Are all extensions of finite fields cyclic?












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My book says all extensions of finite fields are cyclic, but I could not find a proof (maybe I haven't looked hard enough). If it's straightforward, can you tell me why it's true? Thanks :)










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    Please make the body of your messages self-contained. "My books says this is true" is not a particularly informative first sentence. Titles/subject lines are no more part of the content of the message than the publisher's imprint on the spine of the book is part of the novel.
    $endgroup$
    – Arturo Magidin
    May 10 '11 at 16:05
















13












$begingroup$


My book says all extensions of finite fields are cyclic, but I could not find a proof (maybe I haven't looked hard enough). If it's straightforward, can you tell me why it's true? Thanks :)










share|cite|improve this question











$endgroup$








  • 5




    $begingroup$
    Please make the body of your messages self-contained. "My books says this is true" is not a particularly informative first sentence. Titles/subject lines are no more part of the content of the message than the publisher's imprint on the spine of the book is part of the novel.
    $endgroup$
    – Arturo Magidin
    May 10 '11 at 16:05














13












13








13


8



$begingroup$


My book says all extensions of finite fields are cyclic, but I could not find a proof (maybe I haven't looked hard enough). If it's straightforward, can you tell me why it's true? Thanks :)










share|cite|improve this question











$endgroup$




My book says all extensions of finite fields are cyclic, but I could not find a proof (maybe I haven't looked hard enough). If it's straightforward, can you tell me why it's true? Thanks :)







abstract-algebra






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edited May 10 '11 at 16:03









Arturo Magidin

264k34587915




264k34587915










asked May 10 '11 at 5:23









badatmathbadatmath

1,92932042




1,92932042








  • 5




    $begingroup$
    Please make the body of your messages self-contained. "My books says this is true" is not a particularly informative first sentence. Titles/subject lines are no more part of the content of the message than the publisher's imprint on the spine of the book is part of the novel.
    $endgroup$
    – Arturo Magidin
    May 10 '11 at 16:05














  • 5




    $begingroup$
    Please make the body of your messages self-contained. "My books says this is true" is not a particularly informative first sentence. Titles/subject lines are no more part of the content of the message than the publisher's imprint on the spine of the book is part of the novel.
    $endgroup$
    – Arturo Magidin
    May 10 '11 at 16:05








5




5




$begingroup$
Please make the body of your messages self-contained. "My books says this is true" is not a particularly informative first sentence. Titles/subject lines are no more part of the content of the message than the publisher's imprint on the spine of the book is part of the novel.
$endgroup$
– Arturo Magidin
May 10 '11 at 16:05




$begingroup$
Please make the body of your messages self-contained. "My books says this is true" is not a particularly informative first sentence. Titles/subject lines are no more part of the content of the message than the publisher's imprint on the spine of the book is part of the novel.
$endgroup$
– Arturo Magidin
May 10 '11 at 16:05










3 Answers
3






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15












$begingroup$

I'm not sure that the cyclicity of the unit group of a finite field is being used here. (Not that I have any problem using it: see e.g. Section 2 of these notes for a proof.)



Let $K/mathbb{F}_q$ be a field extension of degree $n$, so $# K = q^n$. Let $sigma: K rightarrow K$ be $x mapsto x^q$. Recall:




(Lagrange's Little Theorem): Let $G$ be a finite abelian group of order $n$ and
$g in G$. Then the order of $g$ divides $n$.



The point is that LLT is a special case of Lagrange's Theorem which can be proved by the same argument which proves Fermat's Little Theorem -- i.e., the special case in which $G = mathbb{F}_p^{times}$. So one need not talk about cosets and such...




Let $x in mathbb{F}_q$. I claim that $x^q = x$. This is clear if $x = 0$, and otherwise apply LLT to $x in mathbb{F}_q^{times}$ to get $x^{q-1} = 1$, which implies $x^q = x$.



Therefore $sigma$ is an automorphism of $K/mathbb{F}_q$. As for its order, suppose
$sigma^i$ is equal to the identity: that is, for all $x in K$, $x^{q^i} = x$. We have $P_i(t) = t^{q^i} - t in K[t]$ is a polynomial of degree $q^i$ over the field $K$, so by the Root-Factor Theorem (a consequence of the division algorithm for polynomials), $P_i(t)$ has at most $q^i$ roots. It follows that the order of $sigma$ is equal to $n = log_q(# K)$. Thus the cyclic group generated by $sigma$ is a degree $n$ subgroup of $operatorname{Aut}(K/mathbb{F}_q)$. But by basic Galois theory, for an extension $K/F$ of degree $n$, we have $# operatorname{Aut}(K/F) leq n$, with equality holding if and only if $K/F$ is Galois. Therefore $K/mathbb{F}_q$
is a cyclic Galois extension.



If we like, we can now establish that there is a unique cyclic extension of degree $n$ for any $n in mathbb{Z}^+$: we can take
the splitting field of $t^{q^n} - t$, and splitting fields exist and are unique up to (nonunique) isomorphism over the ground field.



It seems to me that I have not used the cyclicity of $mathbb{F}_q^{times}$ anywhere...






share|cite|improve this answer











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  • 5




    $begingroup$
    sorry for the offtopic nonsense but I think it's great you came back..
    $endgroup$
    – quanta
    May 10 '11 at 19:51






  • 4




    $begingroup$
    @quanta: thanks. Let's see if we can avoid making a big deal out of it...
    $endgroup$
    – Pete L. Clark
    May 10 '11 at 19:54






  • 1




    $begingroup$
    @PeteL.Clark : $G$ need not be abelian for FLT to hold , right ?
    $endgroup$
    – Theorem
    Feb 8 '13 at 14:54










  • $begingroup$
    @Theorem: I'm not sure what "FLT" stands for here. Definitely Lagrange's Theorem holds for all finite groups. But the proof which generalizes the standard proof of Fermat's Little Theorem (hmm, FLT?) works only for finite abelian groups. So far as I know, in order to prove Lagrange's Theorem in the nonabelian case one needs to discuss cosets.
    $endgroup$
    – Pete L. Clark
    Feb 8 '13 at 17:21



















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Any finite extension of a finite field $mathbb{F}_q$ is cyclic. For such an extension $K$ first recall that the Frobenius map $x mapsto x^q$ is an $mathbb{F}_q$-linear endomorphism. If $x^q = y^q$ then $(x - y)^q = 0$, hence $x = y$, so the Frobenius map is injective. Since it is an injective linear map from a finite-dimensional vector space to itself, it is surjective, so it is an automorphism. Its fixed field is the subfield of roots of $x^q - x$, which are precisely the elements of the base field $mathbb{F}_q$. It follows that $K$ is Galois with Galois group the cyclic group generated by $x mapsto x^q$.



(I guess when I say $mathbb{F}_q$ I am being mildly circular. Interpret the above proof as follows: any finite extension of $mathbb{F}_p$ is cyclic, and in fact the above proof shows that they are all of the form $mathbb{F}_{p^n}$ using the fact that any finite subgroup of the multiplicative group of a field is cyclic, so finite fields $mathbb{F}_q$ really do have Frobenius maps like I just claimed they do, and then apply the proof again to $mathbb{F}_q$.)






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  • $begingroup$
    How about algebraic but infinite extensions of a finite field ?
    $endgroup$
    – Fardad Pouran
    Nov 28 '14 at 22:11






  • 1




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    @Fardad: they are "topologically cyclic" in the sense that the subgroup of the Galois group generated by the Frobenius map is dense.
    $endgroup$
    – Qiaochu Yuan
    Nov 28 '14 at 22:20



















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Below is a complete, noncircular simple proof of the result mentioned by Qiaochu that avoids invoking the high-powered structure theorem for finite abelian groups.



Theorem $ $ A finite subgroup $rm:G:$ of the multiplicative group of a field is cyclic.



Proof $ $ The proposition below yields, with $rm,m = maxord(G) = expt(G),,$ that $rm, x^m = 1,$ has $rm:#G:$ roots. Since a polynomial $rm:f:$ over a field satisfies $rm:#roots f le deg f:$ we infer that $rm: #G le m.:$ But maxorder $rm:m le #G:$ since $rm:g^{#G} = 1:$ for all $rm:g in G:$ (Lagrange). $:$ Thus $rm:m = #G = maxord(G),:$ therefore $rm:G:$ has an element of order $rm#G,:$ hence $rm:G:$ is cyclic.



$begin{eqnarray}rm{bf Proposition}qquad maxord(G) &=&,rm expt(G) text{ for a finite abelian group} G, i.e.\[.5em]
rm max { ord(g) : : g in G} &=&,rm min { n>0 : : g^n = 1 forall g in G}end{eqnarray}$



Proof $ $ By the lemma below, $rm: S, =, { ord(g) : :g in G }$ is a finite
set of naturals closed under$rm lcm$.



Hence every $rm s in S:$ is a
divisor of the max elt $rm: m $ [else $rm: lcm(s,m) > m,$],$ $ so $rm m = expt(G)$.



Lemma $ $ A finite abelian group $rm:G:$ has an lcm-closed order set, i.e. with $rm: o(X) = $ order of $rm, X$



$$rm X,Y in G Rightarrow exists Z in G: o(Z) = lcm(o(X),o(Y))$$



Proof $ $ By induction on $rm: o(X), o(Y). $ If it's $:1:$ then trivially $rm:Z = 1$. $ $ Otherwise



write $rm, o(X), =: AP,: o(Y) = BP', P'mid P = p^m > 1, $ prime $rm: p:$ coprime to $rm: A,B.$



Then $rm, o(X^P) = A, o(Y^{P'}) = B. $ By induction there's a $rm: Z:$ with $rm : o(Z) = lcm(A,B)$



thus $rm, o(X^A Z) = P lcm(A,B) = lcm(AP,BP') = lcm(o(X),o(Y)).$






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  • 3




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    I think the OP was asking if the (finite) extension $K/mathbb{F}_q$ is a cyclic extension, namely that $mbox{Gal}(K/mathbb{F}_q)$ is a cyclic group. Does this immediately follow from the fact that $K^times$ is a cyclic group?
    $endgroup$
    – Alon Amit
    May 10 '11 at 16:15










  • $begingroup$
    @Alon I've edited it to clarify that it was meant to support Qiaochu's answer.
    $endgroup$
    – Bill Dubuque
    May 10 '11 at 18:04






  • 5




    $begingroup$
    Sadly, it looks like this nice answer is destined to be one of the gems at the tail end of the voting range. To work against that, I'm voting it up.
    $endgroup$
    – t.b.
    May 10 '11 at 19:02











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3 Answers
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3 Answers
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active

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active

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15












$begingroup$

I'm not sure that the cyclicity of the unit group of a finite field is being used here. (Not that I have any problem using it: see e.g. Section 2 of these notes for a proof.)



Let $K/mathbb{F}_q$ be a field extension of degree $n$, so $# K = q^n$. Let $sigma: K rightarrow K$ be $x mapsto x^q$. Recall:




(Lagrange's Little Theorem): Let $G$ be a finite abelian group of order $n$ and
$g in G$. Then the order of $g$ divides $n$.



The point is that LLT is a special case of Lagrange's Theorem which can be proved by the same argument which proves Fermat's Little Theorem -- i.e., the special case in which $G = mathbb{F}_p^{times}$. So one need not talk about cosets and such...




Let $x in mathbb{F}_q$. I claim that $x^q = x$. This is clear if $x = 0$, and otherwise apply LLT to $x in mathbb{F}_q^{times}$ to get $x^{q-1} = 1$, which implies $x^q = x$.



Therefore $sigma$ is an automorphism of $K/mathbb{F}_q$. As for its order, suppose
$sigma^i$ is equal to the identity: that is, for all $x in K$, $x^{q^i} = x$. We have $P_i(t) = t^{q^i} - t in K[t]$ is a polynomial of degree $q^i$ over the field $K$, so by the Root-Factor Theorem (a consequence of the division algorithm for polynomials), $P_i(t)$ has at most $q^i$ roots. It follows that the order of $sigma$ is equal to $n = log_q(# K)$. Thus the cyclic group generated by $sigma$ is a degree $n$ subgroup of $operatorname{Aut}(K/mathbb{F}_q)$. But by basic Galois theory, for an extension $K/F$ of degree $n$, we have $# operatorname{Aut}(K/F) leq n$, with equality holding if and only if $K/F$ is Galois. Therefore $K/mathbb{F}_q$
is a cyclic Galois extension.



If we like, we can now establish that there is a unique cyclic extension of degree $n$ for any $n in mathbb{Z}^+$: we can take
the splitting field of $t^{q^n} - t$, and splitting fields exist and are unique up to (nonunique) isomorphism over the ground field.



It seems to me that I have not used the cyclicity of $mathbb{F}_q^{times}$ anywhere...






share|cite|improve this answer











$endgroup$









  • 5




    $begingroup$
    sorry for the offtopic nonsense but I think it's great you came back..
    $endgroup$
    – quanta
    May 10 '11 at 19:51






  • 4




    $begingroup$
    @quanta: thanks. Let's see if we can avoid making a big deal out of it...
    $endgroup$
    – Pete L. Clark
    May 10 '11 at 19:54






  • 1




    $begingroup$
    @PeteL.Clark : $G$ need not be abelian for FLT to hold , right ?
    $endgroup$
    – Theorem
    Feb 8 '13 at 14:54










  • $begingroup$
    @Theorem: I'm not sure what "FLT" stands for here. Definitely Lagrange's Theorem holds for all finite groups. But the proof which generalizes the standard proof of Fermat's Little Theorem (hmm, FLT?) works only for finite abelian groups. So far as I know, in order to prove Lagrange's Theorem in the nonabelian case one needs to discuss cosets.
    $endgroup$
    – Pete L. Clark
    Feb 8 '13 at 17:21
















15












$begingroup$

I'm not sure that the cyclicity of the unit group of a finite field is being used here. (Not that I have any problem using it: see e.g. Section 2 of these notes for a proof.)



Let $K/mathbb{F}_q$ be a field extension of degree $n$, so $# K = q^n$. Let $sigma: K rightarrow K$ be $x mapsto x^q$. Recall:




(Lagrange's Little Theorem): Let $G$ be a finite abelian group of order $n$ and
$g in G$. Then the order of $g$ divides $n$.



The point is that LLT is a special case of Lagrange's Theorem which can be proved by the same argument which proves Fermat's Little Theorem -- i.e., the special case in which $G = mathbb{F}_p^{times}$. So one need not talk about cosets and such...




Let $x in mathbb{F}_q$. I claim that $x^q = x$. This is clear if $x = 0$, and otherwise apply LLT to $x in mathbb{F}_q^{times}$ to get $x^{q-1} = 1$, which implies $x^q = x$.



Therefore $sigma$ is an automorphism of $K/mathbb{F}_q$. As for its order, suppose
$sigma^i$ is equal to the identity: that is, for all $x in K$, $x^{q^i} = x$. We have $P_i(t) = t^{q^i} - t in K[t]$ is a polynomial of degree $q^i$ over the field $K$, so by the Root-Factor Theorem (a consequence of the division algorithm for polynomials), $P_i(t)$ has at most $q^i$ roots. It follows that the order of $sigma$ is equal to $n = log_q(# K)$. Thus the cyclic group generated by $sigma$ is a degree $n$ subgroup of $operatorname{Aut}(K/mathbb{F}_q)$. But by basic Galois theory, for an extension $K/F$ of degree $n$, we have $# operatorname{Aut}(K/F) leq n$, with equality holding if and only if $K/F$ is Galois. Therefore $K/mathbb{F}_q$
is a cyclic Galois extension.



If we like, we can now establish that there is a unique cyclic extension of degree $n$ for any $n in mathbb{Z}^+$: we can take
the splitting field of $t^{q^n} - t$, and splitting fields exist and are unique up to (nonunique) isomorphism over the ground field.



It seems to me that I have not used the cyclicity of $mathbb{F}_q^{times}$ anywhere...






share|cite|improve this answer











$endgroup$









  • 5




    $begingroup$
    sorry for the offtopic nonsense but I think it's great you came back..
    $endgroup$
    – quanta
    May 10 '11 at 19:51






  • 4




    $begingroup$
    @quanta: thanks. Let's see if we can avoid making a big deal out of it...
    $endgroup$
    – Pete L. Clark
    May 10 '11 at 19:54






  • 1




    $begingroup$
    @PeteL.Clark : $G$ need not be abelian for FLT to hold , right ?
    $endgroup$
    – Theorem
    Feb 8 '13 at 14:54










  • $begingroup$
    @Theorem: I'm not sure what "FLT" stands for here. Definitely Lagrange's Theorem holds for all finite groups. But the proof which generalizes the standard proof of Fermat's Little Theorem (hmm, FLT?) works only for finite abelian groups. So far as I know, in order to prove Lagrange's Theorem in the nonabelian case one needs to discuss cosets.
    $endgroup$
    – Pete L. Clark
    Feb 8 '13 at 17:21














15












15








15





$begingroup$

I'm not sure that the cyclicity of the unit group of a finite field is being used here. (Not that I have any problem using it: see e.g. Section 2 of these notes for a proof.)



Let $K/mathbb{F}_q$ be a field extension of degree $n$, so $# K = q^n$. Let $sigma: K rightarrow K$ be $x mapsto x^q$. Recall:




(Lagrange's Little Theorem): Let $G$ be a finite abelian group of order $n$ and
$g in G$. Then the order of $g$ divides $n$.



The point is that LLT is a special case of Lagrange's Theorem which can be proved by the same argument which proves Fermat's Little Theorem -- i.e., the special case in which $G = mathbb{F}_p^{times}$. So one need not talk about cosets and such...




Let $x in mathbb{F}_q$. I claim that $x^q = x$. This is clear if $x = 0$, and otherwise apply LLT to $x in mathbb{F}_q^{times}$ to get $x^{q-1} = 1$, which implies $x^q = x$.



Therefore $sigma$ is an automorphism of $K/mathbb{F}_q$. As for its order, suppose
$sigma^i$ is equal to the identity: that is, for all $x in K$, $x^{q^i} = x$. We have $P_i(t) = t^{q^i} - t in K[t]$ is a polynomial of degree $q^i$ over the field $K$, so by the Root-Factor Theorem (a consequence of the division algorithm for polynomials), $P_i(t)$ has at most $q^i$ roots. It follows that the order of $sigma$ is equal to $n = log_q(# K)$. Thus the cyclic group generated by $sigma$ is a degree $n$ subgroup of $operatorname{Aut}(K/mathbb{F}_q)$. But by basic Galois theory, for an extension $K/F$ of degree $n$, we have $# operatorname{Aut}(K/F) leq n$, with equality holding if and only if $K/F$ is Galois. Therefore $K/mathbb{F}_q$
is a cyclic Galois extension.



If we like, we can now establish that there is a unique cyclic extension of degree $n$ for any $n in mathbb{Z}^+$: we can take
the splitting field of $t^{q^n} - t$, and splitting fields exist and are unique up to (nonunique) isomorphism over the ground field.



It seems to me that I have not used the cyclicity of $mathbb{F}_q^{times}$ anywhere...






share|cite|improve this answer











$endgroup$



I'm not sure that the cyclicity of the unit group of a finite field is being used here. (Not that I have any problem using it: see e.g. Section 2 of these notes for a proof.)



Let $K/mathbb{F}_q$ be a field extension of degree $n$, so $# K = q^n$. Let $sigma: K rightarrow K$ be $x mapsto x^q$. Recall:




(Lagrange's Little Theorem): Let $G$ be a finite abelian group of order $n$ and
$g in G$. Then the order of $g$ divides $n$.



The point is that LLT is a special case of Lagrange's Theorem which can be proved by the same argument which proves Fermat's Little Theorem -- i.e., the special case in which $G = mathbb{F}_p^{times}$. So one need not talk about cosets and such...




Let $x in mathbb{F}_q$. I claim that $x^q = x$. This is clear if $x = 0$, and otherwise apply LLT to $x in mathbb{F}_q^{times}$ to get $x^{q-1} = 1$, which implies $x^q = x$.



Therefore $sigma$ is an automorphism of $K/mathbb{F}_q$. As for its order, suppose
$sigma^i$ is equal to the identity: that is, for all $x in K$, $x^{q^i} = x$. We have $P_i(t) = t^{q^i} - t in K[t]$ is a polynomial of degree $q^i$ over the field $K$, so by the Root-Factor Theorem (a consequence of the division algorithm for polynomials), $P_i(t)$ has at most $q^i$ roots. It follows that the order of $sigma$ is equal to $n = log_q(# K)$. Thus the cyclic group generated by $sigma$ is a degree $n$ subgroup of $operatorname{Aut}(K/mathbb{F}_q)$. But by basic Galois theory, for an extension $K/F$ of degree $n$, we have $# operatorname{Aut}(K/F) leq n$, with equality holding if and only if $K/F$ is Galois. Therefore $K/mathbb{F}_q$
is a cyclic Galois extension.



If we like, we can now establish that there is a unique cyclic extension of degree $n$ for any $n in mathbb{Z}^+$: we can take
the splitting field of $t^{q^n} - t$, and splitting fields exist and are unique up to (nonunique) isomorphism over the ground field.



It seems to me that I have not used the cyclicity of $mathbb{F}_q^{times}$ anywhere...







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Dec 8 '16 at 2:09









Martin Sleziak

44.7k10119272




44.7k10119272










answered May 10 '11 at 19:44









Pete L. ClarkPete L. Clark

80.6k9161312




80.6k9161312








  • 5




    $begingroup$
    sorry for the offtopic nonsense but I think it's great you came back..
    $endgroup$
    – quanta
    May 10 '11 at 19:51






  • 4




    $begingroup$
    @quanta: thanks. Let's see if we can avoid making a big deal out of it...
    $endgroup$
    – Pete L. Clark
    May 10 '11 at 19:54






  • 1




    $begingroup$
    @PeteL.Clark : $G$ need not be abelian for FLT to hold , right ?
    $endgroup$
    – Theorem
    Feb 8 '13 at 14:54










  • $begingroup$
    @Theorem: I'm not sure what "FLT" stands for here. Definitely Lagrange's Theorem holds for all finite groups. But the proof which generalizes the standard proof of Fermat's Little Theorem (hmm, FLT?) works only for finite abelian groups. So far as I know, in order to prove Lagrange's Theorem in the nonabelian case one needs to discuss cosets.
    $endgroup$
    – Pete L. Clark
    Feb 8 '13 at 17:21














  • 5




    $begingroup$
    sorry for the offtopic nonsense but I think it's great you came back..
    $endgroup$
    – quanta
    May 10 '11 at 19:51






  • 4




    $begingroup$
    @quanta: thanks. Let's see if we can avoid making a big deal out of it...
    $endgroup$
    – Pete L. Clark
    May 10 '11 at 19:54






  • 1




    $begingroup$
    @PeteL.Clark : $G$ need not be abelian for FLT to hold , right ?
    $endgroup$
    – Theorem
    Feb 8 '13 at 14:54










  • $begingroup$
    @Theorem: I'm not sure what "FLT" stands for here. Definitely Lagrange's Theorem holds for all finite groups. But the proof which generalizes the standard proof of Fermat's Little Theorem (hmm, FLT?) works only for finite abelian groups. So far as I know, in order to prove Lagrange's Theorem in the nonabelian case one needs to discuss cosets.
    $endgroup$
    – Pete L. Clark
    Feb 8 '13 at 17:21








5




5




$begingroup$
sorry for the offtopic nonsense but I think it's great you came back..
$endgroup$
– quanta
May 10 '11 at 19:51




$begingroup$
sorry for the offtopic nonsense but I think it's great you came back..
$endgroup$
– quanta
May 10 '11 at 19:51




4




4




$begingroup$
@quanta: thanks. Let's see if we can avoid making a big deal out of it...
$endgroup$
– Pete L. Clark
May 10 '11 at 19:54




$begingroup$
@quanta: thanks. Let's see if we can avoid making a big deal out of it...
$endgroup$
– Pete L. Clark
May 10 '11 at 19:54




1




1




$begingroup$
@PeteL.Clark : $G$ need not be abelian for FLT to hold , right ?
$endgroup$
– Theorem
Feb 8 '13 at 14:54




$begingroup$
@PeteL.Clark : $G$ need not be abelian for FLT to hold , right ?
$endgroup$
– Theorem
Feb 8 '13 at 14:54












$begingroup$
@Theorem: I'm not sure what "FLT" stands for here. Definitely Lagrange's Theorem holds for all finite groups. But the proof which generalizes the standard proof of Fermat's Little Theorem (hmm, FLT?) works only for finite abelian groups. So far as I know, in order to prove Lagrange's Theorem in the nonabelian case one needs to discuss cosets.
$endgroup$
– Pete L. Clark
Feb 8 '13 at 17:21




$begingroup$
@Theorem: I'm not sure what "FLT" stands for here. Definitely Lagrange's Theorem holds for all finite groups. But the proof which generalizes the standard proof of Fermat's Little Theorem (hmm, FLT?) works only for finite abelian groups. So far as I know, in order to prove Lagrange's Theorem in the nonabelian case one needs to discuss cosets.
$endgroup$
– Pete L. Clark
Feb 8 '13 at 17:21











16












$begingroup$

Any finite extension of a finite field $mathbb{F}_q$ is cyclic. For such an extension $K$ first recall that the Frobenius map $x mapsto x^q$ is an $mathbb{F}_q$-linear endomorphism. If $x^q = y^q$ then $(x - y)^q = 0$, hence $x = y$, so the Frobenius map is injective. Since it is an injective linear map from a finite-dimensional vector space to itself, it is surjective, so it is an automorphism. Its fixed field is the subfield of roots of $x^q - x$, which are precisely the elements of the base field $mathbb{F}_q$. It follows that $K$ is Galois with Galois group the cyclic group generated by $x mapsto x^q$.



(I guess when I say $mathbb{F}_q$ I am being mildly circular. Interpret the above proof as follows: any finite extension of $mathbb{F}_p$ is cyclic, and in fact the above proof shows that they are all of the form $mathbb{F}_{p^n}$ using the fact that any finite subgroup of the multiplicative group of a field is cyclic, so finite fields $mathbb{F}_q$ really do have Frobenius maps like I just claimed they do, and then apply the proof again to $mathbb{F}_q$.)






share|cite|improve this answer









$endgroup$













  • $begingroup$
    How about algebraic but infinite extensions of a finite field ?
    $endgroup$
    – Fardad Pouran
    Nov 28 '14 at 22:11






  • 1




    $begingroup$
    @Fardad: they are "topologically cyclic" in the sense that the subgroup of the Galois group generated by the Frobenius map is dense.
    $endgroup$
    – Qiaochu Yuan
    Nov 28 '14 at 22:20
















16












$begingroup$

Any finite extension of a finite field $mathbb{F}_q$ is cyclic. For such an extension $K$ first recall that the Frobenius map $x mapsto x^q$ is an $mathbb{F}_q$-linear endomorphism. If $x^q = y^q$ then $(x - y)^q = 0$, hence $x = y$, so the Frobenius map is injective. Since it is an injective linear map from a finite-dimensional vector space to itself, it is surjective, so it is an automorphism. Its fixed field is the subfield of roots of $x^q - x$, which are precisely the elements of the base field $mathbb{F}_q$. It follows that $K$ is Galois with Galois group the cyclic group generated by $x mapsto x^q$.



(I guess when I say $mathbb{F}_q$ I am being mildly circular. Interpret the above proof as follows: any finite extension of $mathbb{F}_p$ is cyclic, and in fact the above proof shows that they are all of the form $mathbb{F}_{p^n}$ using the fact that any finite subgroup of the multiplicative group of a field is cyclic, so finite fields $mathbb{F}_q$ really do have Frobenius maps like I just claimed they do, and then apply the proof again to $mathbb{F}_q$.)






share|cite|improve this answer









$endgroup$













  • $begingroup$
    How about algebraic but infinite extensions of a finite field ?
    $endgroup$
    – Fardad Pouran
    Nov 28 '14 at 22:11






  • 1




    $begingroup$
    @Fardad: they are "topologically cyclic" in the sense that the subgroup of the Galois group generated by the Frobenius map is dense.
    $endgroup$
    – Qiaochu Yuan
    Nov 28 '14 at 22:20














16












16








16





$begingroup$

Any finite extension of a finite field $mathbb{F}_q$ is cyclic. For such an extension $K$ first recall that the Frobenius map $x mapsto x^q$ is an $mathbb{F}_q$-linear endomorphism. If $x^q = y^q$ then $(x - y)^q = 0$, hence $x = y$, so the Frobenius map is injective. Since it is an injective linear map from a finite-dimensional vector space to itself, it is surjective, so it is an automorphism. Its fixed field is the subfield of roots of $x^q - x$, which are precisely the elements of the base field $mathbb{F}_q$. It follows that $K$ is Galois with Galois group the cyclic group generated by $x mapsto x^q$.



(I guess when I say $mathbb{F}_q$ I am being mildly circular. Interpret the above proof as follows: any finite extension of $mathbb{F}_p$ is cyclic, and in fact the above proof shows that they are all of the form $mathbb{F}_{p^n}$ using the fact that any finite subgroup of the multiplicative group of a field is cyclic, so finite fields $mathbb{F}_q$ really do have Frobenius maps like I just claimed they do, and then apply the proof again to $mathbb{F}_q$.)






share|cite|improve this answer









$endgroup$



Any finite extension of a finite field $mathbb{F}_q$ is cyclic. For such an extension $K$ first recall that the Frobenius map $x mapsto x^q$ is an $mathbb{F}_q$-linear endomorphism. If $x^q = y^q$ then $(x - y)^q = 0$, hence $x = y$, so the Frobenius map is injective. Since it is an injective linear map from a finite-dimensional vector space to itself, it is surjective, so it is an automorphism. Its fixed field is the subfield of roots of $x^q - x$, which are precisely the elements of the base field $mathbb{F}_q$. It follows that $K$ is Galois with Galois group the cyclic group generated by $x mapsto x^q$.



(I guess when I say $mathbb{F}_q$ I am being mildly circular. Interpret the above proof as follows: any finite extension of $mathbb{F}_p$ is cyclic, and in fact the above proof shows that they are all of the form $mathbb{F}_{p^n}$ using the fact that any finite subgroup of the multiplicative group of a field is cyclic, so finite fields $mathbb{F}_q$ really do have Frobenius maps like I just claimed they do, and then apply the proof again to $mathbb{F}_q$.)







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered May 10 '11 at 5:49









Qiaochu YuanQiaochu Yuan

279k32590935




279k32590935












  • $begingroup$
    How about algebraic but infinite extensions of a finite field ?
    $endgroup$
    – Fardad Pouran
    Nov 28 '14 at 22:11






  • 1




    $begingroup$
    @Fardad: they are "topologically cyclic" in the sense that the subgroup of the Galois group generated by the Frobenius map is dense.
    $endgroup$
    – Qiaochu Yuan
    Nov 28 '14 at 22:20


















  • $begingroup$
    How about algebraic but infinite extensions of a finite field ?
    $endgroup$
    – Fardad Pouran
    Nov 28 '14 at 22:11






  • 1




    $begingroup$
    @Fardad: they are "topologically cyclic" in the sense that the subgroup of the Galois group generated by the Frobenius map is dense.
    $endgroup$
    – Qiaochu Yuan
    Nov 28 '14 at 22:20
















$begingroup$
How about algebraic but infinite extensions of a finite field ?
$endgroup$
– Fardad Pouran
Nov 28 '14 at 22:11




$begingroup$
How about algebraic but infinite extensions of a finite field ?
$endgroup$
– Fardad Pouran
Nov 28 '14 at 22:11




1




1




$begingroup$
@Fardad: they are "topologically cyclic" in the sense that the subgroup of the Galois group generated by the Frobenius map is dense.
$endgroup$
– Qiaochu Yuan
Nov 28 '14 at 22:20




$begingroup$
@Fardad: they are "topologically cyclic" in the sense that the subgroup of the Galois group generated by the Frobenius map is dense.
$endgroup$
– Qiaochu Yuan
Nov 28 '14 at 22:20











14












$begingroup$

Below is a complete, noncircular simple proof of the result mentioned by Qiaochu that avoids invoking the high-powered structure theorem for finite abelian groups.



Theorem $ $ A finite subgroup $rm:G:$ of the multiplicative group of a field is cyclic.



Proof $ $ The proposition below yields, with $rm,m = maxord(G) = expt(G),,$ that $rm, x^m = 1,$ has $rm:#G:$ roots. Since a polynomial $rm:f:$ over a field satisfies $rm:#roots f le deg f:$ we infer that $rm: #G le m.:$ But maxorder $rm:m le #G:$ since $rm:g^{#G} = 1:$ for all $rm:g in G:$ (Lagrange). $:$ Thus $rm:m = #G = maxord(G),:$ therefore $rm:G:$ has an element of order $rm#G,:$ hence $rm:G:$ is cyclic.



$begin{eqnarray}rm{bf Proposition}qquad maxord(G) &=&,rm expt(G) text{ for a finite abelian group} G, i.e.\[.5em]
rm max { ord(g) : : g in G} &=&,rm min { n>0 : : g^n = 1 forall g in G}end{eqnarray}$



Proof $ $ By the lemma below, $rm: S, =, { ord(g) : :g in G }$ is a finite
set of naturals closed under$rm lcm$.



Hence every $rm s in S:$ is a
divisor of the max elt $rm: m $ [else $rm: lcm(s,m) > m,$],$ $ so $rm m = expt(G)$.



Lemma $ $ A finite abelian group $rm:G:$ has an lcm-closed order set, i.e. with $rm: o(X) = $ order of $rm, X$



$$rm X,Y in G Rightarrow exists Z in G: o(Z) = lcm(o(X),o(Y))$$



Proof $ $ By induction on $rm: o(X), o(Y). $ If it's $:1:$ then trivially $rm:Z = 1$. $ $ Otherwise



write $rm, o(X), =: AP,: o(Y) = BP', P'mid P = p^m > 1, $ prime $rm: p:$ coprime to $rm: A,B.$



Then $rm, o(X^P) = A, o(Y^{P'}) = B. $ By induction there's a $rm: Z:$ with $rm : o(Z) = lcm(A,B)$



thus $rm, o(X^A Z) = P lcm(A,B) = lcm(AP,BP') = lcm(o(X),o(Y)).$






share|cite|improve this answer











$endgroup$









  • 3




    $begingroup$
    I think the OP was asking if the (finite) extension $K/mathbb{F}_q$ is a cyclic extension, namely that $mbox{Gal}(K/mathbb{F}_q)$ is a cyclic group. Does this immediately follow from the fact that $K^times$ is a cyclic group?
    $endgroup$
    – Alon Amit
    May 10 '11 at 16:15










  • $begingroup$
    @Alon I've edited it to clarify that it was meant to support Qiaochu's answer.
    $endgroup$
    – Bill Dubuque
    May 10 '11 at 18:04






  • 5




    $begingroup$
    Sadly, it looks like this nice answer is destined to be one of the gems at the tail end of the voting range. To work against that, I'm voting it up.
    $endgroup$
    – t.b.
    May 10 '11 at 19:02
















14












$begingroup$

Below is a complete, noncircular simple proof of the result mentioned by Qiaochu that avoids invoking the high-powered structure theorem for finite abelian groups.



Theorem $ $ A finite subgroup $rm:G:$ of the multiplicative group of a field is cyclic.



Proof $ $ The proposition below yields, with $rm,m = maxord(G) = expt(G),,$ that $rm, x^m = 1,$ has $rm:#G:$ roots. Since a polynomial $rm:f:$ over a field satisfies $rm:#roots f le deg f:$ we infer that $rm: #G le m.:$ But maxorder $rm:m le #G:$ since $rm:g^{#G} = 1:$ for all $rm:g in G:$ (Lagrange). $:$ Thus $rm:m = #G = maxord(G),:$ therefore $rm:G:$ has an element of order $rm#G,:$ hence $rm:G:$ is cyclic.



$begin{eqnarray}rm{bf Proposition}qquad maxord(G) &=&,rm expt(G) text{ for a finite abelian group} G, i.e.\[.5em]
rm max { ord(g) : : g in G} &=&,rm min { n>0 : : g^n = 1 forall g in G}end{eqnarray}$



Proof $ $ By the lemma below, $rm: S, =, { ord(g) : :g in G }$ is a finite
set of naturals closed under$rm lcm$.



Hence every $rm s in S:$ is a
divisor of the max elt $rm: m $ [else $rm: lcm(s,m) > m,$],$ $ so $rm m = expt(G)$.



Lemma $ $ A finite abelian group $rm:G:$ has an lcm-closed order set, i.e. with $rm: o(X) = $ order of $rm, X$



$$rm X,Y in G Rightarrow exists Z in G: o(Z) = lcm(o(X),o(Y))$$



Proof $ $ By induction on $rm: o(X), o(Y). $ If it's $:1:$ then trivially $rm:Z = 1$. $ $ Otherwise



write $rm, o(X), =: AP,: o(Y) = BP', P'mid P = p^m > 1, $ prime $rm: p:$ coprime to $rm: A,B.$



Then $rm, o(X^P) = A, o(Y^{P'}) = B. $ By induction there's a $rm: Z:$ with $rm : o(Z) = lcm(A,B)$



thus $rm, o(X^A Z) = P lcm(A,B) = lcm(AP,BP') = lcm(o(X),o(Y)).$






share|cite|improve this answer











$endgroup$









  • 3




    $begingroup$
    I think the OP was asking if the (finite) extension $K/mathbb{F}_q$ is a cyclic extension, namely that $mbox{Gal}(K/mathbb{F}_q)$ is a cyclic group. Does this immediately follow from the fact that $K^times$ is a cyclic group?
    $endgroup$
    – Alon Amit
    May 10 '11 at 16:15










  • $begingroup$
    @Alon I've edited it to clarify that it was meant to support Qiaochu's answer.
    $endgroup$
    – Bill Dubuque
    May 10 '11 at 18:04






  • 5




    $begingroup$
    Sadly, it looks like this nice answer is destined to be one of the gems at the tail end of the voting range. To work against that, I'm voting it up.
    $endgroup$
    – t.b.
    May 10 '11 at 19:02














14












14








14





$begingroup$

Below is a complete, noncircular simple proof of the result mentioned by Qiaochu that avoids invoking the high-powered structure theorem for finite abelian groups.



Theorem $ $ A finite subgroup $rm:G:$ of the multiplicative group of a field is cyclic.



Proof $ $ The proposition below yields, with $rm,m = maxord(G) = expt(G),,$ that $rm, x^m = 1,$ has $rm:#G:$ roots. Since a polynomial $rm:f:$ over a field satisfies $rm:#roots f le deg f:$ we infer that $rm: #G le m.:$ But maxorder $rm:m le #G:$ since $rm:g^{#G} = 1:$ for all $rm:g in G:$ (Lagrange). $:$ Thus $rm:m = #G = maxord(G),:$ therefore $rm:G:$ has an element of order $rm#G,:$ hence $rm:G:$ is cyclic.



$begin{eqnarray}rm{bf Proposition}qquad maxord(G) &=&,rm expt(G) text{ for a finite abelian group} G, i.e.\[.5em]
rm max { ord(g) : : g in G} &=&,rm min { n>0 : : g^n = 1 forall g in G}end{eqnarray}$



Proof $ $ By the lemma below, $rm: S, =, { ord(g) : :g in G }$ is a finite
set of naturals closed under$rm lcm$.



Hence every $rm s in S:$ is a
divisor of the max elt $rm: m $ [else $rm: lcm(s,m) > m,$],$ $ so $rm m = expt(G)$.



Lemma $ $ A finite abelian group $rm:G:$ has an lcm-closed order set, i.e. with $rm: o(X) = $ order of $rm, X$



$$rm X,Y in G Rightarrow exists Z in G: o(Z) = lcm(o(X),o(Y))$$



Proof $ $ By induction on $rm: o(X), o(Y). $ If it's $:1:$ then trivially $rm:Z = 1$. $ $ Otherwise



write $rm, o(X), =: AP,: o(Y) = BP', P'mid P = p^m > 1, $ prime $rm: p:$ coprime to $rm: A,B.$



Then $rm, o(X^P) = A, o(Y^{P'}) = B. $ By induction there's a $rm: Z:$ with $rm : o(Z) = lcm(A,B)$



thus $rm, o(X^A Z) = P lcm(A,B) = lcm(AP,BP') = lcm(o(X),o(Y)).$






share|cite|improve this answer











$endgroup$



Below is a complete, noncircular simple proof of the result mentioned by Qiaochu that avoids invoking the high-powered structure theorem for finite abelian groups.



Theorem $ $ A finite subgroup $rm:G:$ of the multiplicative group of a field is cyclic.



Proof $ $ The proposition below yields, with $rm,m = maxord(G) = expt(G),,$ that $rm, x^m = 1,$ has $rm:#G:$ roots. Since a polynomial $rm:f:$ over a field satisfies $rm:#roots f le deg f:$ we infer that $rm: #G le m.:$ But maxorder $rm:m le #G:$ since $rm:g^{#G} = 1:$ for all $rm:g in G:$ (Lagrange). $:$ Thus $rm:m = #G = maxord(G),:$ therefore $rm:G:$ has an element of order $rm#G,:$ hence $rm:G:$ is cyclic.



$begin{eqnarray}rm{bf Proposition}qquad maxord(G) &=&,rm expt(G) text{ for a finite abelian group} G, i.e.\[.5em]
rm max { ord(g) : : g in G} &=&,rm min { n>0 : : g^n = 1 forall g in G}end{eqnarray}$



Proof $ $ By the lemma below, $rm: S, =, { ord(g) : :g in G }$ is a finite
set of naturals closed under$rm lcm$.



Hence every $rm s in S:$ is a
divisor of the max elt $rm: m $ [else $rm: lcm(s,m) > m,$],$ $ so $rm m = expt(G)$.



Lemma $ $ A finite abelian group $rm:G:$ has an lcm-closed order set, i.e. with $rm: o(X) = $ order of $rm, X$



$$rm X,Y in G Rightarrow exists Z in G: o(Z) = lcm(o(X),o(Y))$$



Proof $ $ By induction on $rm: o(X), o(Y). $ If it's $:1:$ then trivially $rm:Z = 1$. $ $ Otherwise



write $rm, o(X), =: AP,: o(Y) = BP', P'mid P = p^m > 1, $ prime $rm: p:$ coprime to $rm: A,B.$



Then $rm, o(X^P) = A, o(Y^{P'}) = B. $ By induction there's a $rm: Z:$ with $rm : o(Z) = lcm(A,B)$



thus $rm, o(X^A Z) = P lcm(A,B) = lcm(AP,BP') = lcm(o(X),o(Y)).$







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Dec 27 '18 at 2:34

























answered May 10 '11 at 15:16









Bill DubuqueBill Dubuque

211k29193647




211k29193647








  • 3




    $begingroup$
    I think the OP was asking if the (finite) extension $K/mathbb{F}_q$ is a cyclic extension, namely that $mbox{Gal}(K/mathbb{F}_q)$ is a cyclic group. Does this immediately follow from the fact that $K^times$ is a cyclic group?
    $endgroup$
    – Alon Amit
    May 10 '11 at 16:15










  • $begingroup$
    @Alon I've edited it to clarify that it was meant to support Qiaochu's answer.
    $endgroup$
    – Bill Dubuque
    May 10 '11 at 18:04






  • 5




    $begingroup$
    Sadly, it looks like this nice answer is destined to be one of the gems at the tail end of the voting range. To work against that, I'm voting it up.
    $endgroup$
    – t.b.
    May 10 '11 at 19:02














  • 3




    $begingroup$
    I think the OP was asking if the (finite) extension $K/mathbb{F}_q$ is a cyclic extension, namely that $mbox{Gal}(K/mathbb{F}_q)$ is a cyclic group. Does this immediately follow from the fact that $K^times$ is a cyclic group?
    $endgroup$
    – Alon Amit
    May 10 '11 at 16:15










  • $begingroup$
    @Alon I've edited it to clarify that it was meant to support Qiaochu's answer.
    $endgroup$
    – Bill Dubuque
    May 10 '11 at 18:04






  • 5




    $begingroup$
    Sadly, it looks like this nice answer is destined to be one of the gems at the tail end of the voting range. To work against that, I'm voting it up.
    $endgroup$
    – t.b.
    May 10 '11 at 19:02








3




3




$begingroup$
I think the OP was asking if the (finite) extension $K/mathbb{F}_q$ is a cyclic extension, namely that $mbox{Gal}(K/mathbb{F}_q)$ is a cyclic group. Does this immediately follow from the fact that $K^times$ is a cyclic group?
$endgroup$
– Alon Amit
May 10 '11 at 16:15




$begingroup$
I think the OP was asking if the (finite) extension $K/mathbb{F}_q$ is a cyclic extension, namely that $mbox{Gal}(K/mathbb{F}_q)$ is a cyclic group. Does this immediately follow from the fact that $K^times$ is a cyclic group?
$endgroup$
– Alon Amit
May 10 '11 at 16:15












$begingroup$
@Alon I've edited it to clarify that it was meant to support Qiaochu's answer.
$endgroup$
– Bill Dubuque
May 10 '11 at 18:04




$begingroup$
@Alon I've edited it to clarify that it was meant to support Qiaochu's answer.
$endgroup$
– Bill Dubuque
May 10 '11 at 18:04




5




5




$begingroup$
Sadly, it looks like this nice answer is destined to be one of the gems at the tail end of the voting range. To work against that, I'm voting it up.
$endgroup$
– t.b.
May 10 '11 at 19:02




$begingroup$
Sadly, it looks like this nice answer is destined to be one of the gems at the tail end of the voting range. To work against that, I'm voting it up.
$endgroup$
– t.b.
May 10 '11 at 19:02


















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