Relation between a finite group and the Galois group for the field extension generated by the character table entries Announcing the arrival of Valued Associate #679: Cesar Manara Planned maintenance scheduled April 17/18, 2019 at 00:00UTC (8:00pm US/Eastern)To prove that the sum of the roots of the characteristic polynomial of a square matrix is equal to the trace of the matrixAn explicit calculation of Galois groupLink between representation theory and Galois theory: Trivial representation in field towers.Systematically describing the Galois Group and Intermediate FieldsField extension $mathbb Q(f)/mathbb Q$ and its Galois groupAutomorphism of $mathbbQ(zeta_n)/mathbbQ$Galois group of the extension $E:= mathbbQ(i, sqrt2, sqrt3, sqrt[4]2)$Solving the Character table for $A_4$ and derived algebra characteristics.Constructing semidirect product out of finite fields and Galois groups and the permutation groups they induceQuestion about Galois Extension and fixed field by a Subgroup of the Galois groupWhat is a primitive character of a Galois groups of a finite cyclic extension of local fields?
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Relation between a finite group and the Galois group for the field extension generated by the character table entries
Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 17/18, 2019 at 00:00UTC (8:00pm US/Eastern)To prove that the sum of the roots of the characteristic polynomial of a square matrix is equal to the trace of the matrixAn explicit calculation of Galois groupLink between representation theory and Galois theory: Trivial representation in field towers.Systematically describing the Galois Group and Intermediate FieldsField extension $mathbb Q(f)/mathbb Q$ and its Galois groupAutomorphism of $mathbbQ(zeta_n)/mathbbQ$Galois group of the extension $E:= mathbbQ(i, sqrt2, sqrt3, sqrt[4]2)$Solving the Character table for $A_4$ and derived algebra characteristics.Constructing semidirect product out of finite fields and Galois groups and the permutation groups they induceQuestion about Galois Extension and fixed field by a Subgroup of the Galois groupWhat is a primitive character of a Galois groups of a finite cyclic extension of local fields?
$begingroup$
Let $E$ be the extension of $mathbbQ$ generated by the character table entries of a finite group $G$.
Let $F$ be the Galois closure of $E$.
Examples:
- $G=C_n$, $E=F=mathbbQ(zeta_n)$ and $Gal(F/mathbbQ) simeq (mathbbZ/nmathbbZ)^times simeq Out(G)=Aut(G)$,
- $G=S_n$, $E=F=mathbbQ$ and so $C_1 = Gal(F/mathbbQ) hookrightarrow Out(G)$ and $Aut(G)$,
- $G=A_n$ with $3 le n le 5$, $E=F$ and $Gal(F/mathbbQ) simeq Out(G) simeq C_2 hookrightarrow Aut(G)$,
- $G=A_6$, $E=F=mathbbQ(sqrt5)$ and $Gal(F/mathbbQ) simeq C_2 hookrightarrow Out(G)simeq C_2 times C_2 hookrightarrow Aut(G)$.
Question: Is it true in general that $Gal(F/mathbbQ)$ or $Aut(E/mathbbQ) hookrightarrow Out(G)$ or $Aut(G)$?
If not, is there at least a non-trivial homomorphism from $Gal(F/mathbbQ)$ to $Aut(G)$? Or any relation?
Bonus question: Is there an example with $E subsetneq F$?
group-theory finite-groups representation-theory galois-theory characters
$endgroup$
|
show 7 more comments
$begingroup$
Let $E$ be the extension of $mathbbQ$ generated by the character table entries of a finite group $G$.
Let $F$ be the Galois closure of $E$.
Examples:
- $G=C_n$, $E=F=mathbbQ(zeta_n)$ and $Gal(F/mathbbQ) simeq (mathbbZ/nmathbbZ)^times simeq Out(G)=Aut(G)$,
- $G=S_n$, $E=F=mathbbQ$ and so $C_1 = Gal(F/mathbbQ) hookrightarrow Out(G)$ and $Aut(G)$,
- $G=A_n$ with $3 le n le 5$, $E=F$ and $Gal(F/mathbbQ) simeq Out(G) simeq C_2 hookrightarrow Aut(G)$,
- $G=A_6$, $E=F=mathbbQ(sqrt5)$ and $Gal(F/mathbbQ) simeq C_2 hookrightarrow Out(G)simeq C_2 times C_2 hookrightarrow Aut(G)$.
Question: Is it true in general that $Gal(F/mathbbQ)$ or $Aut(E/mathbbQ) hookrightarrow Out(G)$ or $Aut(G)$?
If not, is there at least a non-trivial homomorphism from $Gal(F/mathbbQ)$ to $Aut(G)$? Or any relation?
Bonus question: Is there an example with $E subsetneq F$?
group-theory finite-groups representation-theory galois-theory characters
$endgroup$
1
$begingroup$
$n=|G|$ an homomorphism $rho : G to GL_m(BbbC)$ and $det(xI-rho(g))= prod_j (x-alpha_g,j)$ then $rho(g)^n = I$ implies the minimal polynomial divides $x^n-1$ so that $alpha_g,j = zeta_n^e_g,j$ and $Tr(rho(g)) = sum_j alpha_g,j in BbbQ(zeta_n)$ and $E/BbbQ$ is abelian
$endgroup$
– reuns
Mar 25 at 21:15
$begingroup$
@reuns: Can we deduce that $Gal(E/mathbbQ)$ is a subgroup of $G$, or something like that?
$endgroup$
– Sebastien Palcoux
Mar 25 at 21:22
$begingroup$
No, it is a quotient (thus a subgroup since we are in finite abelian group) of $BbbZ/nZ^times = Gal(BbbQ(zeta_n)/Q)$ and $BbbZ/e Z^times$ where $e$ is the exponent of $G$. Also $E$ is called the minimal splitting field
$endgroup$
– reuns
Mar 25 at 21:25
$begingroup$
The character table of dihedral group $D_6$ non-abelian with $6$ elements is integer valued so you can't deduce $E$ only from $n$ or $e$
$endgroup$
– reuns
Mar 25 at 21:40
$begingroup$
There are nonisomorphic groups with the same characters so you can't hope to deduce $G$ from $E$ or $F$
$endgroup$
– Max
Mar 25 at 21:45
|
show 7 more comments
$begingroup$
Let $E$ be the extension of $mathbbQ$ generated by the character table entries of a finite group $G$.
Let $F$ be the Galois closure of $E$.
Examples:
- $G=C_n$, $E=F=mathbbQ(zeta_n)$ and $Gal(F/mathbbQ) simeq (mathbbZ/nmathbbZ)^times simeq Out(G)=Aut(G)$,
- $G=S_n$, $E=F=mathbbQ$ and so $C_1 = Gal(F/mathbbQ) hookrightarrow Out(G)$ and $Aut(G)$,
- $G=A_n$ with $3 le n le 5$, $E=F$ and $Gal(F/mathbbQ) simeq Out(G) simeq C_2 hookrightarrow Aut(G)$,
- $G=A_6$, $E=F=mathbbQ(sqrt5)$ and $Gal(F/mathbbQ) simeq C_2 hookrightarrow Out(G)simeq C_2 times C_2 hookrightarrow Aut(G)$.
Question: Is it true in general that $Gal(F/mathbbQ)$ or $Aut(E/mathbbQ) hookrightarrow Out(G)$ or $Aut(G)$?
If not, is there at least a non-trivial homomorphism from $Gal(F/mathbbQ)$ to $Aut(G)$? Or any relation?
Bonus question: Is there an example with $E subsetneq F$?
group-theory finite-groups representation-theory galois-theory characters
$endgroup$
Let $E$ be the extension of $mathbbQ$ generated by the character table entries of a finite group $G$.
Let $F$ be the Galois closure of $E$.
Examples:
- $G=C_n$, $E=F=mathbbQ(zeta_n)$ and $Gal(F/mathbbQ) simeq (mathbbZ/nmathbbZ)^times simeq Out(G)=Aut(G)$,
- $G=S_n$, $E=F=mathbbQ$ and so $C_1 = Gal(F/mathbbQ) hookrightarrow Out(G)$ and $Aut(G)$,
- $G=A_n$ with $3 le n le 5$, $E=F$ and $Gal(F/mathbbQ) simeq Out(G) simeq C_2 hookrightarrow Aut(G)$,
- $G=A_6$, $E=F=mathbbQ(sqrt5)$ and $Gal(F/mathbbQ) simeq C_2 hookrightarrow Out(G)simeq C_2 times C_2 hookrightarrow Aut(G)$.
Question: Is it true in general that $Gal(F/mathbbQ)$ or $Aut(E/mathbbQ) hookrightarrow Out(G)$ or $Aut(G)$?
If not, is there at least a non-trivial homomorphism from $Gal(F/mathbbQ)$ to $Aut(G)$? Or any relation?
Bonus question: Is there an example with $E subsetneq F$?
group-theory finite-groups representation-theory galois-theory characters
group-theory finite-groups representation-theory galois-theory characters
edited Mar 27 at 7:37
Sebastien Palcoux
asked Mar 25 at 20:01
Sebastien PalcouxSebastien Palcoux
2,336928
2,336928
1
$begingroup$
$n=|G|$ an homomorphism $rho : G to GL_m(BbbC)$ and $det(xI-rho(g))= prod_j (x-alpha_g,j)$ then $rho(g)^n = I$ implies the minimal polynomial divides $x^n-1$ so that $alpha_g,j = zeta_n^e_g,j$ and $Tr(rho(g)) = sum_j alpha_g,j in BbbQ(zeta_n)$ and $E/BbbQ$ is abelian
$endgroup$
– reuns
Mar 25 at 21:15
$begingroup$
@reuns: Can we deduce that $Gal(E/mathbbQ)$ is a subgroup of $G$, or something like that?
$endgroup$
– Sebastien Palcoux
Mar 25 at 21:22
$begingroup$
No, it is a quotient (thus a subgroup since we are in finite abelian group) of $BbbZ/nZ^times = Gal(BbbQ(zeta_n)/Q)$ and $BbbZ/e Z^times$ where $e$ is the exponent of $G$. Also $E$ is called the minimal splitting field
$endgroup$
– reuns
Mar 25 at 21:25
$begingroup$
The character table of dihedral group $D_6$ non-abelian with $6$ elements is integer valued so you can't deduce $E$ only from $n$ or $e$
$endgroup$
– reuns
Mar 25 at 21:40
$begingroup$
There are nonisomorphic groups with the same characters so you can't hope to deduce $G$ from $E$ or $F$
$endgroup$
– Max
Mar 25 at 21:45
|
show 7 more comments
1
$begingroup$
$n=|G|$ an homomorphism $rho : G to GL_m(BbbC)$ and $det(xI-rho(g))= prod_j (x-alpha_g,j)$ then $rho(g)^n = I$ implies the minimal polynomial divides $x^n-1$ so that $alpha_g,j = zeta_n^e_g,j$ and $Tr(rho(g)) = sum_j alpha_g,j in BbbQ(zeta_n)$ and $E/BbbQ$ is abelian
$endgroup$
– reuns
Mar 25 at 21:15
$begingroup$
@reuns: Can we deduce that $Gal(E/mathbbQ)$ is a subgroup of $G$, or something like that?
$endgroup$
– Sebastien Palcoux
Mar 25 at 21:22
$begingroup$
No, it is a quotient (thus a subgroup since we are in finite abelian group) of $BbbZ/nZ^times = Gal(BbbQ(zeta_n)/Q)$ and $BbbZ/e Z^times$ where $e$ is the exponent of $G$. Also $E$ is called the minimal splitting field
$endgroup$
– reuns
Mar 25 at 21:25
$begingroup$
The character table of dihedral group $D_6$ non-abelian with $6$ elements is integer valued so you can't deduce $E$ only from $n$ or $e$
$endgroup$
– reuns
Mar 25 at 21:40
$begingroup$
There are nonisomorphic groups with the same characters so you can't hope to deduce $G$ from $E$ or $F$
$endgroup$
– Max
Mar 25 at 21:45
1
1
$begingroup$
$n=|G|$ an homomorphism $rho : G to GL_m(BbbC)$ and $det(xI-rho(g))= prod_j (x-alpha_g,j)$ then $rho(g)^n = I$ implies the minimal polynomial divides $x^n-1$ so that $alpha_g,j = zeta_n^e_g,j$ and $Tr(rho(g)) = sum_j alpha_g,j in BbbQ(zeta_n)$ and $E/BbbQ$ is abelian
$endgroup$
– reuns
Mar 25 at 21:15
$begingroup$
$n=|G|$ an homomorphism $rho : G to GL_m(BbbC)$ and $det(xI-rho(g))= prod_j (x-alpha_g,j)$ then $rho(g)^n = I$ implies the minimal polynomial divides $x^n-1$ so that $alpha_g,j = zeta_n^e_g,j$ and $Tr(rho(g)) = sum_j alpha_g,j in BbbQ(zeta_n)$ and $E/BbbQ$ is abelian
$endgroup$
– reuns
Mar 25 at 21:15
$begingroup$
@reuns: Can we deduce that $Gal(E/mathbbQ)$ is a subgroup of $G$, or something like that?
$endgroup$
– Sebastien Palcoux
Mar 25 at 21:22
$begingroup$
@reuns: Can we deduce that $Gal(E/mathbbQ)$ is a subgroup of $G$, or something like that?
$endgroup$
– Sebastien Palcoux
Mar 25 at 21:22
$begingroup$
No, it is a quotient (thus a subgroup since we are in finite abelian group) of $BbbZ/nZ^times = Gal(BbbQ(zeta_n)/Q)$ and $BbbZ/e Z^times$ where $e$ is the exponent of $G$. Also $E$ is called the minimal splitting field
$endgroup$
– reuns
Mar 25 at 21:25
$begingroup$
No, it is a quotient (thus a subgroup since we are in finite abelian group) of $BbbZ/nZ^times = Gal(BbbQ(zeta_n)/Q)$ and $BbbZ/e Z^times$ where $e$ is the exponent of $G$. Also $E$ is called the minimal splitting field
$endgroup$
– reuns
Mar 25 at 21:25
$begingroup$
The character table of dihedral group $D_6$ non-abelian with $6$ elements is integer valued so you can't deduce $E$ only from $n$ or $e$
$endgroup$
– reuns
Mar 25 at 21:40
$begingroup$
The character table of dihedral group $D_6$ non-abelian with $6$ elements is integer valued so you can't deduce $E$ only from $n$ or $e$
$endgroup$
– reuns
Mar 25 at 21:40
$begingroup$
There are nonisomorphic groups with the same characters so you can't hope to deduce $G$ from $E$ or $F$
$endgroup$
– Max
Mar 25 at 21:45
$begingroup$
There are nonisomorphic groups with the same characters so you can't hope to deduce $G$ from $E$ or $F$
$endgroup$
– Max
Mar 25 at 21:45
|
show 7 more comments
1 Answer
1
active
oldest
votes
$begingroup$
For the Mathieu group $G=M_11$, we have $E=mathbbQ(isqrt2,isqrt11)$ (see the character table here p. 89), whereas $Out(G)=C_1$. It follows that $Gal(F/mathbbQ) not hookrightarrow Out(G)$.
Otherwise the following sentence comes from this page:
The outer automorphism group acts on the character table by permuting
columns (conjugacy classes) and accordingly rows, which gives another
symmetry to the table.
$endgroup$
add a comment |
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$begingroup$
For the Mathieu group $G=M_11$, we have $E=mathbbQ(isqrt2,isqrt11)$ (see the character table here p. 89), whereas $Out(G)=C_1$. It follows that $Gal(F/mathbbQ) not hookrightarrow Out(G)$.
Otherwise the following sentence comes from this page:
The outer automorphism group acts on the character table by permuting
columns (conjugacy classes) and accordingly rows, which gives another
symmetry to the table.
$endgroup$
add a comment |
$begingroup$
For the Mathieu group $G=M_11$, we have $E=mathbbQ(isqrt2,isqrt11)$ (see the character table here p. 89), whereas $Out(G)=C_1$. It follows that $Gal(F/mathbbQ) not hookrightarrow Out(G)$.
Otherwise the following sentence comes from this page:
The outer automorphism group acts on the character table by permuting
columns (conjugacy classes) and accordingly rows, which gives another
symmetry to the table.
$endgroup$
add a comment |
$begingroup$
For the Mathieu group $G=M_11$, we have $E=mathbbQ(isqrt2,isqrt11)$ (see the character table here p. 89), whereas $Out(G)=C_1$. It follows that $Gal(F/mathbbQ) not hookrightarrow Out(G)$.
Otherwise the following sentence comes from this page:
The outer automorphism group acts on the character table by permuting
columns (conjugacy classes) and accordingly rows, which gives another
symmetry to the table.
$endgroup$
For the Mathieu group $G=M_11$, we have $E=mathbbQ(isqrt2,isqrt11)$ (see the character table here p. 89), whereas $Out(G)=C_1$. It follows that $Gal(F/mathbbQ) not hookrightarrow Out(G)$.
Otherwise the following sentence comes from this page:
The outer automorphism group acts on the character table by permuting
columns (conjugacy classes) and accordingly rows, which gives another
symmetry to the table.
answered Mar 29 at 11:25
Sebastien PalcouxSebastien Palcoux
2,336928
2,336928
add a comment |
add a comment |
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1
$begingroup$
$n=|G|$ an homomorphism $rho : G to GL_m(BbbC)$ and $det(xI-rho(g))= prod_j (x-alpha_g,j)$ then $rho(g)^n = I$ implies the minimal polynomial divides $x^n-1$ so that $alpha_g,j = zeta_n^e_g,j$ and $Tr(rho(g)) = sum_j alpha_g,j in BbbQ(zeta_n)$ and $E/BbbQ$ is abelian
$endgroup$
– reuns
Mar 25 at 21:15
$begingroup$
@reuns: Can we deduce that $Gal(E/mathbbQ)$ is a subgroup of $G$, or something like that?
$endgroup$
– Sebastien Palcoux
Mar 25 at 21:22
$begingroup$
No, it is a quotient (thus a subgroup since we are in finite abelian group) of $BbbZ/nZ^times = Gal(BbbQ(zeta_n)/Q)$ and $BbbZ/e Z^times$ where $e$ is the exponent of $G$. Also $E$ is called the minimal splitting field
$endgroup$
– reuns
Mar 25 at 21:25
$begingroup$
The character table of dihedral group $D_6$ non-abelian with $6$ elements is integer valued so you can't deduce $E$ only from $n$ or $e$
$endgroup$
– reuns
Mar 25 at 21:40
$begingroup$
There are nonisomorphic groups with the same characters so you can't hope to deduce $G$ from $E$ or $F$
$endgroup$
– Max
Mar 25 at 21:45