Let $R subseteq S$ be two local PIDs with the same field of fractions, then $R=S$.
$begingroup$
Let $R$ and $S$ be two local principal ideal domains with the same field of fractions $K$. I want to show that if $Rsubseteq S$ then $R=S$.
I will denote as $mathfrak{m}_R=(m_R)$ and $mathfrak{m}_S=(m_S)$ the unique maximal ideal of the local rings $R$ and $S$ respectively. Then every non-unit of $R$ is of the form $x=m_R^nu$ for some $ninmathbb{N} $ and some unit $uin R$. Same goes for $S$. I though of using the following Lemma to attack this problem.
$textbf{Lemma}$ $colon$ Let $R$ be a local PID with field of fractions $K$. Let $S$ be any local domain with $Rsubseteq Ssubseteq K$. If $mathfrak{m}_R subseteq mathfrak{m}_S$, then $R=S$
I know that for local $R$ it is true that its maximal ideal consists of all the non-units. Also, if $R$ is a PID then, $xin K$ implies that $xin R$ or $x^{-1} in R$ (or both). Let $xin mathfrak{m}_R$, thus $x^{-1} notin R$. Now $x^{-1}$ cannot be in $mathfrak{m}_S$ because then, since $xin mathfrak{m}_RRightarrow x in R Rightarrow xin S$ we would have that $1=xx^{-1} in mathfrak{m}_S$ which is a contradiction. How can i prove that in fact $x^{-1}$ can't be in $S$?
abstract-algebra ring-theory commutative-algebra localization
$endgroup$
add a comment |
$begingroup$
Let $R$ and $S$ be two local principal ideal domains with the same field of fractions $K$. I want to show that if $Rsubseteq S$ then $R=S$.
I will denote as $mathfrak{m}_R=(m_R)$ and $mathfrak{m}_S=(m_S)$ the unique maximal ideal of the local rings $R$ and $S$ respectively. Then every non-unit of $R$ is of the form $x=m_R^nu$ for some $ninmathbb{N} $ and some unit $uin R$. Same goes for $S$. I though of using the following Lemma to attack this problem.
$textbf{Lemma}$ $colon$ Let $R$ be a local PID with field of fractions $K$. Let $S$ be any local domain with $Rsubseteq Ssubseteq K$. If $mathfrak{m}_R subseteq mathfrak{m}_S$, then $R=S$
I know that for local $R$ it is true that its maximal ideal consists of all the non-units. Also, if $R$ is a PID then, $xin K$ implies that $xin R$ or $x^{-1} in R$ (or both). Let $xin mathfrak{m}_R$, thus $x^{-1} notin R$. Now $x^{-1}$ cannot be in $mathfrak{m}_S$ because then, since $xin mathfrak{m}_RRightarrow x in R Rightarrow xin S$ we would have that $1=xx^{-1} in mathfrak{m}_S$ which is a contradiction. How can i prove that in fact $x^{-1}$ can't be in $S$?
abstract-algebra ring-theory commutative-algebra localization
$endgroup$
add a comment |
$begingroup$
Let $R$ and $S$ be two local principal ideal domains with the same field of fractions $K$. I want to show that if $Rsubseteq S$ then $R=S$.
I will denote as $mathfrak{m}_R=(m_R)$ and $mathfrak{m}_S=(m_S)$ the unique maximal ideal of the local rings $R$ and $S$ respectively. Then every non-unit of $R$ is of the form $x=m_R^nu$ for some $ninmathbb{N} $ and some unit $uin R$. Same goes for $S$. I though of using the following Lemma to attack this problem.
$textbf{Lemma}$ $colon$ Let $R$ be a local PID with field of fractions $K$. Let $S$ be any local domain with $Rsubseteq Ssubseteq K$. If $mathfrak{m}_R subseteq mathfrak{m}_S$, then $R=S$
I know that for local $R$ it is true that its maximal ideal consists of all the non-units. Also, if $R$ is a PID then, $xin K$ implies that $xin R$ or $x^{-1} in R$ (or both). Let $xin mathfrak{m}_R$, thus $x^{-1} notin R$. Now $x^{-1}$ cannot be in $mathfrak{m}_S$ because then, since $xin mathfrak{m}_RRightarrow x in R Rightarrow xin S$ we would have that $1=xx^{-1} in mathfrak{m}_S$ which is a contradiction. How can i prove that in fact $x^{-1}$ can't be in $S$?
abstract-algebra ring-theory commutative-algebra localization
$endgroup$
Let $R$ and $S$ be two local principal ideal domains with the same field of fractions $K$. I want to show that if $Rsubseteq S$ then $R=S$.
I will denote as $mathfrak{m}_R=(m_R)$ and $mathfrak{m}_S=(m_S)$ the unique maximal ideal of the local rings $R$ and $S$ respectively. Then every non-unit of $R$ is of the form $x=m_R^nu$ for some $ninmathbb{N} $ and some unit $uin R$. Same goes for $S$. I though of using the following Lemma to attack this problem.
$textbf{Lemma}$ $colon$ Let $R$ be a local PID with field of fractions $K$. Let $S$ be any local domain with $Rsubseteq Ssubseteq K$. If $mathfrak{m}_R subseteq mathfrak{m}_S$, then $R=S$
I know that for local $R$ it is true that its maximal ideal consists of all the non-units. Also, if $R$ is a PID then, $xin K$ implies that $xin R$ or $x^{-1} in R$ (or both). Let $xin mathfrak{m}_R$, thus $x^{-1} notin R$. Now $x^{-1}$ cannot be in $mathfrak{m}_S$ because then, since $xin mathfrak{m}_RRightarrow x in R Rightarrow xin S$ we would have that $1=xx^{-1} in mathfrak{m}_S$ which is a contradiction. How can i prove that in fact $x^{-1}$ can't be in $S$?
abstract-algebra ring-theory commutative-algebra localization
abstract-algebra ring-theory commutative-algebra localization
edited Dec 8 '18 at 16:00
Badam Baplan
4,531722
4,531722
asked Dec 8 '18 at 12:52
CorneliusCornelius
1957
1957
add a comment |
add a comment |
2 Answers
2
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oldest
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$begingroup$
First show that $S=T^{-1}R$ where $T$ is a multiplicative subset of $R$. This is true whenever $R$ is a PID. Now use the fact that R is local PID to show that $T^{-1}R = R $ or $K$
$endgroup$
$begingroup$
Thanks! Your answer was enlightening. The intuition i thought of is that if an element $xin mathfrak{m}_R$ ie $x=m_R^nu$ is a unit in $S$ then, $m_R$ is invertible in $S$ and hence, every element of $R$ is invertible in $S$.
$endgroup$
– Cornelius
Dec 11 '18 at 10:28
add a comment |
$begingroup$
This is not true: Take $S = K$ the field of fractions of $R$.
This is the only counter-example: if $R subseteq S subseteq K$ and $S$ contains an element of the form $u m_{R}^{n}$ with $u in R^{times}$ and $n < 0$, it follows that $S = K$. Thus either $S = R$ or $S = K$.
$endgroup$
1
$begingroup$
The exercise as i wrote it, is Exercise 2.4 in Lorenzini's book An invitation in Arithmetic Geometry I see why you say it is not true. Can it be true if we assume that $Rsubseteq S subsetneq K$
$endgroup$
– Cornelius
Dec 8 '18 at 13:28
$begingroup$
I can't see why an element of the form $um_R^n$ with $u in R^{times}$ and $n>0$ is in $S$ implies that $S=K$. Can you be more specific ?
$endgroup$
– Cornelius
Dec 8 '18 at 17:11
$begingroup$
Every element of $K$ is of the form $u m_R^n$ with $n in mathbb Z$. If $m_R^{n_0} in S$ with $n_0 < 0$, then for each $n < 0$, $um_R^{n} = um_R^{n-N n_0} cdot (m_R^{n_0})^{N} in R cdot S = S$, by choosing $N$ large enough so that $n-Nn_0 geq 0$.
$endgroup$
– barto
Dec 8 '18 at 17:15
add a comment |
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2 Answers
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active
oldest
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2 Answers
2
active
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$begingroup$
First show that $S=T^{-1}R$ where $T$ is a multiplicative subset of $R$. This is true whenever $R$ is a PID. Now use the fact that R is local PID to show that $T^{-1}R = R $ or $K$
$endgroup$
$begingroup$
Thanks! Your answer was enlightening. The intuition i thought of is that if an element $xin mathfrak{m}_R$ ie $x=m_R^nu$ is a unit in $S$ then, $m_R$ is invertible in $S$ and hence, every element of $R$ is invertible in $S$.
$endgroup$
– Cornelius
Dec 11 '18 at 10:28
add a comment |
$begingroup$
First show that $S=T^{-1}R$ where $T$ is a multiplicative subset of $R$. This is true whenever $R$ is a PID. Now use the fact that R is local PID to show that $T^{-1}R = R $ or $K$
$endgroup$
$begingroup$
Thanks! Your answer was enlightening. The intuition i thought of is that if an element $xin mathfrak{m}_R$ ie $x=m_R^nu$ is a unit in $S$ then, $m_R$ is invertible in $S$ and hence, every element of $R$ is invertible in $S$.
$endgroup$
– Cornelius
Dec 11 '18 at 10:28
add a comment |
$begingroup$
First show that $S=T^{-1}R$ where $T$ is a multiplicative subset of $R$. This is true whenever $R$ is a PID. Now use the fact that R is local PID to show that $T^{-1}R = R $ or $K$
$endgroup$
First show that $S=T^{-1}R$ where $T$ is a multiplicative subset of $R$. This is true whenever $R$ is a PID. Now use the fact that R is local PID to show that $T^{-1}R = R $ or $K$
answered Dec 11 '18 at 10:08
Soumik GhoshSoumik Ghosh
19610
19610
$begingroup$
Thanks! Your answer was enlightening. The intuition i thought of is that if an element $xin mathfrak{m}_R$ ie $x=m_R^nu$ is a unit in $S$ then, $m_R$ is invertible in $S$ and hence, every element of $R$ is invertible in $S$.
$endgroup$
– Cornelius
Dec 11 '18 at 10:28
add a comment |
$begingroup$
Thanks! Your answer was enlightening. The intuition i thought of is that if an element $xin mathfrak{m}_R$ ie $x=m_R^nu$ is a unit in $S$ then, $m_R$ is invertible in $S$ and hence, every element of $R$ is invertible in $S$.
$endgroup$
– Cornelius
Dec 11 '18 at 10:28
$begingroup$
Thanks! Your answer was enlightening. The intuition i thought of is that if an element $xin mathfrak{m}_R$ ie $x=m_R^nu$ is a unit in $S$ then, $m_R$ is invertible in $S$ and hence, every element of $R$ is invertible in $S$.
$endgroup$
– Cornelius
Dec 11 '18 at 10:28
$begingroup$
Thanks! Your answer was enlightening. The intuition i thought of is that if an element $xin mathfrak{m}_R$ ie $x=m_R^nu$ is a unit in $S$ then, $m_R$ is invertible in $S$ and hence, every element of $R$ is invertible in $S$.
$endgroup$
– Cornelius
Dec 11 '18 at 10:28
add a comment |
$begingroup$
This is not true: Take $S = K$ the field of fractions of $R$.
This is the only counter-example: if $R subseteq S subseteq K$ and $S$ contains an element of the form $u m_{R}^{n}$ with $u in R^{times}$ and $n < 0$, it follows that $S = K$. Thus either $S = R$ or $S = K$.
$endgroup$
1
$begingroup$
The exercise as i wrote it, is Exercise 2.4 in Lorenzini's book An invitation in Arithmetic Geometry I see why you say it is not true. Can it be true if we assume that $Rsubseteq S subsetneq K$
$endgroup$
– Cornelius
Dec 8 '18 at 13:28
$begingroup$
I can't see why an element of the form $um_R^n$ with $u in R^{times}$ and $n>0$ is in $S$ implies that $S=K$. Can you be more specific ?
$endgroup$
– Cornelius
Dec 8 '18 at 17:11
$begingroup$
Every element of $K$ is of the form $u m_R^n$ with $n in mathbb Z$. If $m_R^{n_0} in S$ with $n_0 < 0$, then for each $n < 0$, $um_R^{n} = um_R^{n-N n_0} cdot (m_R^{n_0})^{N} in R cdot S = S$, by choosing $N$ large enough so that $n-Nn_0 geq 0$.
$endgroup$
– barto
Dec 8 '18 at 17:15
add a comment |
$begingroup$
This is not true: Take $S = K$ the field of fractions of $R$.
This is the only counter-example: if $R subseteq S subseteq K$ and $S$ contains an element of the form $u m_{R}^{n}$ with $u in R^{times}$ and $n < 0$, it follows that $S = K$. Thus either $S = R$ or $S = K$.
$endgroup$
1
$begingroup$
The exercise as i wrote it, is Exercise 2.4 in Lorenzini's book An invitation in Arithmetic Geometry I see why you say it is not true. Can it be true if we assume that $Rsubseteq S subsetneq K$
$endgroup$
– Cornelius
Dec 8 '18 at 13:28
$begingroup$
I can't see why an element of the form $um_R^n$ with $u in R^{times}$ and $n>0$ is in $S$ implies that $S=K$. Can you be more specific ?
$endgroup$
– Cornelius
Dec 8 '18 at 17:11
$begingroup$
Every element of $K$ is of the form $u m_R^n$ with $n in mathbb Z$. If $m_R^{n_0} in S$ with $n_0 < 0$, then for each $n < 0$, $um_R^{n} = um_R^{n-N n_0} cdot (m_R^{n_0})^{N} in R cdot S = S$, by choosing $N$ large enough so that $n-Nn_0 geq 0$.
$endgroup$
– barto
Dec 8 '18 at 17:15
add a comment |
$begingroup$
This is not true: Take $S = K$ the field of fractions of $R$.
This is the only counter-example: if $R subseteq S subseteq K$ and $S$ contains an element of the form $u m_{R}^{n}$ with $u in R^{times}$ and $n < 0$, it follows that $S = K$. Thus either $S = R$ or $S = K$.
$endgroup$
This is not true: Take $S = K$ the field of fractions of $R$.
This is the only counter-example: if $R subseteq S subseteq K$ and $S$ contains an element of the form $u m_{R}^{n}$ with $u in R^{times}$ and $n < 0$, it follows that $S = K$. Thus either $S = R$ or $S = K$.
edited Dec 8 '18 at 13:38
answered Dec 8 '18 at 13:13
bartobarto
13.7k32682
13.7k32682
1
$begingroup$
The exercise as i wrote it, is Exercise 2.4 in Lorenzini's book An invitation in Arithmetic Geometry I see why you say it is not true. Can it be true if we assume that $Rsubseteq S subsetneq K$
$endgroup$
– Cornelius
Dec 8 '18 at 13:28
$begingroup$
I can't see why an element of the form $um_R^n$ with $u in R^{times}$ and $n>0$ is in $S$ implies that $S=K$. Can you be more specific ?
$endgroup$
– Cornelius
Dec 8 '18 at 17:11
$begingroup$
Every element of $K$ is of the form $u m_R^n$ with $n in mathbb Z$. If $m_R^{n_0} in S$ with $n_0 < 0$, then for each $n < 0$, $um_R^{n} = um_R^{n-N n_0} cdot (m_R^{n_0})^{N} in R cdot S = S$, by choosing $N$ large enough so that $n-Nn_0 geq 0$.
$endgroup$
– barto
Dec 8 '18 at 17:15
add a comment |
1
$begingroup$
The exercise as i wrote it, is Exercise 2.4 in Lorenzini's book An invitation in Arithmetic Geometry I see why you say it is not true. Can it be true if we assume that $Rsubseteq S subsetneq K$
$endgroup$
– Cornelius
Dec 8 '18 at 13:28
$begingroup$
I can't see why an element of the form $um_R^n$ with $u in R^{times}$ and $n>0$ is in $S$ implies that $S=K$. Can you be more specific ?
$endgroup$
– Cornelius
Dec 8 '18 at 17:11
$begingroup$
Every element of $K$ is of the form $u m_R^n$ with $n in mathbb Z$. If $m_R^{n_0} in S$ with $n_0 < 0$, then for each $n < 0$, $um_R^{n} = um_R^{n-N n_0} cdot (m_R^{n_0})^{N} in R cdot S = S$, by choosing $N$ large enough so that $n-Nn_0 geq 0$.
$endgroup$
– barto
Dec 8 '18 at 17:15
1
1
$begingroup$
The exercise as i wrote it, is Exercise 2.4 in Lorenzini's book An invitation in Arithmetic Geometry I see why you say it is not true. Can it be true if we assume that $Rsubseteq S subsetneq K$
$endgroup$
– Cornelius
Dec 8 '18 at 13:28
$begingroup$
The exercise as i wrote it, is Exercise 2.4 in Lorenzini's book An invitation in Arithmetic Geometry I see why you say it is not true. Can it be true if we assume that $Rsubseteq S subsetneq K$
$endgroup$
– Cornelius
Dec 8 '18 at 13:28
$begingroup$
I can't see why an element of the form $um_R^n$ with $u in R^{times}$ and $n>0$ is in $S$ implies that $S=K$. Can you be more specific ?
$endgroup$
– Cornelius
Dec 8 '18 at 17:11
$begingroup$
I can't see why an element of the form $um_R^n$ with $u in R^{times}$ and $n>0$ is in $S$ implies that $S=K$. Can you be more specific ?
$endgroup$
– Cornelius
Dec 8 '18 at 17:11
$begingroup$
Every element of $K$ is of the form $u m_R^n$ with $n in mathbb Z$. If $m_R^{n_0} in S$ with $n_0 < 0$, then for each $n < 0$, $um_R^{n} = um_R^{n-N n_0} cdot (m_R^{n_0})^{N} in R cdot S = S$, by choosing $N$ large enough so that $n-Nn_0 geq 0$.
$endgroup$
– barto
Dec 8 '18 at 17:15
$begingroup$
Every element of $K$ is of the form $u m_R^n$ with $n in mathbb Z$. If $m_R^{n_0} in S$ with $n_0 < 0$, then for each $n < 0$, $um_R^{n} = um_R^{n-N n_0} cdot (m_R^{n_0})^{N} in R cdot S = S$, by choosing $N$ large enough so that $n-Nn_0 geq 0$.
$endgroup$
– barto
Dec 8 '18 at 17:15
add a comment |
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