Why $mathbb{C}(f(t),g(t))=mathbb{C}(t)$ implies that $gcd(f(t)-a,g(t)-b)=t-c$, for some $a,b,c in mathbb{C}$?












5












$begingroup$


Assume that $f(t),g(t) in mathbb{C}[t]$ satisfy the following two conditions:



(1) $deg(f) geq 2$ and $deg(g) geq 2$.



(2) $mathbb{C}(f(t),g(t))=mathbb{C}(t)$.




In this question it was mentioned that in that case, there exist
$a,b,c in mathbb{C}$ such that $gcd(f(t)-a,g(t)-b)=t-c$.




Unfortunately, I do not see why this is true.



Perhaps Theorem 2.1 (about resultants) or this question (about subresultants) may somehow help (perhaps no).



Edit: Just to make sure:




Is it true that there exist infinitely many $a in mathbb{C}$ and infinitely many $b in mathbb{C}$ such that
$gcd(f(t)-a,g(t)-b)=t-c$, for (infinitely many) $c in mathbb{C}$?




Choose $c in mathbb{C}$ such that $f'(c) neq 0$ etc. (as in the answer). Clearly, there are infinitely many such $c$'s. Let $a:=f(c)$ and $b:=g(c)$.



Asumme that there exist finitely many $a in mathbb{C}$ or finitely many $b in mathbb{C}$
such that $gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



W.l.o.g., there exist finitely many $a in mathbb{C}$
such that $gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



By the pigeon hole principle, there exist $a_0$ (among those finitely many $a$'s), such that for infinitely many $c$'s, we have $a_0=f(c)$.



This is impossible from the following reason:
Let $h(t):=f(t)-a_0$. Then $h(c)=f(c)-a_0=0$, so $c$ is a root of $h(t)$, and trivially every polynomial can have only finitely many different roots.



So after all, I think that I have proved that there exist infinitely many $a in mathbb{C}$ and infinitely many $b in mathbb{C}$ such that
$gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



Any hints are welcome!










share|cite|improve this question











$endgroup$

















    5












    $begingroup$


    Assume that $f(t),g(t) in mathbb{C}[t]$ satisfy the following two conditions:



    (1) $deg(f) geq 2$ and $deg(g) geq 2$.



    (2) $mathbb{C}(f(t),g(t))=mathbb{C}(t)$.




    In this question it was mentioned that in that case, there exist
    $a,b,c in mathbb{C}$ such that $gcd(f(t)-a,g(t)-b)=t-c$.




    Unfortunately, I do not see why this is true.



    Perhaps Theorem 2.1 (about resultants) or this question (about subresultants) may somehow help (perhaps no).



    Edit: Just to make sure:




    Is it true that there exist infinitely many $a in mathbb{C}$ and infinitely many $b in mathbb{C}$ such that
    $gcd(f(t)-a,g(t)-b)=t-c$, for (infinitely many) $c in mathbb{C}$?




    Choose $c in mathbb{C}$ such that $f'(c) neq 0$ etc. (as in the answer). Clearly, there are infinitely many such $c$'s. Let $a:=f(c)$ and $b:=g(c)$.



    Asumme that there exist finitely many $a in mathbb{C}$ or finitely many $b in mathbb{C}$
    such that $gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



    W.l.o.g., there exist finitely many $a in mathbb{C}$
    such that $gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



    By the pigeon hole principle, there exist $a_0$ (among those finitely many $a$'s), such that for infinitely many $c$'s, we have $a_0=f(c)$.



    This is impossible from the following reason:
    Let $h(t):=f(t)-a_0$. Then $h(c)=f(c)-a_0=0$, so $c$ is a root of $h(t)$, and trivially every polynomial can have only finitely many different roots.



    So after all, I think that I have proved that there exist infinitely many $a in mathbb{C}$ and infinitely many $b in mathbb{C}$ such that
    $gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



    Any hints are welcome!










    share|cite|improve this question











    $endgroup$















      5












      5








      5


      2



      $begingroup$


      Assume that $f(t),g(t) in mathbb{C}[t]$ satisfy the following two conditions:



      (1) $deg(f) geq 2$ and $deg(g) geq 2$.



      (2) $mathbb{C}(f(t),g(t))=mathbb{C}(t)$.




      In this question it was mentioned that in that case, there exist
      $a,b,c in mathbb{C}$ such that $gcd(f(t)-a,g(t)-b)=t-c$.




      Unfortunately, I do not see why this is true.



      Perhaps Theorem 2.1 (about resultants) or this question (about subresultants) may somehow help (perhaps no).



      Edit: Just to make sure:




      Is it true that there exist infinitely many $a in mathbb{C}$ and infinitely many $b in mathbb{C}$ such that
      $gcd(f(t)-a,g(t)-b)=t-c$, for (infinitely many) $c in mathbb{C}$?




      Choose $c in mathbb{C}$ such that $f'(c) neq 0$ etc. (as in the answer). Clearly, there are infinitely many such $c$'s. Let $a:=f(c)$ and $b:=g(c)$.



      Asumme that there exist finitely many $a in mathbb{C}$ or finitely many $b in mathbb{C}$
      such that $gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



      W.l.o.g., there exist finitely many $a in mathbb{C}$
      such that $gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



      By the pigeon hole principle, there exist $a_0$ (among those finitely many $a$'s), such that for infinitely many $c$'s, we have $a_0=f(c)$.



      This is impossible from the following reason:
      Let $h(t):=f(t)-a_0$. Then $h(c)=f(c)-a_0=0$, so $c$ is a root of $h(t)$, and trivially every polynomial can have only finitely many different roots.



      So after all, I think that I have proved that there exist infinitely many $a in mathbb{C}$ and infinitely many $b in mathbb{C}$ such that
      $gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



      Any hints are welcome!










      share|cite|improve this question











      $endgroup$




      Assume that $f(t),g(t) in mathbb{C}[t]$ satisfy the following two conditions:



      (1) $deg(f) geq 2$ and $deg(g) geq 2$.



      (2) $mathbb{C}(f(t),g(t))=mathbb{C}(t)$.




      In this question it was mentioned that in that case, there exist
      $a,b,c in mathbb{C}$ such that $gcd(f(t)-a,g(t)-b)=t-c$.




      Unfortunately, I do not see why this is true.



      Perhaps Theorem 2.1 (about resultants) or this question (about subresultants) may somehow help (perhaps no).



      Edit: Just to make sure:




      Is it true that there exist infinitely many $a in mathbb{C}$ and infinitely many $b in mathbb{C}$ such that
      $gcd(f(t)-a,g(t)-b)=t-c$, for (infinitely many) $c in mathbb{C}$?




      Choose $c in mathbb{C}$ such that $f'(c) neq 0$ etc. (as in the answer). Clearly, there are infinitely many such $c$'s. Let $a:=f(c)$ and $b:=g(c)$.



      Asumme that there exist finitely many $a in mathbb{C}$ or finitely many $b in mathbb{C}$
      such that $gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



      W.l.o.g., there exist finitely many $a in mathbb{C}$
      such that $gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



      By the pigeon hole principle, there exist $a_0$ (among those finitely many $a$'s), such that for infinitely many $c$'s, we have $a_0=f(c)$.



      This is impossible from the following reason:
      Let $h(t):=f(t)-a_0$. Then $h(c)=f(c)-a_0=0$, so $c$ is a root of $h(t)$, and trivially every polynomial can have only finitely many different roots.



      So after all, I think that I have proved that there exist infinitely many $a in mathbb{C}$ and infinitely many $b in mathbb{C}$ such that
      $gcd(f(t)-a,g(t)-b)=t-c$, $c in mathbb{C}$.



      Any hints are welcome!







      polynomials commutative-algebra field-theory greatest-common-divisor






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Dec 24 '18 at 10:26







      user237522

















      asked Dec 15 '18 at 22:08









      user237522user237522

      2,1631617




      2,1631617






















          1 Answer
          1






          active

          oldest

          votes


















          4












          $begingroup$

          We may assume that $f$ and $g$ are monic.
          There exists some nonzero two-variable polynomials $P,Q$ such that $P(f(t),g(t))=tQ(f(t),g(t))$, and $Q(f,g)(t)=0$ only finitely many times (else the composition $P/Q (f,g)$ is not defined because $Q(f,g)=0$).



          Let $c$ be such that $f’(c) neq 0$, and there exists no $d$ such that $Q(f(d),g(d))=0$ and $f(d)=f(c)$.



          Then $f(t)-f(c)$ and $g(t)-g(c)$ have only $c$ as a common root, because any root $d$ satisfies $(f,g)(d)=(f,g)(c)$, thus $d=(P/Q)(f(d),g(d))=(P/Q)(f(c),g(c))=c$. Moreover, $c$ is a simple root of $f-f(c)$. So the gcd of the polynomials is $t-c$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Thank you very much! I understand that there exist two-variable polynomials $P$ and $Q$ such that $frac{P(f(t),g(t))}{Q(f(t),g(t))}=t$. Please, why $Q in mathbb{C}^{times}$?
            $endgroup$
            – user237522
            Dec 15 '18 at 22:51












          • $begingroup$
            I mistook the parentheses for brackets. The main point should hold though: $(f,g)$ is injective because of that relation.
            $endgroup$
            – Mindlack
            Dec 15 '18 at 22:54










          • $begingroup$
            Thanks; please, could you slightly elaborate your answer?
            $endgroup$
            – user237522
            Dec 15 '18 at 22:55












          • $begingroup$
            I edited. Is there enough detail?
            $endgroup$
            – Mindlack
            Dec 15 '18 at 22:59










          • $begingroup$
            Thank you. I think I now understand your answer, very nice!
            $endgroup$
            – user237522
            Dec 15 '18 at 23:07











          Your Answer





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          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          4












          $begingroup$

          We may assume that $f$ and $g$ are monic.
          There exists some nonzero two-variable polynomials $P,Q$ such that $P(f(t),g(t))=tQ(f(t),g(t))$, and $Q(f,g)(t)=0$ only finitely many times (else the composition $P/Q (f,g)$ is not defined because $Q(f,g)=0$).



          Let $c$ be such that $f’(c) neq 0$, and there exists no $d$ such that $Q(f(d),g(d))=0$ and $f(d)=f(c)$.



          Then $f(t)-f(c)$ and $g(t)-g(c)$ have only $c$ as a common root, because any root $d$ satisfies $(f,g)(d)=(f,g)(c)$, thus $d=(P/Q)(f(d),g(d))=(P/Q)(f(c),g(c))=c$. Moreover, $c$ is a simple root of $f-f(c)$. So the gcd of the polynomials is $t-c$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Thank you very much! I understand that there exist two-variable polynomials $P$ and $Q$ such that $frac{P(f(t),g(t))}{Q(f(t),g(t))}=t$. Please, why $Q in mathbb{C}^{times}$?
            $endgroup$
            – user237522
            Dec 15 '18 at 22:51












          • $begingroup$
            I mistook the parentheses for brackets. The main point should hold though: $(f,g)$ is injective because of that relation.
            $endgroup$
            – Mindlack
            Dec 15 '18 at 22:54










          • $begingroup$
            Thanks; please, could you slightly elaborate your answer?
            $endgroup$
            – user237522
            Dec 15 '18 at 22:55












          • $begingroup$
            I edited. Is there enough detail?
            $endgroup$
            – Mindlack
            Dec 15 '18 at 22:59










          • $begingroup$
            Thank you. I think I now understand your answer, very nice!
            $endgroup$
            – user237522
            Dec 15 '18 at 23:07
















          4












          $begingroup$

          We may assume that $f$ and $g$ are monic.
          There exists some nonzero two-variable polynomials $P,Q$ such that $P(f(t),g(t))=tQ(f(t),g(t))$, and $Q(f,g)(t)=0$ only finitely many times (else the composition $P/Q (f,g)$ is not defined because $Q(f,g)=0$).



          Let $c$ be such that $f’(c) neq 0$, and there exists no $d$ such that $Q(f(d),g(d))=0$ and $f(d)=f(c)$.



          Then $f(t)-f(c)$ and $g(t)-g(c)$ have only $c$ as a common root, because any root $d$ satisfies $(f,g)(d)=(f,g)(c)$, thus $d=(P/Q)(f(d),g(d))=(P/Q)(f(c),g(c))=c$. Moreover, $c$ is a simple root of $f-f(c)$. So the gcd of the polynomials is $t-c$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Thank you very much! I understand that there exist two-variable polynomials $P$ and $Q$ such that $frac{P(f(t),g(t))}{Q(f(t),g(t))}=t$. Please, why $Q in mathbb{C}^{times}$?
            $endgroup$
            – user237522
            Dec 15 '18 at 22:51












          • $begingroup$
            I mistook the parentheses for brackets. The main point should hold though: $(f,g)$ is injective because of that relation.
            $endgroup$
            – Mindlack
            Dec 15 '18 at 22:54










          • $begingroup$
            Thanks; please, could you slightly elaborate your answer?
            $endgroup$
            – user237522
            Dec 15 '18 at 22:55












          • $begingroup$
            I edited. Is there enough detail?
            $endgroup$
            – Mindlack
            Dec 15 '18 at 22:59










          • $begingroup$
            Thank you. I think I now understand your answer, very nice!
            $endgroup$
            – user237522
            Dec 15 '18 at 23:07














          4












          4








          4





          $begingroup$

          We may assume that $f$ and $g$ are monic.
          There exists some nonzero two-variable polynomials $P,Q$ such that $P(f(t),g(t))=tQ(f(t),g(t))$, and $Q(f,g)(t)=0$ only finitely many times (else the composition $P/Q (f,g)$ is not defined because $Q(f,g)=0$).



          Let $c$ be such that $f’(c) neq 0$, and there exists no $d$ such that $Q(f(d),g(d))=0$ and $f(d)=f(c)$.



          Then $f(t)-f(c)$ and $g(t)-g(c)$ have only $c$ as a common root, because any root $d$ satisfies $(f,g)(d)=(f,g)(c)$, thus $d=(P/Q)(f(d),g(d))=(P/Q)(f(c),g(c))=c$. Moreover, $c$ is a simple root of $f-f(c)$. So the gcd of the polynomials is $t-c$.






          share|cite|improve this answer











          $endgroup$



          We may assume that $f$ and $g$ are monic.
          There exists some nonzero two-variable polynomials $P,Q$ such that $P(f(t),g(t))=tQ(f(t),g(t))$, and $Q(f,g)(t)=0$ only finitely many times (else the composition $P/Q (f,g)$ is not defined because $Q(f,g)=0$).



          Let $c$ be such that $f’(c) neq 0$, and there exists no $d$ such that $Q(f(d),g(d))=0$ and $f(d)=f(c)$.



          Then $f(t)-f(c)$ and $g(t)-g(c)$ have only $c$ as a common root, because any root $d$ satisfies $(f,g)(d)=(f,g)(c)$, thus $d=(P/Q)(f(d),g(d))=(P/Q)(f(c),g(c))=c$. Moreover, $c$ is a simple root of $f-f(c)$. So the gcd of the polynomials is $t-c$.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Dec 15 '18 at 22:59

























          answered Dec 15 '18 at 22:47









          MindlackMindlack

          4,885211




          4,885211












          • $begingroup$
            Thank you very much! I understand that there exist two-variable polynomials $P$ and $Q$ such that $frac{P(f(t),g(t))}{Q(f(t),g(t))}=t$. Please, why $Q in mathbb{C}^{times}$?
            $endgroup$
            – user237522
            Dec 15 '18 at 22:51












          • $begingroup$
            I mistook the parentheses for brackets. The main point should hold though: $(f,g)$ is injective because of that relation.
            $endgroup$
            – Mindlack
            Dec 15 '18 at 22:54










          • $begingroup$
            Thanks; please, could you slightly elaborate your answer?
            $endgroup$
            – user237522
            Dec 15 '18 at 22:55












          • $begingroup$
            I edited. Is there enough detail?
            $endgroup$
            – Mindlack
            Dec 15 '18 at 22:59










          • $begingroup$
            Thank you. I think I now understand your answer, very nice!
            $endgroup$
            – user237522
            Dec 15 '18 at 23:07


















          • $begingroup$
            Thank you very much! I understand that there exist two-variable polynomials $P$ and $Q$ such that $frac{P(f(t),g(t))}{Q(f(t),g(t))}=t$. Please, why $Q in mathbb{C}^{times}$?
            $endgroup$
            – user237522
            Dec 15 '18 at 22:51












          • $begingroup$
            I mistook the parentheses for brackets. The main point should hold though: $(f,g)$ is injective because of that relation.
            $endgroup$
            – Mindlack
            Dec 15 '18 at 22:54










          • $begingroup$
            Thanks; please, could you slightly elaborate your answer?
            $endgroup$
            – user237522
            Dec 15 '18 at 22:55












          • $begingroup$
            I edited. Is there enough detail?
            $endgroup$
            – Mindlack
            Dec 15 '18 at 22:59










          • $begingroup$
            Thank you. I think I now understand your answer, very nice!
            $endgroup$
            – user237522
            Dec 15 '18 at 23:07
















          $begingroup$
          Thank you very much! I understand that there exist two-variable polynomials $P$ and $Q$ such that $frac{P(f(t),g(t))}{Q(f(t),g(t))}=t$. Please, why $Q in mathbb{C}^{times}$?
          $endgroup$
          – user237522
          Dec 15 '18 at 22:51






          $begingroup$
          Thank you very much! I understand that there exist two-variable polynomials $P$ and $Q$ such that $frac{P(f(t),g(t))}{Q(f(t),g(t))}=t$. Please, why $Q in mathbb{C}^{times}$?
          $endgroup$
          – user237522
          Dec 15 '18 at 22:51














          $begingroup$
          I mistook the parentheses for brackets. The main point should hold though: $(f,g)$ is injective because of that relation.
          $endgroup$
          – Mindlack
          Dec 15 '18 at 22:54




          $begingroup$
          I mistook the parentheses for brackets. The main point should hold though: $(f,g)$ is injective because of that relation.
          $endgroup$
          – Mindlack
          Dec 15 '18 at 22:54












          $begingroup$
          Thanks; please, could you slightly elaborate your answer?
          $endgroup$
          – user237522
          Dec 15 '18 at 22:55






          $begingroup$
          Thanks; please, could you slightly elaborate your answer?
          $endgroup$
          – user237522
          Dec 15 '18 at 22:55














          $begingroup$
          I edited. Is there enough detail?
          $endgroup$
          – Mindlack
          Dec 15 '18 at 22:59




          $begingroup$
          I edited. Is there enough detail?
          $endgroup$
          – Mindlack
          Dec 15 '18 at 22:59












          $begingroup$
          Thank you. I think I now understand your answer, very nice!
          $endgroup$
          – user237522
          Dec 15 '18 at 23:07




          $begingroup$
          Thank you. I think I now understand your answer, very nice!
          $endgroup$
          – user237522
          Dec 15 '18 at 23:07


















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