Make a short exact sequence of abstract groups into a short exact sequence of topological groups (motivated...












4












$begingroup$


Let $1 to H_1 to G to H_2 to 1$ be a short exact sequence of abstract groups.



Question: If $H_1$, $H_2$ have fixed topologies, can we endow $G$ with a topology such that the sequence above becomes a short exact sequence of topological groups? If yes, is this topology then uniquely determined?



Motivation: I want to understand the topology of the Weil group $W_K$ which is a dense subgroup of the absolute Galois group $G_K$ (which has this weird profinite topology I find hard to get). However, we have a short exact sequence of abstract groups



$$1 to I_K to W_K to langle operatorname{Frob}_k rangle to 1$$
where $I_K$ denotes the inertia subgroup of $G_K$ and $operatorname{Frob}_k : x mapsto x^{|k|}$ is the Frobenius element of the absolute Galois group $G_k$ of the residue field $k$ of $K$.



If I understand it correctly, this sequence is not a short exact sequence of topological groups if we give $W_K$ the subspace topology of $G_K$. However, if we give $I_K$ and $langle operatorname{Frob}_k rangle$ the subspace topologies of $G_K$ and $G_k$ respectively, we can give $W_K$ a unique topology such that this sequence becomes a short exact sequence of topological groups.



I heard that this causes $W_K$ to have a finer topology than the usual subspace topology, $I_K$ to be open and the maximal compact subgroup of $W_K$ etc. but I do not really understand that.



Could you please explain this to me? Thank you!










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$endgroup$








  • 3




    $begingroup$
    You don't want $langle operatorname{Frob}_k rangle$ to have the suspace topology from $G_k$, but instead the discrete topology.
    $endgroup$
    – MatheinBoulomenos
    Jan 6 at 21:32
















4












$begingroup$


Let $1 to H_1 to G to H_2 to 1$ be a short exact sequence of abstract groups.



Question: If $H_1$, $H_2$ have fixed topologies, can we endow $G$ with a topology such that the sequence above becomes a short exact sequence of topological groups? If yes, is this topology then uniquely determined?



Motivation: I want to understand the topology of the Weil group $W_K$ which is a dense subgroup of the absolute Galois group $G_K$ (which has this weird profinite topology I find hard to get). However, we have a short exact sequence of abstract groups



$$1 to I_K to W_K to langle operatorname{Frob}_k rangle to 1$$
where $I_K$ denotes the inertia subgroup of $G_K$ and $operatorname{Frob}_k : x mapsto x^{|k|}$ is the Frobenius element of the absolute Galois group $G_k$ of the residue field $k$ of $K$.



If I understand it correctly, this sequence is not a short exact sequence of topological groups if we give $W_K$ the subspace topology of $G_K$. However, if we give $I_K$ and $langle operatorname{Frob}_k rangle$ the subspace topologies of $G_K$ and $G_k$ respectively, we can give $W_K$ a unique topology such that this sequence becomes a short exact sequence of topological groups.



I heard that this causes $W_K$ to have a finer topology than the usual subspace topology, $I_K$ to be open and the maximal compact subgroup of $W_K$ etc. but I do not really understand that.



Could you please explain this to me? Thank you!










share|cite|improve this question











$endgroup$








  • 3




    $begingroup$
    You don't want $langle operatorname{Frob}_k rangle$ to have the suspace topology from $G_k$, but instead the discrete topology.
    $endgroup$
    – MatheinBoulomenos
    Jan 6 at 21:32














4












4








4


1



$begingroup$


Let $1 to H_1 to G to H_2 to 1$ be a short exact sequence of abstract groups.



Question: If $H_1$, $H_2$ have fixed topologies, can we endow $G$ with a topology such that the sequence above becomes a short exact sequence of topological groups? If yes, is this topology then uniquely determined?



Motivation: I want to understand the topology of the Weil group $W_K$ which is a dense subgroup of the absolute Galois group $G_K$ (which has this weird profinite topology I find hard to get). However, we have a short exact sequence of abstract groups



$$1 to I_K to W_K to langle operatorname{Frob}_k rangle to 1$$
where $I_K$ denotes the inertia subgroup of $G_K$ and $operatorname{Frob}_k : x mapsto x^{|k|}$ is the Frobenius element of the absolute Galois group $G_k$ of the residue field $k$ of $K$.



If I understand it correctly, this sequence is not a short exact sequence of topological groups if we give $W_K$ the subspace topology of $G_K$. However, if we give $I_K$ and $langle operatorname{Frob}_k rangle$ the subspace topologies of $G_K$ and $G_k$ respectively, we can give $W_K$ a unique topology such that this sequence becomes a short exact sequence of topological groups.



I heard that this causes $W_K$ to have a finer topology than the usual subspace topology, $I_K$ to be open and the maximal compact subgroup of $W_K$ etc. but I do not really understand that.



Could you please explain this to me? Thank you!










share|cite|improve this question











$endgroup$




Let $1 to H_1 to G to H_2 to 1$ be a short exact sequence of abstract groups.



Question: If $H_1$, $H_2$ have fixed topologies, can we endow $G$ with a topology such that the sequence above becomes a short exact sequence of topological groups? If yes, is this topology then uniquely determined?



Motivation: I want to understand the topology of the Weil group $W_K$ which is a dense subgroup of the absolute Galois group $G_K$ (which has this weird profinite topology I find hard to get). However, we have a short exact sequence of abstract groups



$$1 to I_K to W_K to langle operatorname{Frob}_k rangle to 1$$
where $I_K$ denotes the inertia subgroup of $G_K$ and $operatorname{Frob}_k : x mapsto x^{|k|}$ is the Frobenius element of the absolute Galois group $G_k$ of the residue field $k$ of $K$.



If I understand it correctly, this sequence is not a short exact sequence of topological groups if we give $W_K$ the subspace topology of $G_K$. However, if we give $I_K$ and $langle operatorname{Frob}_k rangle$ the subspace topologies of $G_K$ and $G_k$ respectively, we can give $W_K$ a unique topology such that this sequence becomes a short exact sequence of topological groups.



I heard that this causes $W_K$ to have a finer topology than the usual subspace topology, $I_K$ to be open and the maximal compact subgroup of $W_K$ etc. but I do not really understand that.



Could you please explain this to me? Thank you!







abstract-algebra general-topology group-theory algebraic-number-theory topological-groups






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edited Jan 6 at 13:12







Diglett

















asked Jan 6 at 12:41









DiglettDiglett

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  • 3




    $begingroup$
    You don't want $langle operatorname{Frob}_k rangle$ to have the suspace topology from $G_k$, but instead the discrete topology.
    $endgroup$
    – MatheinBoulomenos
    Jan 6 at 21:32














  • 3




    $begingroup$
    You don't want $langle operatorname{Frob}_k rangle$ to have the suspace topology from $G_k$, but instead the discrete topology.
    $endgroup$
    – MatheinBoulomenos
    Jan 6 at 21:32








3




3




$begingroup$
You don't want $langle operatorname{Frob}_k rangle$ to have the suspace topology from $G_k$, but instead the discrete topology.
$endgroup$
– MatheinBoulomenos
Jan 6 at 21:32




$begingroup$
You don't want $langle operatorname{Frob}_k rangle$ to have the suspace topology from $G_k$, but instead the discrete topology.
$endgroup$
– MatheinBoulomenos
Jan 6 at 21:32










1 Answer
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oldest

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$begingroup$

Not saying anything about the Weil group at this point. Trying to settle the first question instead with the following construction that smacks of cheating.





Let $i:H_1to G$ be the monomorphism, and $p:Gto H_2$ be the epimorphism in this short exact sequence. Exactness means that $N:=i(H_1)=ker(p)unlhd G$. If I got it right $H_1$ and $H_2$ are topological groups, and you want to give a topology to $G$ so that $G$ becomes a topological group, and the mappings $i,p$ are continuous.



Unless I made a silly mistake this can be achieved by giving $G$ the topology defined by letting the cosets of $N$ to form a basis. In other words, the open sets of $G$ are the arbitrary unions of cosets of $N$. This is trivially a topology $tau$. Furthermore:





  1. $p:(G,tau)to H_2$ is continuous for much the same reason that any mapping from a discrete space to any topological space is continuous. The preimage of any subset of $H_2$ is a union of cosets of $N$.


  2. $i:H_1to(G,tau)$ is continuous for much the same reason that any map to a space with a trivial topology is continuous. The preimage of any open subset $intau$ is either all of $H_1$ or the empty set.


  3. $(G,tau)$ is a topological group. For if $x,yin G$ are arbitrary, then the product map $m:Gtimes Gto G, m(x,y)=xy$ satisfies $m(xN,yN)subseteq xyN$. Any open set containing $xy$ contains the coset $xyN$, so continuity of $m$ follows. Continuity of the inverse mapping is verified in the same way.




Seems to me that you really wanted to ask a slightly different question. I'm afraid I cannot suggest one. Anwyway, it seems morally certain to me that the topology $G$ needs to have to satisfy items 1 to 3 is not unique at all.






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    $begingroup$

    Not saying anything about the Weil group at this point. Trying to settle the first question instead with the following construction that smacks of cheating.





    Let $i:H_1to G$ be the monomorphism, and $p:Gto H_2$ be the epimorphism in this short exact sequence. Exactness means that $N:=i(H_1)=ker(p)unlhd G$. If I got it right $H_1$ and $H_2$ are topological groups, and you want to give a topology to $G$ so that $G$ becomes a topological group, and the mappings $i,p$ are continuous.



    Unless I made a silly mistake this can be achieved by giving $G$ the topology defined by letting the cosets of $N$ to form a basis. In other words, the open sets of $G$ are the arbitrary unions of cosets of $N$. This is trivially a topology $tau$. Furthermore:





    1. $p:(G,tau)to H_2$ is continuous for much the same reason that any mapping from a discrete space to any topological space is continuous. The preimage of any subset of $H_2$ is a union of cosets of $N$.


    2. $i:H_1to(G,tau)$ is continuous for much the same reason that any map to a space with a trivial topology is continuous. The preimage of any open subset $intau$ is either all of $H_1$ or the empty set.


    3. $(G,tau)$ is a topological group. For if $x,yin G$ are arbitrary, then the product map $m:Gtimes Gto G, m(x,y)=xy$ satisfies $m(xN,yN)subseteq xyN$. Any open set containing $xy$ contains the coset $xyN$, so continuity of $m$ follows. Continuity of the inverse mapping is verified in the same way.




    Seems to me that you really wanted to ask a slightly different question. I'm afraid I cannot suggest one. Anwyway, it seems morally certain to me that the topology $G$ needs to have to satisfy items 1 to 3 is not unique at all.






    share|cite|improve this answer









    $endgroup$


















      0












      $begingroup$

      Not saying anything about the Weil group at this point. Trying to settle the first question instead with the following construction that smacks of cheating.





      Let $i:H_1to G$ be the monomorphism, and $p:Gto H_2$ be the epimorphism in this short exact sequence. Exactness means that $N:=i(H_1)=ker(p)unlhd G$. If I got it right $H_1$ and $H_2$ are topological groups, and you want to give a topology to $G$ so that $G$ becomes a topological group, and the mappings $i,p$ are continuous.



      Unless I made a silly mistake this can be achieved by giving $G$ the topology defined by letting the cosets of $N$ to form a basis. In other words, the open sets of $G$ are the arbitrary unions of cosets of $N$. This is trivially a topology $tau$. Furthermore:





      1. $p:(G,tau)to H_2$ is continuous for much the same reason that any mapping from a discrete space to any topological space is continuous. The preimage of any subset of $H_2$ is a union of cosets of $N$.


      2. $i:H_1to(G,tau)$ is continuous for much the same reason that any map to a space with a trivial topology is continuous. The preimage of any open subset $intau$ is either all of $H_1$ or the empty set.


      3. $(G,tau)$ is a topological group. For if $x,yin G$ are arbitrary, then the product map $m:Gtimes Gto G, m(x,y)=xy$ satisfies $m(xN,yN)subseteq xyN$. Any open set containing $xy$ contains the coset $xyN$, so continuity of $m$ follows. Continuity of the inverse mapping is verified in the same way.




      Seems to me that you really wanted to ask a slightly different question. I'm afraid I cannot suggest one. Anwyway, it seems morally certain to me that the topology $G$ needs to have to satisfy items 1 to 3 is not unique at all.






      share|cite|improve this answer









      $endgroup$
















        0












        0








        0





        $begingroup$

        Not saying anything about the Weil group at this point. Trying to settle the first question instead with the following construction that smacks of cheating.





        Let $i:H_1to G$ be the monomorphism, and $p:Gto H_2$ be the epimorphism in this short exact sequence. Exactness means that $N:=i(H_1)=ker(p)unlhd G$. If I got it right $H_1$ and $H_2$ are topological groups, and you want to give a topology to $G$ so that $G$ becomes a topological group, and the mappings $i,p$ are continuous.



        Unless I made a silly mistake this can be achieved by giving $G$ the topology defined by letting the cosets of $N$ to form a basis. In other words, the open sets of $G$ are the arbitrary unions of cosets of $N$. This is trivially a topology $tau$. Furthermore:





        1. $p:(G,tau)to H_2$ is continuous for much the same reason that any mapping from a discrete space to any topological space is continuous. The preimage of any subset of $H_2$ is a union of cosets of $N$.


        2. $i:H_1to(G,tau)$ is continuous for much the same reason that any map to a space with a trivial topology is continuous. The preimage of any open subset $intau$ is either all of $H_1$ or the empty set.


        3. $(G,tau)$ is a topological group. For if $x,yin G$ are arbitrary, then the product map $m:Gtimes Gto G, m(x,y)=xy$ satisfies $m(xN,yN)subseteq xyN$. Any open set containing $xy$ contains the coset $xyN$, so continuity of $m$ follows. Continuity of the inverse mapping is verified in the same way.




        Seems to me that you really wanted to ask a slightly different question. I'm afraid I cannot suggest one. Anwyway, it seems morally certain to me that the topology $G$ needs to have to satisfy items 1 to 3 is not unique at all.






        share|cite|improve this answer









        $endgroup$



        Not saying anything about the Weil group at this point. Trying to settle the first question instead with the following construction that smacks of cheating.





        Let $i:H_1to G$ be the monomorphism, and $p:Gto H_2$ be the epimorphism in this short exact sequence. Exactness means that $N:=i(H_1)=ker(p)unlhd G$. If I got it right $H_1$ and $H_2$ are topological groups, and you want to give a topology to $G$ so that $G$ becomes a topological group, and the mappings $i,p$ are continuous.



        Unless I made a silly mistake this can be achieved by giving $G$ the topology defined by letting the cosets of $N$ to form a basis. In other words, the open sets of $G$ are the arbitrary unions of cosets of $N$. This is trivially a topology $tau$. Furthermore:





        1. $p:(G,tau)to H_2$ is continuous for much the same reason that any mapping from a discrete space to any topological space is continuous. The preimage of any subset of $H_2$ is a union of cosets of $N$.


        2. $i:H_1to(G,tau)$ is continuous for much the same reason that any map to a space with a trivial topology is continuous. The preimage of any open subset $intau$ is either all of $H_1$ or the empty set.


        3. $(G,tau)$ is a topological group. For if $x,yin G$ are arbitrary, then the product map $m:Gtimes Gto G, m(x,y)=xy$ satisfies $m(xN,yN)subseteq xyN$. Any open set containing $xy$ contains the coset $xyN$, so continuity of $m$ follows. Continuity of the inverse mapping is verified in the same way.




        Seems to me that you really wanted to ask a slightly different question. I'm afraid I cannot suggest one. Anwyway, it seems morally certain to me that the topology $G$ needs to have to satisfy items 1 to 3 is not unique at all.







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        answered Jan 6 at 19:07









        Jyrki LahtonenJyrki Lahtonen

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