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?










3












$begingroup$


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 $E subsetneq F$?










share|cite|improve this question











$endgroup$







  • 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















3












$begingroup$


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 $E subsetneq F$?










share|cite|improve this question











$endgroup$







  • 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













3












3








3


2



$begingroup$


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 $E subsetneq F$?










share|cite|improve this question











$endgroup$




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






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








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












  • 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










1 Answer
1






active

oldest

votes


















0












$begingroup$

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.







share|cite|improve this answer









$endgroup$













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    0












    $begingroup$

    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.







    share|cite|improve this answer









    $endgroup$

















      0












      $begingroup$

      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.







      share|cite|improve this answer









      $endgroup$















        0












        0








        0





        $begingroup$

        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.







        share|cite|improve this answer









        $endgroup$



        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.








        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Mar 29 at 11:25









        Sebastien PalcouxSebastien Palcoux

        2,336928




        2,336928



























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