=mathbb R times 0$.


    3. After proving that $([x-1+(x^2-1)])$ and $([x+1+(x^2-1)])$ are the only maximal ideals in the quotient ring, we can say $((0,1))=0times mathbb R, ((1,0))=mathbb R times 0$ are the only maximal ideals in $mathbb R^2$.



    Is this correct?



    To recap, I answered the maximal ideals of $mathbb R[x]/(x^2-1)$ instead of for $mathbb R^2$. Then I converted the ideals from $mathbb R[x]/(x^2-1)$ to $mathbb R^2$.










    share|cite|improve this question











    $endgroup$















      0












      0








      0





      $begingroup$


      This is a letter of an exericse in Artin Algebra and has been asked and answered here as well as by Brian Bi here and Takumi Murayama here. I have a different approach.



      enter image description here



      An earlier exercise is to prove $mathbb R^2 cong mathbb R[x]/(x^2-1)$. One way to do that is with the surjective homomorphism $varphi:mathbb R[x] to mathbb R^2$ defined by $varphi(x)=(1,-1)$. This solution is in the answer here and in the answer by Takumi Murayama.



      enter image description here




      1. The maximal ideals in $mathbb R[x]/(x^2-1)$ include the principal ideals $([x-1+(x^2-1)])$ and $([x+1+(x^2-1)])$.


      2. The corresponding maximal ideals in $mathbb R^2$ are principal ideals generated by



      $$overlinevarphi([x-1+(x^2-1)])=(overlinevarphi circ pi) (x-1) = varphi(x-1)=(0,-2)$$



      and



      $$overlinevarphi([x+1+(x^2-1)])=(overlinevarphi circ pi) (x+1) = varphi(x+1)=(2,0)$$



      where $overline varphi$ is the isomorphism from $R[x]/(x^2-1) to mathbb R^2$.




      1. The principal ideal generated by $(0,pm 2)$ is the same as the one generated by $(0,1)$. Also, the one by $(pm 2,0)$ is the same as the one by $(1,0)$.


      2. We know that $((0,1))=0times mathbb R, ((1,0))=mathbb R times 0$.


      3. After proving that $([x-1+(x^2-1)])$ and $([x+1+(x^2-1)])$ are the only maximal ideals in the quotient ring, we can say $((0,1))=0times mathbb R, ((1,0))=mathbb R times 0$ are the only maximal ideals in $mathbb R^2$.



      Is this correct?



      To recap, I answered the maximal ideals of $mathbb R[x]/(x^2-1)$ instead of for $mathbb R^2$. Then I converted the ideals from $mathbb R[x]/(x^2-1)$ to $mathbb R^2$.










      share|cite|improve this question











      $endgroup$




      This is a letter of an exericse in Artin Algebra and has been asked and answered here as well as by Brian Bi here and Takumi Murayama here. I have a different approach.



      enter image description here



      An earlier exercise is to prove $mathbb R^2 cong mathbb R[x]/(x^2-1)$. One way to do that is with the surjective homomorphism $varphi:mathbb R[x] to mathbb R^2$ defined by $varphi(x)=(1,-1)$. This solution is in the answer here and in the answer by Takumi Murayama.



      enter image description here




      1. The maximal ideals in $mathbb R[x]/(x^2-1)$ include the principal ideals $([x-1+(x^2-1)])$ and $([x+1+(x^2-1)])$.


      2. The corresponding maximal ideals in $mathbb R^2$ are principal ideals generated by



      $$overlinevarphi([x-1+(x^2-1)])=(overlinevarphi circ pi) (x-1) = varphi(x-1)=(0,-2)$$



      and



      $$overlinevarphi([x+1+(x^2-1)])=(overlinevarphi circ pi) (x+1) = varphi(x+1)=(2,0)$$



      where $overline varphi$ is the isomorphism from $R[x]/(x^2-1) to mathbb R^2$.




      1. The principal ideal generated by $(0,pm 2)$ is the same as the one generated by $(0,1)$. Also, the one by $(pm 2,0)$ is the same as the one by $(1,0)$.


      2. We know that $((0,1))=0times mathbb R, ((1,0))=mathbb R times 0$.


      3. After proving that $([x-1+(x^2-1)])$ and $([x+1+(x^2-1)])$ are the only maximal ideals in the quotient ring, we can say $((0,1))=0times mathbb R, ((1,0))=mathbb R times 0$ are the only maximal ideals in $mathbb R^2$.



      Is this correct?



      To recap, I answered the maximal ideals of $mathbb R[x]/(x^2-1)$ instead of for $mathbb R^2$. Then I converted the ideals from $mathbb R[x]/(x^2-1)$ to $mathbb R^2$.







      abstract-algebra proof-verification ring-theory ideals maximal-and-prime-ideals






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      share|cite|improve this question













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      edited Dec 26 '18 at 21:15









      user26857

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      asked Dec 22 '18 at 9:03







      user198044





























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          0












          $begingroup$

          Yes, your argument is fine (modulo that you still need to show that $(x+1)$ and $(x-1)$ are the only maximal ideals of $Bbb R[x]/(x^2 - 1)$). If you have an isomorphism $f : Rto S,$ then the maximal ideals of $S$ are precisely the images via $f$ of maximal ideals of $R.$



          I only have two comments: you can probably remove some of the steps and some of the notation, but if you really want to spell everything out, that's fine. Secondly, there's a minor typo in step 2: $overline{varphi}circpi(x+1) = varphi(x + 1),$ not $varphi(x-1).$



          Another comment not related to the correctness of the proof: as in your first link, it is also not too difficult to argue directly that if you have a finite product of fields $A = k_1timesdotstimes k_n,$ the maximal ideals are precisely the ideals $$mathfrak{m}_i := {(x_1,dots, x_n)in Amid x_i = 0},$$
          and you could generalize without too much trouble to a direct product of finitely many commutative rings. For infinite products of fields, the answer is stranger, as you might expect.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            The last part sounds like the introduction to algebraic geometry, so I won't try to understand it now. I will understand it soon though. Thank you!
            $endgroup$
            – user198044
            Dec 25 '18 at 20:28






          • 1




            $begingroup$
            @JackBauer it's not, it's a basic fact of rings. Ideals in a (finite) product are products of ideals
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 21:22












          • $begingroup$
            @leibnewtz I assume they were talking about the infinite product result (which falls more under weird results involving set theory and other foundations related stuff I think)
            $endgroup$
            – Stahl
            Dec 25 '18 at 21:28










          • $begingroup$
            @Stahl My impression was that they were mixing up the result you cited with the Nullstellensatz, but who knows
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 21:53










          • $begingroup$
            @leibnewtz Oh, is it this one? If it's in Dummit Foote but not Artin, that explains why people say to do the exercises in Dummit Foote but read Artin.
            $endgroup$
            – user198044
            Dec 29 '18 at 11:59













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          1 Answer
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          1 Answer
          1






          active

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          active

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          0












          $begingroup$

          Yes, your argument is fine (modulo that you still need to show that $(x+1)$ and $(x-1)$ are the only maximal ideals of $Bbb R[x]/(x^2 - 1)$). If you have an isomorphism $f : Rto S,$ then the maximal ideals of $S$ are precisely the images via $f$ of maximal ideals of $R.$



          I only have two comments: you can probably remove some of the steps and some of the notation, but if you really want to spell everything out, that's fine. Secondly, there's a minor typo in step 2: $overline{varphi}circpi(x+1) = varphi(x + 1),$ not $varphi(x-1).$



          Another comment not related to the correctness of the proof: as in your first link, it is also not too difficult to argue directly that if you have a finite product of fields $A = k_1timesdotstimes k_n,$ the maximal ideals are precisely the ideals $$mathfrak{m}_i := {(x_1,dots, x_n)in Amid x_i = 0},$$
          and you could generalize without too much trouble to a direct product of finitely many commutative rings. For infinite products of fields, the answer is stranger, as you might expect.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            The last part sounds like the introduction to algebraic geometry, so I won't try to understand it now. I will understand it soon though. Thank you!
            $endgroup$
            – user198044
            Dec 25 '18 at 20:28






          • 1




            $begingroup$
            @JackBauer it's not, it's a basic fact of rings. Ideals in a (finite) product are products of ideals
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 21:22












          • $begingroup$
            @leibnewtz I assume they were talking about the infinite product result (which falls more under weird results involving set theory and other foundations related stuff I think)
            $endgroup$
            – Stahl
            Dec 25 '18 at 21:28










          • $begingroup$
            @Stahl My impression was that they were mixing up the result you cited with the Nullstellensatz, but who knows
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 21:53










          • $begingroup$
            @leibnewtz Oh, is it this one? If it's in Dummit Foote but not Artin, that explains why people say to do the exercises in Dummit Foote but read Artin.
            $endgroup$
            – user198044
            Dec 29 '18 at 11:59


















          0












          $begingroup$

          Yes, your argument is fine (modulo that you still need to show that $(x+1)$ and $(x-1)$ are the only maximal ideals of $Bbb R[x]/(x^2 - 1)$). If you have an isomorphism $f : Rto S,$ then the maximal ideals of $S$ are precisely the images via $f$ of maximal ideals of $R.$



          I only have two comments: you can probably remove some of the steps and some of the notation, but if you really want to spell everything out, that's fine. Secondly, there's a minor typo in step 2: $overline{varphi}circpi(x+1) = varphi(x + 1),$ not $varphi(x-1).$



          Another comment not related to the correctness of the proof: as in your first link, it is also not too difficult to argue directly that if you have a finite product of fields $A = k_1timesdotstimes k_n,$ the maximal ideals are precisely the ideals $$mathfrak{m}_i := {(x_1,dots, x_n)in Amid x_i = 0},$$
          and you could generalize without too much trouble to a direct product of finitely many commutative rings. For infinite products of fields, the answer is stranger, as you might expect.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            The last part sounds like the introduction to algebraic geometry, so I won't try to understand it now. I will understand it soon though. Thank you!
            $endgroup$
            – user198044
            Dec 25 '18 at 20:28






          • 1




            $begingroup$
            @JackBauer it's not, it's a basic fact of rings. Ideals in a (finite) product are products of ideals
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 21:22












          • $begingroup$
            @leibnewtz I assume they were talking about the infinite product result (which falls more under weird results involving set theory and other foundations related stuff I think)
            $endgroup$
            – Stahl
            Dec 25 '18 at 21:28










          • $begingroup$
            @Stahl My impression was that they were mixing up the result you cited with the Nullstellensatz, but who knows
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 21:53










          • $begingroup$
            @leibnewtz Oh, is it this one? If it's in Dummit Foote but not Artin, that explains why people say to do the exercises in Dummit Foote but read Artin.
            $endgroup$
            – user198044
            Dec 29 '18 at 11:59
















          0












          0








          0





          $begingroup$

          Yes, your argument is fine (modulo that you still need to show that $(x+1)$ and $(x-1)$ are the only maximal ideals of $Bbb R[x]/(x^2 - 1)$). If you have an isomorphism $f : Rto S,$ then the maximal ideals of $S$ are precisely the images via $f$ of maximal ideals of $R.$



          I only have two comments: you can probably remove some of the steps and some of the notation, but if you really want to spell everything out, that's fine. Secondly, there's a minor typo in step 2: $overline{varphi}circpi(x+1) = varphi(x + 1),$ not $varphi(x-1).$



          Another comment not related to the correctness of the proof: as in your first link, it is also not too difficult to argue directly that if you have a finite product of fields $A = k_1timesdotstimes k_n,$ the maximal ideals are precisely the ideals $$mathfrak{m}_i := {(x_1,dots, x_n)in Amid x_i = 0},$$
          and you could generalize without too much trouble to a direct product of finitely many commutative rings. For infinite products of fields, the answer is stranger, as you might expect.






          share|cite|improve this answer









          $endgroup$



          Yes, your argument is fine (modulo that you still need to show that $(x+1)$ and $(x-1)$ are the only maximal ideals of $Bbb R[x]/(x^2 - 1)$). If you have an isomorphism $f : Rto S,$ then the maximal ideals of $S$ are precisely the images via $f$ of maximal ideals of $R.$



          I only have two comments: you can probably remove some of the steps and some of the notation, but if you really want to spell everything out, that's fine. Secondly, there's a minor typo in step 2: $overline{varphi}circpi(x+1) = varphi(x + 1),$ not $varphi(x-1).$



          Another comment not related to the correctness of the proof: as in your first link, it is also not too difficult to argue directly that if you have a finite product of fields $A = k_1timesdotstimes k_n,$ the maximal ideals are precisely the ideals $$mathfrak{m}_i := {(x_1,dots, x_n)in Amid x_i = 0},$$
          and you could generalize without too much trouble to a direct product of finitely many commutative rings. For infinite products of fields, the answer is stranger, as you might expect.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Dec 22 '18 at 17:10









          StahlStahl

          16.7k43455




          16.7k43455












          • $begingroup$
            The last part sounds like the introduction to algebraic geometry, so I won't try to understand it now. I will understand it soon though. Thank you!
            $endgroup$
            – user198044
            Dec 25 '18 at 20:28






          • 1




            $begingroup$
            @JackBauer it's not, it's a basic fact of rings. Ideals in a (finite) product are products of ideals
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 21:22












          • $begingroup$
            @leibnewtz I assume they were talking about the infinite product result (which falls more under weird results involving set theory and other foundations related stuff I think)
            $endgroup$
            – Stahl
            Dec 25 '18 at 21:28










          • $begingroup$
            @Stahl My impression was that they were mixing up the result you cited with the Nullstellensatz, but who knows
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 21:53










          • $begingroup$
            @leibnewtz Oh, is it this one? If it's in Dummit Foote but not Artin, that explains why people say to do the exercises in Dummit Foote but read Artin.
            $endgroup$
            – user198044
            Dec 29 '18 at 11:59




















          • $begingroup$
            The last part sounds like the introduction to algebraic geometry, so I won't try to understand it now. I will understand it soon though. Thank you!
            $endgroup$
            – user198044
            Dec 25 '18 at 20:28






          • 1




            $begingroup$
            @JackBauer it's not, it's a basic fact of rings. Ideals in a (finite) product are products of ideals
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 21:22












          • $begingroup$
            @leibnewtz I assume they were talking about the infinite product result (which falls more under weird results involving set theory and other foundations related stuff I think)
            $endgroup$
            – Stahl
            Dec 25 '18 at 21:28










          • $begingroup$
            @Stahl My impression was that they were mixing up the result you cited with the Nullstellensatz, but who knows
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 21:53










          • $begingroup$
            @leibnewtz Oh, is it this one? If it's in Dummit Foote but not Artin, that explains why people say to do the exercises in Dummit Foote but read Artin.
            $endgroup$
            – user198044
            Dec 29 '18 at 11:59


















          $begingroup$
          The last part sounds like the introduction to algebraic geometry, so I won't try to understand it now. I will understand it soon though. Thank you!
          $endgroup$
          – user198044
          Dec 25 '18 at 20:28




          $begingroup$
          The last part sounds like the introduction to algebraic geometry, so I won't try to understand it now. I will understand it soon though. Thank you!
          $endgroup$
          – user198044
          Dec 25 '18 at 20:28




          1




          1




          $begingroup$
          @JackBauer it's not, it's a basic fact of rings. Ideals in a (finite) product are products of ideals
          $endgroup$
          – leibnewtz
          Dec 25 '18 at 21:22






          $begingroup$
          @JackBauer it's not, it's a basic fact of rings. Ideals in a (finite) product are products of ideals
          $endgroup$
          – leibnewtz
          Dec 25 '18 at 21:22














          $begingroup$
          @leibnewtz I assume they were talking about the infinite product result (which falls more under weird results involving set theory and other foundations related stuff I think)
          $endgroup$
          – Stahl
          Dec 25 '18 at 21:28




          $begingroup$
          @leibnewtz I assume they were talking about the infinite product result (which falls more under weird results involving set theory and other foundations related stuff I think)
          $endgroup$
          – Stahl
          Dec 25 '18 at 21:28












          $begingroup$
          @Stahl My impression was that they were mixing up the result you cited with the Nullstellensatz, but who knows
          $endgroup$
          – leibnewtz
          Dec 25 '18 at 21:53




          $begingroup$
          @Stahl My impression was that they were mixing up the result you cited with the Nullstellensatz, but who knows
          $endgroup$
          – leibnewtz
          Dec 25 '18 at 21:53












          $begingroup$
          @leibnewtz Oh, is it this one? If it's in Dummit Foote but not Artin, that explains why people say to do the exercises in Dummit Foote but read Artin.
          $endgroup$
          – user198044
          Dec 29 '18 at 11:59






          $begingroup$
          @leibnewtz Oh, is it this one? If it's in Dummit Foote but not Artin, that explains why people say to do the exercises in Dummit Foote but read Artin.
          $endgroup$
          – user198044
          Dec 29 '18 at 11:59




















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