Why $mathbb{C}(f(t),g(t))=mathbb{C}(t)$ implies that $gcd(f(t)-a,g(t)-b)=t-c$, for some $a,b,c in mathbb{C}$?
$begingroup$
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!
polynomials commutative-algebra field-theory greatest-common-divisor
$endgroup$
add a comment |
$begingroup$
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!
polynomials commutative-algebra field-theory greatest-common-divisor
$endgroup$
add a comment |
$begingroup$
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!
polynomials commutative-algebra field-theory greatest-common-divisor
$endgroup$
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
polynomials commutative-algebra field-theory greatest-common-divisor
edited Dec 24 '18 at 10:26
user237522
asked Dec 15 '18 at 22:08
user237522user237522
2,1631617
2,1631617
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
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 $t-c$.
$endgroup$
$begingroup$
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
|
show 1 more comment
Your Answer
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1 Answer
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1 Answer
1
active
oldest
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active
oldest
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active
oldest
votes
$begingroup$
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 $t-c$.
$endgroup$
$begingroup$
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
|
show 1 more comment
$begingroup$
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 $t-c$.
$endgroup$
$begingroup$
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
|
show 1 more comment
$begingroup$
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 $t-c$.
$endgroup$
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 $t-c$.
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
|
show 1 more comment
$begingroup$
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
|
show 1 more comment
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