Return to Answer

As KConrad points out, you perhaps mean to say that $F$ is a finite extension of $\mathbf{Q}_2$ or of $\mathbf{F}_2((x))$, and that the quadratic extesnions $E|F$ is separable (and hence galoisian) in the second case.

With this interpretation of the question, $N_{E|F}(E^\times)$ is a closed subgroup of index $2$ in $F^\times$, and every closed subgroup of index $2$ in $F^\times$ is of this form. In particular, the ramification index $e_{E|F}$ does not determine the subgroup in question.

For more on this, see the relevant chapter in Serre's Corps locaux (=Local fields) or the book by Fesenko and Vostokov, among many other places.

Addendum. In the same vein as David Speyer's example, it might also be instructive to work out the case $F=\mathbf{F}_2((x))$,by using "Artin-Schreier theory" instead of "Kummer theory". Cyclic quadratic extensions of $F$ correspond to $\mathbf{F}_2$-lines $D\subset F/\wp(F)$, where $\wp$ is the endomorhphism $t\mapsto t^2-t$ of the additive group of $F$. The quotient $F/\wp(F)$ carries a natural filtration, coming from the filtration on the additive group $F$, so every line $D$ has a level in this filtration. The ramification index of the cyclic quadratic extension $E_D=F(\wp^{-1}(D))$ of $F$ corresponding to $D$ depends only on this level.

It would be interesting to work out the norm subgroup $N_{E_D|F}(E_D^\times)\subset F^\times$ for each line $D$.

By the way, the Second Extended Edition of Fesenko-Vostokov is available online.

Addendum 2. See this answer for a generalisation from $p=2$ to arbitrary primes $p$.

As KConrad points out, you perhaps mean to say that $F$ is a finite extension of $\mathbf{Q}_2$ or of $\mathbf{F}_2((x))$, and that the quadratic extesnions $E|F$ is separable (and hence galoisian) in the second case.

With this interpretation of the question, $N_{E|F}(E^\times)$ is a closed subgroup of index $2$ in $F^\times$, and every closed subgroup of index $2$ in $F^\times$ is of this form. In particular, the ramification index $e_{E|F}$ does not determine the subgroup in question.

For more on this, see the relevant chapter in Serre's Corps locaux (=Local fields) or the book by Fesenko and Vostokov, among many other places.

Addendum. In the same vein as David Speyer's example, it might also be instructive to work out the case $F=\mathbf{F}_2((x))$,by using "Artin-Schreier theory" instead of "Kummer theory". Cyclic quadratic extensions of $F$ correspond to $\mathbf{F}_2$-lines $D\subset F/\wp(F)$, where $\wp$ is the endomorhphism $t\mapsto t^2-t$ of the additive group of $F$. The quotient $F/\wp(F)$ carries a natural filtration, coming from the filtration on the additive group $F$, so every line $D$ has a level in this filtration. The ramification index of the cyclic quadratic extension $E_D=F(\wp^{-1}(D))$ of $F$ corresponding to $D$ depends only on this level.

It would be interesting to work out the norm subgroup $N_{E_D|F}(E_D^\times)\subset F^\times$ for each line $D$.

By the way, the Second Extended Edition of Fesenko-Vostokov is available online.

Addendum 2. See this answer for a generalisation from $p=2$ to arbitrary primes $p$.

As KConrad points out, you perhaps mean to say that $F$ is a finite extension of $\mathbf{Q}_2$ or of $\mathbf{F}_2((x))$, and that the quadratic extesnions $E|F$ is separable (and hence galoisian) in the second case.

With this interpretation of the question, $N_{E|F}(E^\times)$ is a closed subgroup of index $2$ in $F^\times$, and every closed subgroup of index $2$ in $F^\times$ is of this form. In particular, the ramification index $e_{E|F}$ does not determine the subgroup in question.

For more on this, see the relevant chapter in Serre's Corps locaux (=Local fields) or the book by Fesenko and Vostokov, among many other places.

Addendum In the same vein as David Speyer's example, it might also be instructive to work out the case $F=\mathbf{F}_2((x))$,by using "Artin-Schreier theory" instead of "Kummer theory". Cyclic quadratic extensions of $F$ correspond to $\mathbf{F}_2$-lines $D\subset F/\wp(F)$, where $\wp$ is the endomorhphism $t\mapsto t^2-t$ of the additive group of $F$. The quotient $F/\wp(F)$ carries a natural filtration, coming from the filtration on the additive group $F$, so every line $D$ has a level in this filtration. The ramification index of the cyclic quadratic extension $E_D=F(\wp^{-1}(D)$ of $F$ corresponding to $D$ depends only on this level.

It would be interesting to work out the norm subgroup $N_{E_D|F}(E_D^\times)\subset F^\times$ for each line $D$.

By the way, the Second Extended Edition of Fesenko-Vostokov is available online.

As KConrad points out, you perhaps mean to say that $F$ is a finite extension of $\mathbf{Q}_2$ or of $\mathbf{F}_2((x))$, and that the quadratic extesnions $E|F$ is separable (and hence galoisian) in the second case.

With this interpretation of the question, $N_{E|F}(E^\times)$ is a closed subgroup of index $2$ in $F^\times$, and every closed subgroup of index $2$ in $F^\times$ is of this form. In particular, the ramification index $e_{E|F}$ does not determine the subgroup in question.

For more on this, see the relevant chapter in Serre's Corps locaux (=Local fields) or the book by Fesenko and Vostokov, among many other places.

Addendum. In the same vein as David Speyer's example, it might also be instructive to work out the case $F=\mathbf{F}_2((x))$,by using "Artin-Schreier theory" instead of "Kummer theory". Cyclic quadratic extensions of $F$ correspond to $\mathbf{F}_2$-lines $D\subset F/\wp(F)$, where $\wp$ is the endomorhphism $t\mapsto t^2-t$ of the additive group of $F$. The quotient $F/\wp(F)$ carries a natural filtration, coming from the filtration on the additive group $F$, so every line $D$ has a level in this filtration. The ramification index of the cyclic quadratic extension $E_D=F(\wp^{-1}(D))$ of $F$ corresponding to $D$ depends only on this level.

It would be interesting to work out the norm subgroup $N_{E_D|F}(E_D^\times)\subset F^\times$ for each line $D$.

By the way, the Second Extended Edition of Fesenko-Vostokov is available online.

Addendum 2. See this answer for a generalisation from $p=2$ to arbitrary primes $p$.

As KConrad points out, you perhaps mean to say that $F$ is a finite extension of $\mathbf{Q}_2$ or of $\mathbf{F}_2((x))$, and that the quadratic extesnions $E|F$ is separable (and hence galoisian) in the second case.

With this interpretation of the question, $N_{E|F}(E^\times)$ is a closed subgroup of index $2$ in $F^\times$, and every closed subgroup of index $2$ in $F^\times$ is of this form. In particular, the ramification index $e_{E|F}$ does not determine the subgroup in question.

For more on this, see the relevant chapter in Serre's Corps locaux (=Local fields) or the book by Fesenko and Vostokov, among many other places.

Addendum In the same vein as David Speyer's example, it might also be instructive to work out the case $F=\mathbf{F}_2((x))$,by using "Artin-Schreier theory" instead of "Kummer theory". Cyclic quadratic extensions of $F$ correspond to $\mathbf{F}_2$-lines $D\subset F/\wp(F)$, where $\wp$ is the endomorhphism $t\mapsto t^2-t$ of the additive group of $F$. The quotient $F/wp(F)$ carries a natural filtration, coming from the filtration on the additive group $F$, so every line $D$ has a level in this filtration. The ramification index of the cyclic quadratic extension $E_D=F(\wp^{-1}(D)$ of $F$ corresponding to $D$ depends only on this level.

It would be interesting to work out the norm subgroup of $N_{E_D|F}(E_D^\times)\subset F^\times$ for each line $D$.

By the way, the Second Extended Edition of Fesenko-Vostokov is available online.

As KConrad points out, you perhaps mean to say that $F$ is a finite extension of $\mathbf{Q}_2$ or of $\mathbf{F}_2((x))$, and that the quadratic extesnions $E|F$ is separable (and hence galoisian) in the second case.

With this interpretation of the question, $N_{E|F}(E^\times)$ is a closed subgroup of index $2$ in $F^\times$, and every closed subgroup of index $2$ in $F^\times$ is of this form. In particular, the ramification index $e_{E|F}$ does not determine the subgroup in question.

For more on this, see the relevant chapter in Serre's Corps locaux (=Local fields) or the book by Fesenko and Vostokov, among many other places.

Addendum In the same vein as David Speyer's example, it might also be instructive to work out the case $F=\mathbf{F}_2((x))$,by using "Artin-Schreier theory" instead of "Kummer theory". Cyclic quadratic extensions of $F$ correspond to $\mathbf{F}_2$-lines $D\subset F/\wp(F)$, where $\wp$ is the endomorhphism $t\mapsto t^2-t$ of the additive group of $F$. The quotient $F/\wp(F)$ carries a natural filtration, coming from the filtration on the additive group $F$, so every line $D$ has a level in this filtration. The ramification index of the cyclic quadratic extension $E_D=F(\wp^{-1}(D)$ of $F$ corresponding to $D$ depends only on this level.

It would be interesting to work out the norm subgroup $N_{E_D|F}(E_D^\times)\subset F^\times$ for each line $D$.

By the way, the Second Extended Edition of Fesenko-Vostokov is available online.