When $langle F(x,y)y+u(x)(x-c), G(x,y)y+v(x)(x-c) rangle$ is a maximal ideal of $mathbb{C}[x,y]$?












0












$begingroup$


Let $F=F(x,y),G=G(x,y) in mathbb{C}[x,y]$, $u=u(x),v=v(x) in mathbb{C}[x]$, $c in mathbb{C}$.



Denote:



$A=F(x,y)y+u(x)(x-c)=Fy+u(x-c)$,



$B=G(x,y)y+v(x)(x-c)=Gy+v(x-c)$.




When $I_{A,B}:=langle A,B rangle$ is a maximal ideal of $mathbb{C}[x,y]$?




It is well-known that maximal ideals of $mathbb{C}[x,y]$ are of the form: $langle x-a,y-b rangle$, $a,b in mathbb{C}$, see this question.
Observe that, for example, $langle x-a, y-b+(x-a)^5 rangle$ is maximal, since $langle x-a, y-b+(x-a)^5 rangle = langle x-a,y-b rangle$.



Remarks:
1. Notice that $I_{A,B} subseteq langle x-c, y rangle$.




  1. What if we further assume that $gcd(u,v)=gcd(u(x),v(x))=1$?


Any hints and comments are welcome!










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$endgroup$

















    0












    $begingroup$


    Let $F=F(x,y),G=G(x,y) in mathbb{C}[x,y]$, $u=u(x),v=v(x) in mathbb{C}[x]$, $c in mathbb{C}$.



    Denote:



    $A=F(x,y)y+u(x)(x-c)=Fy+u(x-c)$,



    $B=G(x,y)y+v(x)(x-c)=Gy+v(x-c)$.




    When $I_{A,B}:=langle A,B rangle$ is a maximal ideal of $mathbb{C}[x,y]$?




    It is well-known that maximal ideals of $mathbb{C}[x,y]$ are of the form: $langle x-a,y-b rangle$, $a,b in mathbb{C}$, see this question.
    Observe that, for example, $langle x-a, y-b+(x-a)^5 rangle$ is maximal, since $langle x-a, y-b+(x-a)^5 rangle = langle x-a,y-b rangle$.



    Remarks:
    1. Notice that $I_{A,B} subseteq langle x-c, y rangle$.




    1. What if we further assume that $gcd(u,v)=gcd(u(x),v(x))=1$?


    Any hints and comments are welcome!










    share|cite|improve this question











    $endgroup$















      0












      0








      0


      1



      $begingroup$


      Let $F=F(x,y),G=G(x,y) in mathbb{C}[x,y]$, $u=u(x),v=v(x) in mathbb{C}[x]$, $c in mathbb{C}$.



      Denote:



      $A=F(x,y)y+u(x)(x-c)=Fy+u(x-c)$,



      $B=G(x,y)y+v(x)(x-c)=Gy+v(x-c)$.




      When $I_{A,B}:=langle A,B rangle$ is a maximal ideal of $mathbb{C}[x,y]$?




      It is well-known that maximal ideals of $mathbb{C}[x,y]$ are of the form: $langle x-a,y-b rangle$, $a,b in mathbb{C}$, see this question.
      Observe that, for example, $langle x-a, y-b+(x-a)^5 rangle$ is maximal, since $langle x-a, y-b+(x-a)^5 rangle = langle x-a,y-b rangle$.



      Remarks:
      1. Notice that $I_{A,B} subseteq langle x-c, y rangle$.




      1. What if we further assume that $gcd(u,v)=gcd(u(x),v(x))=1$?


      Any hints and comments are welcome!










      share|cite|improve this question











      $endgroup$




      Let $F=F(x,y),G=G(x,y) in mathbb{C}[x,y]$, $u=u(x),v=v(x) in mathbb{C}[x]$, $c in mathbb{C}$.



      Denote:



      $A=F(x,y)y+u(x)(x-c)=Fy+u(x-c)$,



      $B=G(x,y)y+v(x)(x-c)=Gy+v(x-c)$.




      When $I_{A,B}:=langle A,B rangle$ is a maximal ideal of $mathbb{C}[x,y]$?




      It is well-known that maximal ideals of $mathbb{C}[x,y]$ are of the form: $langle x-a,y-b rangle$, $a,b in mathbb{C}$, see this question.
      Observe that, for example, $langle x-a, y-b+(x-a)^5 rangle$ is maximal, since $langle x-a, y-b+(x-a)^5 rangle = langle x-a,y-b rangle$.



      Remarks:
      1. Notice that $I_{A,B} subseteq langle x-c, y rangle$.




      1. What if we further assume that $gcd(u,v)=gcd(u(x),v(x))=1$?


      Any hints and comments are welcome!







      algebraic-geometry ring-theory commutative-algebra ideals






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      share|cite|improve this question




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      edited Dec 12 '18 at 19:40







      user237522

















      asked Dec 12 '18 at 19:34









      user237522user237522

      2,1691617




      2,1691617






















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          $begingroup$

          This is true if and only if $Fv-Guin Bbb{C}$.



          To see why, let
          $$ M = newcommandbmat{begin{pmatrix}}newcommandemat{end{pmatrix}}bmat F & u\G
          & vemat.$$

          Then $$bmat A \ B emat = M bmat y \ x-c emat.$$



          Then if $(A,B)=(y,x-c)$, we have that $y=e_{11}A + e_{12}B$ and $(x-c)=e_{21}A + e_{22}B$.
          Then the matrix
          $$ E = bmat e_{11} & e_{12} \ e_{21} & e_{22} emat $$ satisfies
          $$ Ebmat A \ B emat = bmat y \ x-c emat.$$
          Thus $$ EMbmat y \ x-c emat = bmat y \ x-c emat.$$



          Expanding this gives
          $$ bmat e_{11} F +e_{12} G & e_{11}u+e_{12}v \ e_{21}F+e_{22}G & e_{21}u+e_{22}vematbmat y \ x-cemat = bmat y \ x-c emat.$$



          Then
          $$(e_{11}F+e_{12}G)y+(e_{11}u+e_{12}v)(x-c) = y,$$
          which tells us immediately that $e_{11}u+e_{12}v=0$, and thus that
          $e_{11}F+e_{12}G=1$.
          Similarly, the fact that $$(e_{21}F+e_{22}G)y + (e_{21}u+e_{22}v)(x-c) = (x-c)$$
          tells us that $e_{21}F+e_{22}G=0$ and $e_{21}u+e_{22}v=1$.



          Hence $F$ and $G$ are relatively prime (and not just relatively prime, they generate the unit ideal), and $u$ and $v$ also generate the unit ideal.



          Moreover, since $e_{11}u+e_{12}v=0$, we have $e_{11}u=-e_{12}v$, so since $u$ and $v$ are relatively prime, $e_{11} = h_1v$ and $e_{12}=-h_1u$. Then since
          $e_{11}F+e_{12}G=1$, we have $h_1vF-h_1uG=1$. Therefore $Fv-Gu=1/h_1in Bbb{C}$.



          Conversely if $Fv-GuinBbb{C}$, then $M$ is invertible, so $A$ and $B$ generate $(y,x-c)$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you very much! Your answer is very nice.
            $endgroup$
            – user237522
            Dec 12 '18 at 21:44










          • $begingroup$
            Please, do you think that it is easy to find a (sufficient and necessary) condition for $I_{A,B}$ to be a radical ideal? where $I$ is a radical ideal if $a^n in I$ implies that $a in I$.
            $endgroup$
            – user237522
            Dec 13 '18 at 14:38













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          $begingroup$

          This is true if and only if $Fv-Guin Bbb{C}$.



          To see why, let
          $$ M = newcommandbmat{begin{pmatrix}}newcommandemat{end{pmatrix}}bmat F & u\G
          & vemat.$$

          Then $$bmat A \ B emat = M bmat y \ x-c emat.$$



          Then if $(A,B)=(y,x-c)$, we have that $y=e_{11}A + e_{12}B$ and $(x-c)=e_{21}A + e_{22}B$.
          Then the matrix
          $$ E = bmat e_{11} & e_{12} \ e_{21} & e_{22} emat $$ satisfies
          $$ Ebmat A \ B emat = bmat y \ x-c emat.$$
          Thus $$ EMbmat y \ x-c emat = bmat y \ x-c emat.$$



          Expanding this gives
          $$ bmat e_{11} F +e_{12} G & e_{11}u+e_{12}v \ e_{21}F+e_{22}G & e_{21}u+e_{22}vematbmat y \ x-cemat = bmat y \ x-c emat.$$



          Then
          $$(e_{11}F+e_{12}G)y+(e_{11}u+e_{12}v)(x-c) = y,$$
          which tells us immediately that $e_{11}u+e_{12}v=0$, and thus that
          $e_{11}F+e_{12}G=1$.
          Similarly, the fact that $$(e_{21}F+e_{22}G)y + (e_{21}u+e_{22}v)(x-c) = (x-c)$$
          tells us that $e_{21}F+e_{22}G=0$ and $e_{21}u+e_{22}v=1$.



          Hence $F$ and $G$ are relatively prime (and not just relatively prime, they generate the unit ideal), and $u$ and $v$ also generate the unit ideal.



          Moreover, since $e_{11}u+e_{12}v=0$, we have $e_{11}u=-e_{12}v$, so since $u$ and $v$ are relatively prime, $e_{11} = h_1v$ and $e_{12}=-h_1u$. Then since
          $e_{11}F+e_{12}G=1$, we have $h_1vF-h_1uG=1$. Therefore $Fv-Gu=1/h_1in Bbb{C}$.



          Conversely if $Fv-GuinBbb{C}$, then $M$ is invertible, so $A$ and $B$ generate $(y,x-c)$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you very much! Your answer is very nice.
            $endgroup$
            – user237522
            Dec 12 '18 at 21:44










          • $begingroup$
            Please, do you think that it is easy to find a (sufficient and necessary) condition for $I_{A,B}$ to be a radical ideal? where $I$ is a radical ideal if $a^n in I$ implies that $a in I$.
            $endgroup$
            – user237522
            Dec 13 '18 at 14:38


















          1












          $begingroup$

          This is true if and only if $Fv-Guin Bbb{C}$.



          To see why, let
          $$ M = newcommandbmat{begin{pmatrix}}newcommandemat{end{pmatrix}}bmat F & u\G
          & vemat.$$

          Then $$bmat A \ B emat = M bmat y \ x-c emat.$$



          Then if $(A,B)=(y,x-c)$, we have that $y=e_{11}A + e_{12}B$ and $(x-c)=e_{21}A + e_{22}B$.
          Then the matrix
          $$ E = bmat e_{11} & e_{12} \ e_{21} & e_{22} emat $$ satisfies
          $$ Ebmat A \ B emat = bmat y \ x-c emat.$$
          Thus $$ EMbmat y \ x-c emat = bmat y \ x-c emat.$$



          Expanding this gives
          $$ bmat e_{11} F +e_{12} G & e_{11}u+e_{12}v \ e_{21}F+e_{22}G & e_{21}u+e_{22}vematbmat y \ x-cemat = bmat y \ x-c emat.$$



          Then
          $$(e_{11}F+e_{12}G)y+(e_{11}u+e_{12}v)(x-c) = y,$$
          which tells us immediately that $e_{11}u+e_{12}v=0$, and thus that
          $e_{11}F+e_{12}G=1$.
          Similarly, the fact that $$(e_{21}F+e_{22}G)y + (e_{21}u+e_{22}v)(x-c) = (x-c)$$
          tells us that $e_{21}F+e_{22}G=0$ and $e_{21}u+e_{22}v=1$.



          Hence $F$ and $G$ are relatively prime (and not just relatively prime, they generate the unit ideal), and $u$ and $v$ also generate the unit ideal.



          Moreover, since $e_{11}u+e_{12}v=0$, we have $e_{11}u=-e_{12}v$, so since $u$ and $v$ are relatively prime, $e_{11} = h_1v$ and $e_{12}=-h_1u$. Then since
          $e_{11}F+e_{12}G=1$, we have $h_1vF-h_1uG=1$. Therefore $Fv-Gu=1/h_1in Bbb{C}$.



          Conversely if $Fv-GuinBbb{C}$, then $M$ is invertible, so $A$ and $B$ generate $(y,x-c)$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you very much! Your answer is very nice.
            $endgroup$
            – user237522
            Dec 12 '18 at 21:44










          • $begingroup$
            Please, do you think that it is easy to find a (sufficient and necessary) condition for $I_{A,B}$ to be a radical ideal? where $I$ is a radical ideal if $a^n in I$ implies that $a in I$.
            $endgroup$
            – user237522
            Dec 13 '18 at 14:38
















          1












          1








          1





          $begingroup$

          This is true if and only if $Fv-Guin Bbb{C}$.



          To see why, let
          $$ M = newcommandbmat{begin{pmatrix}}newcommandemat{end{pmatrix}}bmat F & u\G
          & vemat.$$

          Then $$bmat A \ B emat = M bmat y \ x-c emat.$$



          Then if $(A,B)=(y,x-c)$, we have that $y=e_{11}A + e_{12}B$ and $(x-c)=e_{21}A + e_{22}B$.
          Then the matrix
          $$ E = bmat e_{11} & e_{12} \ e_{21} & e_{22} emat $$ satisfies
          $$ Ebmat A \ B emat = bmat y \ x-c emat.$$
          Thus $$ EMbmat y \ x-c emat = bmat y \ x-c emat.$$



          Expanding this gives
          $$ bmat e_{11} F +e_{12} G & e_{11}u+e_{12}v \ e_{21}F+e_{22}G & e_{21}u+e_{22}vematbmat y \ x-cemat = bmat y \ x-c emat.$$



          Then
          $$(e_{11}F+e_{12}G)y+(e_{11}u+e_{12}v)(x-c) = y,$$
          which tells us immediately that $e_{11}u+e_{12}v=0$, and thus that
          $e_{11}F+e_{12}G=1$.
          Similarly, the fact that $$(e_{21}F+e_{22}G)y + (e_{21}u+e_{22}v)(x-c) = (x-c)$$
          tells us that $e_{21}F+e_{22}G=0$ and $e_{21}u+e_{22}v=1$.



          Hence $F$ and $G$ are relatively prime (and not just relatively prime, they generate the unit ideal), and $u$ and $v$ also generate the unit ideal.



          Moreover, since $e_{11}u+e_{12}v=0$, we have $e_{11}u=-e_{12}v$, so since $u$ and $v$ are relatively prime, $e_{11} = h_1v$ and $e_{12}=-h_1u$. Then since
          $e_{11}F+e_{12}G=1$, we have $h_1vF-h_1uG=1$. Therefore $Fv-Gu=1/h_1in Bbb{C}$.



          Conversely if $Fv-GuinBbb{C}$, then $M$ is invertible, so $A$ and $B$ generate $(y,x-c)$.






          share|cite|improve this answer









          $endgroup$



          This is true if and only if $Fv-Guin Bbb{C}$.



          To see why, let
          $$ M = newcommandbmat{begin{pmatrix}}newcommandemat{end{pmatrix}}bmat F & u\G
          & vemat.$$

          Then $$bmat A \ B emat = M bmat y \ x-c emat.$$



          Then if $(A,B)=(y,x-c)$, we have that $y=e_{11}A + e_{12}B$ and $(x-c)=e_{21}A + e_{22}B$.
          Then the matrix
          $$ E = bmat e_{11} & e_{12} \ e_{21} & e_{22} emat $$ satisfies
          $$ Ebmat A \ B emat = bmat y \ x-c emat.$$
          Thus $$ EMbmat y \ x-c emat = bmat y \ x-c emat.$$



          Expanding this gives
          $$ bmat e_{11} F +e_{12} G & e_{11}u+e_{12}v \ e_{21}F+e_{22}G & e_{21}u+e_{22}vematbmat y \ x-cemat = bmat y \ x-c emat.$$



          Then
          $$(e_{11}F+e_{12}G)y+(e_{11}u+e_{12}v)(x-c) = y,$$
          which tells us immediately that $e_{11}u+e_{12}v=0$, and thus that
          $e_{11}F+e_{12}G=1$.
          Similarly, the fact that $$(e_{21}F+e_{22}G)y + (e_{21}u+e_{22}v)(x-c) = (x-c)$$
          tells us that $e_{21}F+e_{22}G=0$ and $e_{21}u+e_{22}v=1$.



          Hence $F$ and $G$ are relatively prime (and not just relatively prime, they generate the unit ideal), and $u$ and $v$ also generate the unit ideal.



          Moreover, since $e_{11}u+e_{12}v=0$, we have $e_{11}u=-e_{12}v$, so since $u$ and $v$ are relatively prime, $e_{11} = h_1v$ and $e_{12}=-h_1u$. Then since
          $e_{11}F+e_{12}G=1$, we have $h_1vF-h_1uG=1$. Therefore $Fv-Gu=1/h_1in Bbb{C}$.



          Conversely if $Fv-GuinBbb{C}$, then $M$ is invertible, so $A$ and $B$ generate $(y,x-c)$.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Dec 12 '18 at 19:58









          jgonjgon

          14.2k22042




          14.2k22042












          • $begingroup$
            Thank you very much! Your answer is very nice.
            $endgroup$
            – user237522
            Dec 12 '18 at 21:44










          • $begingroup$
            Please, do you think that it is easy to find a (sufficient and necessary) condition for $I_{A,B}$ to be a radical ideal? where $I$ is a radical ideal if $a^n in I$ implies that $a in I$.
            $endgroup$
            – user237522
            Dec 13 '18 at 14:38




















          • $begingroup$
            Thank you very much! Your answer is very nice.
            $endgroup$
            – user237522
            Dec 12 '18 at 21:44










          • $begingroup$
            Please, do you think that it is easy to find a (sufficient and necessary) condition for $I_{A,B}$ to be a radical ideal? where $I$ is a radical ideal if $a^n in I$ implies that $a in I$.
            $endgroup$
            – user237522
            Dec 13 '18 at 14:38


















          $begingroup$
          Thank you very much! Your answer is very nice.
          $endgroup$
          – user237522
          Dec 12 '18 at 21:44




          $begingroup$
          Thank you very much! Your answer is very nice.
          $endgroup$
          – user237522
          Dec 12 '18 at 21:44












          $begingroup$
          Please, do you think that it is easy to find a (sufficient and necessary) condition for $I_{A,B}$ to be a radical ideal? where $I$ is a radical ideal if $a^n in I$ implies that $a in I$.
          $endgroup$
          – user237522
          Dec 13 '18 at 14:38






          $begingroup$
          Please, do you think that it is easy to find a (sufficient and necessary) condition for $I_{A,B}$ to be a radical ideal? where $I$ is a radical ideal if $a^n in I$ implies that $a in I$.
          $endgroup$
          – user237522
          Dec 13 '18 at 14:38




















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