Integral Closure, Galois extension,and Dedekind Domain
Let
$A$: Dedekind domain, $K$: $operatorname{Frac}(A)$, $B$: Dedekind domain with $A subset B$, $L$: $operatorname{Frac}(B)$
Let $L/K$: galois extension with galois group: $G$.
$B^G={b in B mid sigma(b)=b text{ for all } sigma in G}=A$
$implies B$ is the integral closure of $A$ in $L$
Is this true?
I already prove the converse, but not sure if this holds.
Thank you in advance.
abstract-algebra number-theory ring-theory field-theory galois-theory
|
show 3 more comments
Let
$A$: Dedekind domain, $K$: $operatorname{Frac}(A)$, $B$: Dedekind domain with $A subset B$, $L$: $operatorname{Frac}(B)$
Let $L/K$: galois extension with galois group: $G$.
$B^G={b in B mid sigma(b)=b text{ for all } sigma in G}=A$
$implies B$ is the integral closure of $A$ in $L$
Is this true?
I already prove the converse, but not sure if this holds.
Thank you in advance.
abstract-algebra number-theory ring-theory field-theory galois-theory
I think it's not true. Try to take B=A. Then the fixed ring is A, but unless the extension is trivial, this is not the integral closure.
– Madarb
Dec 9 at 7:38
when A=B, then L=K. Since A is dedekind, A is integrally closed and B(=A) is the integral closure of A in L holds, i think.
– Kento
Dec 9 at 7:56
I don't think I understood what you say. The claim you want to prove, as I understand it, is that if B is a subring of L, for which the fixed ring under the action of G is A, then B is the integral closure. It is not the true if you take B=A. If I got you wrong, please explain what you wanted to prove :)
– Madarb
Dec 9 at 8:00
1
you understand it right. but i do not get that it does not hold when B=A. when B=A,the extension L/K is trivial, which leads to the conclusion. i think. please tell me if i get something wrong.
– Kento
Dec 9 at 8:07
1
@reuns I think you don't necessarily have $sigma(B) = B$. A possible counterexample is given below.
– pisco
Dec 9 at 12:53
|
show 3 more comments
Let
$A$: Dedekind domain, $K$: $operatorname{Frac}(A)$, $B$: Dedekind domain with $A subset B$, $L$: $operatorname{Frac}(B)$
Let $L/K$: galois extension with galois group: $G$.
$B^G={b in B mid sigma(b)=b text{ for all } sigma in G}=A$
$implies B$ is the integral closure of $A$ in $L$
Is this true?
I already prove the converse, but not sure if this holds.
Thank you in advance.
abstract-algebra number-theory ring-theory field-theory galois-theory
Let
$A$: Dedekind domain, $K$: $operatorname{Frac}(A)$, $B$: Dedekind domain with $A subset B$, $L$: $operatorname{Frac}(B)$
Let $L/K$: galois extension with galois group: $G$.
$B^G={b in B mid sigma(b)=b text{ for all } sigma in G}=A$
$implies B$ is the integral closure of $A$ in $L$
Is this true?
I already prove the converse, but not sure if this holds.
Thank you in advance.
abstract-algebra number-theory ring-theory field-theory galois-theory
abstract-algebra number-theory ring-theory field-theory galois-theory
edited Dec 9 at 8:45
Alex Vong
1,294819
1,294819
asked Dec 9 at 7:24
Kento
473
473
I think it's not true. Try to take B=A. Then the fixed ring is A, but unless the extension is trivial, this is not the integral closure.
– Madarb
Dec 9 at 7:38
when A=B, then L=K. Since A is dedekind, A is integrally closed and B(=A) is the integral closure of A in L holds, i think.
– Kento
Dec 9 at 7:56
I don't think I understood what you say. The claim you want to prove, as I understand it, is that if B is a subring of L, for which the fixed ring under the action of G is A, then B is the integral closure. It is not the true if you take B=A. If I got you wrong, please explain what you wanted to prove :)
– Madarb
Dec 9 at 8:00
1
you understand it right. but i do not get that it does not hold when B=A. when B=A,the extension L/K is trivial, which leads to the conclusion. i think. please tell me if i get something wrong.
– Kento
Dec 9 at 8:07
1
@reuns I think you don't necessarily have $sigma(B) = B$. A possible counterexample is given below.
– pisco
Dec 9 at 12:53
|
show 3 more comments
I think it's not true. Try to take B=A. Then the fixed ring is A, but unless the extension is trivial, this is not the integral closure.
– Madarb
Dec 9 at 7:38
when A=B, then L=K. Since A is dedekind, A is integrally closed and B(=A) is the integral closure of A in L holds, i think.
– Kento
Dec 9 at 7:56
I don't think I understood what you say. The claim you want to prove, as I understand it, is that if B is a subring of L, for which the fixed ring under the action of G is A, then B is the integral closure. It is not the true if you take B=A. If I got you wrong, please explain what you wanted to prove :)
– Madarb
Dec 9 at 8:00
1
you understand it right. but i do not get that it does not hold when B=A. when B=A,the extension L/K is trivial, which leads to the conclusion. i think. please tell me if i get something wrong.
– Kento
Dec 9 at 8:07
1
@reuns I think you don't necessarily have $sigma(B) = B$. A possible counterexample is given below.
– pisco
Dec 9 at 12:53
I think it's not true. Try to take B=A. Then the fixed ring is A, but unless the extension is trivial, this is not the integral closure.
– Madarb
Dec 9 at 7:38
I think it's not true. Try to take B=A. Then the fixed ring is A, but unless the extension is trivial, this is not the integral closure.
– Madarb
Dec 9 at 7:38
when A=B, then L=K. Since A is dedekind, A is integrally closed and B(=A) is the integral closure of A in L holds, i think.
– Kento
Dec 9 at 7:56
when A=B, then L=K. Since A is dedekind, A is integrally closed and B(=A) is the integral closure of A in L holds, i think.
– Kento
Dec 9 at 7:56
I don't think I understood what you say. The claim you want to prove, as I understand it, is that if B is a subring of L, for which the fixed ring under the action of G is A, then B is the integral closure. It is not the true if you take B=A. If I got you wrong, please explain what you wanted to prove :)
– Madarb
Dec 9 at 8:00
I don't think I understood what you say. The claim you want to prove, as I understand it, is that if B is a subring of L, for which the fixed ring under the action of G is A, then B is the integral closure. It is not the true if you take B=A. If I got you wrong, please explain what you wanted to prove :)
– Madarb
Dec 9 at 8:00
1
1
you understand it right. but i do not get that it does not hold when B=A. when B=A,the extension L/K is trivial, which leads to the conclusion. i think. please tell me if i get something wrong.
– Kento
Dec 9 at 8:07
you understand it right. but i do not get that it does not hold when B=A. when B=A,the extension L/K is trivial, which leads to the conclusion. i think. please tell me if i get something wrong.
– Kento
Dec 9 at 8:07
1
1
@reuns I think you don't necessarily have $sigma(B) = B$. A possible counterexample is given below.
– pisco
Dec 9 at 12:53
@reuns I think you don't necessarily have $sigma(B) = B$. A possible counterexample is given below.
– pisco
Dec 9 at 12:53
|
show 3 more comments
1 Answer
1
active
oldest
votes
This is not true.
Let $L/K$ be a Galois extension of number field, let $mathfrak{p}$ be a prime ideal of $mathcal{O}_K$ that splits into more than one primes in $mathcal{O}_L$:
$$mathfrak{p}mathcal{O}_L = mathfrak{P}_1 cdots mathfrak{P}_r$$
Let $A = (mathcal{O}_K)_{mathfrak{p}}$, the localization at $mathfrak{p}$ and $B = (mathcal{O}_L)_{mathfrak{P}_1}$. Both are Dedekind domains. It is easily seen that $B^G = A$, but the integral closure of $A$ in $L$ is the localization of $mathcal{O}_L$ at all $mathfrak{P}_1, cdots, mathfrak{P}_r$, which is a proper subset of $B$.
1
So $K = mathbf{Q}, L = mathbf{Q}(i),p=5= (2+i)(2-i), A=mathbf{Z}_{(5)} = {frac{u}{v}, (u,v) in mathbf{Z}^2, 5 nmid v }$, $B = mathbf{Z}[i]_{(2+i)} = {frac{u}{v}, (u,v) in mathbf{Z}[i]^2, v not in (2+i)mathbf{Z}[i] }$, $sigma(c+id) = c-id, G= {sigma^2,sigma}$ then $frac1{5^n} not in B$ so $B^G = mathbf{Z}_{(5)}$ but $frac{2+i}{5}=frac{1}{2-i} in B$ is not integral over $mathbf{Z}_{(5)}$. $A,B$ are Dedekind domains since they have only one prime ideal. The problem is that $sigma(B) ne B$.
– reuns
Dec 9 at 23:22
And replacing $B$ by $bigcap_{sigma in G} sigma(B)$ makes the claim true
– reuns
Dec 10 at 18:19
add a comment |
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This is not true.
Let $L/K$ be a Galois extension of number field, let $mathfrak{p}$ be a prime ideal of $mathcal{O}_K$ that splits into more than one primes in $mathcal{O}_L$:
$$mathfrak{p}mathcal{O}_L = mathfrak{P}_1 cdots mathfrak{P}_r$$
Let $A = (mathcal{O}_K)_{mathfrak{p}}$, the localization at $mathfrak{p}$ and $B = (mathcal{O}_L)_{mathfrak{P}_1}$. Both are Dedekind domains. It is easily seen that $B^G = A$, but the integral closure of $A$ in $L$ is the localization of $mathcal{O}_L$ at all $mathfrak{P}_1, cdots, mathfrak{P}_r$, which is a proper subset of $B$.
1
So $K = mathbf{Q}, L = mathbf{Q}(i),p=5= (2+i)(2-i), A=mathbf{Z}_{(5)} = {frac{u}{v}, (u,v) in mathbf{Z}^2, 5 nmid v }$, $B = mathbf{Z}[i]_{(2+i)} = {frac{u}{v}, (u,v) in mathbf{Z}[i]^2, v not in (2+i)mathbf{Z}[i] }$, $sigma(c+id) = c-id, G= {sigma^2,sigma}$ then $frac1{5^n} not in B$ so $B^G = mathbf{Z}_{(5)}$ but $frac{2+i}{5}=frac{1}{2-i} in B$ is not integral over $mathbf{Z}_{(5)}$. $A,B$ are Dedekind domains since they have only one prime ideal. The problem is that $sigma(B) ne B$.
– reuns
Dec 9 at 23:22
And replacing $B$ by $bigcap_{sigma in G} sigma(B)$ makes the claim true
– reuns
Dec 10 at 18:19
add a comment |
This is not true.
Let $L/K$ be a Galois extension of number field, let $mathfrak{p}$ be a prime ideal of $mathcal{O}_K$ that splits into more than one primes in $mathcal{O}_L$:
$$mathfrak{p}mathcal{O}_L = mathfrak{P}_1 cdots mathfrak{P}_r$$
Let $A = (mathcal{O}_K)_{mathfrak{p}}$, the localization at $mathfrak{p}$ and $B = (mathcal{O}_L)_{mathfrak{P}_1}$. Both are Dedekind domains. It is easily seen that $B^G = A$, but the integral closure of $A$ in $L$ is the localization of $mathcal{O}_L$ at all $mathfrak{P}_1, cdots, mathfrak{P}_r$, which is a proper subset of $B$.
1
So $K = mathbf{Q}, L = mathbf{Q}(i),p=5= (2+i)(2-i), A=mathbf{Z}_{(5)} = {frac{u}{v}, (u,v) in mathbf{Z}^2, 5 nmid v }$, $B = mathbf{Z}[i]_{(2+i)} = {frac{u}{v}, (u,v) in mathbf{Z}[i]^2, v not in (2+i)mathbf{Z}[i] }$, $sigma(c+id) = c-id, G= {sigma^2,sigma}$ then $frac1{5^n} not in B$ so $B^G = mathbf{Z}_{(5)}$ but $frac{2+i}{5}=frac{1}{2-i} in B$ is not integral over $mathbf{Z}_{(5)}$. $A,B$ are Dedekind domains since they have only one prime ideal. The problem is that $sigma(B) ne B$.
– reuns
Dec 9 at 23:22
And replacing $B$ by $bigcap_{sigma in G} sigma(B)$ makes the claim true
– reuns
Dec 10 at 18:19
add a comment |
This is not true.
Let $L/K$ be a Galois extension of number field, let $mathfrak{p}$ be a prime ideal of $mathcal{O}_K$ that splits into more than one primes in $mathcal{O}_L$:
$$mathfrak{p}mathcal{O}_L = mathfrak{P}_1 cdots mathfrak{P}_r$$
Let $A = (mathcal{O}_K)_{mathfrak{p}}$, the localization at $mathfrak{p}$ and $B = (mathcal{O}_L)_{mathfrak{P}_1}$. Both are Dedekind domains. It is easily seen that $B^G = A$, but the integral closure of $A$ in $L$ is the localization of $mathcal{O}_L$ at all $mathfrak{P}_1, cdots, mathfrak{P}_r$, which is a proper subset of $B$.
This is not true.
Let $L/K$ be a Galois extension of number field, let $mathfrak{p}$ be a prime ideal of $mathcal{O}_K$ that splits into more than one primes in $mathcal{O}_L$:
$$mathfrak{p}mathcal{O}_L = mathfrak{P}_1 cdots mathfrak{P}_r$$
Let $A = (mathcal{O}_K)_{mathfrak{p}}$, the localization at $mathfrak{p}$ and $B = (mathcal{O}_L)_{mathfrak{P}_1}$. Both are Dedekind domains. It is easily seen that $B^G = A$, but the integral closure of $A$ in $L$ is the localization of $mathcal{O}_L$ at all $mathfrak{P}_1, cdots, mathfrak{P}_r$, which is a proper subset of $B$.
edited Dec 10 at 16:53
answered Dec 9 at 12:20
pisco
11.5k21742
11.5k21742
1
So $K = mathbf{Q}, L = mathbf{Q}(i),p=5= (2+i)(2-i), A=mathbf{Z}_{(5)} = {frac{u}{v}, (u,v) in mathbf{Z}^2, 5 nmid v }$, $B = mathbf{Z}[i]_{(2+i)} = {frac{u}{v}, (u,v) in mathbf{Z}[i]^2, v not in (2+i)mathbf{Z}[i] }$, $sigma(c+id) = c-id, G= {sigma^2,sigma}$ then $frac1{5^n} not in B$ so $B^G = mathbf{Z}_{(5)}$ but $frac{2+i}{5}=frac{1}{2-i} in B$ is not integral over $mathbf{Z}_{(5)}$. $A,B$ are Dedekind domains since they have only one prime ideal. The problem is that $sigma(B) ne B$.
– reuns
Dec 9 at 23:22
And replacing $B$ by $bigcap_{sigma in G} sigma(B)$ makes the claim true
– reuns
Dec 10 at 18:19
add a comment |
1
So $K = mathbf{Q}, L = mathbf{Q}(i),p=5= (2+i)(2-i), A=mathbf{Z}_{(5)} = {frac{u}{v}, (u,v) in mathbf{Z}^2, 5 nmid v }$, $B = mathbf{Z}[i]_{(2+i)} = {frac{u}{v}, (u,v) in mathbf{Z}[i]^2, v not in (2+i)mathbf{Z}[i] }$, $sigma(c+id) = c-id, G= {sigma^2,sigma}$ then $frac1{5^n} not in B$ so $B^G = mathbf{Z}_{(5)}$ but $frac{2+i}{5}=frac{1}{2-i} in B$ is not integral over $mathbf{Z}_{(5)}$. $A,B$ are Dedekind domains since they have only one prime ideal. The problem is that $sigma(B) ne B$.
– reuns
Dec 9 at 23:22
And replacing $B$ by $bigcap_{sigma in G} sigma(B)$ makes the claim true
– reuns
Dec 10 at 18:19
1
1
So $K = mathbf{Q}, L = mathbf{Q}(i),p=5= (2+i)(2-i), A=mathbf{Z}_{(5)} = {frac{u}{v}, (u,v) in mathbf{Z}^2, 5 nmid v }$, $B = mathbf{Z}[i]_{(2+i)} = {frac{u}{v}, (u,v) in mathbf{Z}[i]^2, v not in (2+i)mathbf{Z}[i] }$, $sigma(c+id) = c-id, G= {sigma^2,sigma}$ then $frac1{5^n} not in B$ so $B^G = mathbf{Z}_{(5)}$ but $frac{2+i}{5}=frac{1}{2-i} in B$ is not integral over $mathbf{Z}_{(5)}$. $A,B$ are Dedekind domains since they have only one prime ideal. The problem is that $sigma(B) ne B$.
– reuns
Dec 9 at 23:22
So $K = mathbf{Q}, L = mathbf{Q}(i),p=5= (2+i)(2-i), A=mathbf{Z}_{(5)} = {frac{u}{v}, (u,v) in mathbf{Z}^2, 5 nmid v }$, $B = mathbf{Z}[i]_{(2+i)} = {frac{u}{v}, (u,v) in mathbf{Z}[i]^2, v not in (2+i)mathbf{Z}[i] }$, $sigma(c+id) = c-id, G= {sigma^2,sigma}$ then $frac1{5^n} not in B$ so $B^G = mathbf{Z}_{(5)}$ but $frac{2+i}{5}=frac{1}{2-i} in B$ is not integral over $mathbf{Z}_{(5)}$. $A,B$ are Dedekind domains since they have only one prime ideal. The problem is that $sigma(B) ne B$.
– reuns
Dec 9 at 23:22
And replacing $B$ by $bigcap_{sigma in G} sigma(B)$ makes the claim true
– reuns
Dec 10 at 18:19
And replacing $B$ by $bigcap_{sigma in G} sigma(B)$ makes the claim true
– reuns
Dec 10 at 18:19
add a comment |
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I think it's not true. Try to take B=A. Then the fixed ring is A, but unless the extension is trivial, this is not the integral closure.
– Madarb
Dec 9 at 7:38
when A=B, then L=K. Since A is dedekind, A is integrally closed and B(=A) is the integral closure of A in L holds, i think.
– Kento
Dec 9 at 7:56
I don't think I understood what you say. The claim you want to prove, as I understand it, is that if B is a subring of L, for which the fixed ring under the action of G is A, then B is the integral closure. It is not the true if you take B=A. If I got you wrong, please explain what you wanted to prove :)
– Madarb
Dec 9 at 8:00
1
you understand it right. but i do not get that it does not hold when B=A. when B=A,the extension L/K is trivial, which leads to the conclusion. i think. please tell me if i get something wrong.
– Kento
Dec 9 at 8:07
1
@reuns I think you don't necessarily have $sigma(B) = B$. A possible counterexample is given below.
– pisco
Dec 9 at 12:53