Non cyclic group of order $p^3$ satisfies $G simeq H rtimes_{theta}K$












7












$begingroup$


Let $G$ be a non-cyclic group of order $p^3$ for an odd prime $p$. Prove that $G simeq H rtimes_{theta}K$, where $H$ is a normal subgroup of $G$ of order $p^2$, $K$ is a subgroup of order $p$, and $theta : K to Aut(H)$ is a homomorphism.





I managed to prove that there exists a normal subgroup $H$ of order $p^2$. Then I took some $g in G-H$. If $g$ is of order $p$, I am done since $G simeq H rtimes langle g rangle $. But what if all $g in G - H$ are of order $p^2 $?










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    Impossible that all the non-trivial elements in $;G;$ are of order $;p^2;$ since there exists a subgroup of order $;p;$ ...
    $endgroup$
    – DonAntonio
    Dec 23 '18 at 22:10










  • $begingroup$
    Sorry, I meant all $g in G - H$. I edited my question
    $endgroup$
    – user401516
    Dec 23 '18 at 22:18








  • 1




    $begingroup$
    Observe that the conclusion would be false when $p=2$. The quaternion group $Q_8$. In other words, the fact that $p$ is odd must come into play somehow.
    $endgroup$
    – Jyrki Lahtonen
    Dec 24 '18 at 6:36








  • 1




    $begingroup$
    The claim seems to follow from the observations made by the OP and the argument given in this old answer by Arturo Magidin.
    $endgroup$
    – Jyrki Lahtonen
    Dec 24 '18 at 7:21
















7












$begingroup$


Let $G$ be a non-cyclic group of order $p^3$ for an odd prime $p$. Prove that $G simeq H rtimes_{theta}K$, where $H$ is a normal subgroup of $G$ of order $p^2$, $K$ is a subgroup of order $p$, and $theta : K to Aut(H)$ is a homomorphism.





I managed to prove that there exists a normal subgroup $H$ of order $p^2$. Then I took some $g in G-H$. If $g$ is of order $p$, I am done since $G simeq H rtimes langle g rangle $. But what if all $g in G - H$ are of order $p^2 $?










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    Impossible that all the non-trivial elements in $;G;$ are of order $;p^2;$ since there exists a subgroup of order $;p;$ ...
    $endgroup$
    – DonAntonio
    Dec 23 '18 at 22:10










  • $begingroup$
    Sorry, I meant all $g in G - H$. I edited my question
    $endgroup$
    – user401516
    Dec 23 '18 at 22:18








  • 1




    $begingroup$
    Observe that the conclusion would be false when $p=2$. The quaternion group $Q_8$. In other words, the fact that $p$ is odd must come into play somehow.
    $endgroup$
    – Jyrki Lahtonen
    Dec 24 '18 at 6:36








  • 1




    $begingroup$
    The claim seems to follow from the observations made by the OP and the argument given in this old answer by Arturo Magidin.
    $endgroup$
    – Jyrki Lahtonen
    Dec 24 '18 at 7:21














7












7








7


2



$begingroup$


Let $G$ be a non-cyclic group of order $p^3$ for an odd prime $p$. Prove that $G simeq H rtimes_{theta}K$, where $H$ is a normal subgroup of $G$ of order $p^2$, $K$ is a subgroup of order $p$, and $theta : K to Aut(H)$ is a homomorphism.





I managed to prove that there exists a normal subgroup $H$ of order $p^2$. Then I took some $g in G-H$. If $g$ is of order $p$, I am done since $G simeq H rtimes langle g rangle $. But what if all $g in G - H$ are of order $p^2 $?










share|cite|improve this question











$endgroup$




Let $G$ be a non-cyclic group of order $p^3$ for an odd prime $p$. Prove that $G simeq H rtimes_{theta}K$, where $H$ is a normal subgroup of $G$ of order $p^2$, $K$ is a subgroup of order $p$, and $theta : K to Aut(H)$ is a homomorphism.





I managed to prove that there exists a normal subgroup $H$ of order $p^2$. Then I took some $g in G-H$. If $g$ is of order $p$, I am done since $G simeq H rtimes langle g rangle $. But what if all $g in G - H$ are of order $p^2 $?







abstract-algebra group-theory finite-groups semidirect-product






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 24 '18 at 2:11









the_fox

2,89021537




2,89021537










asked Dec 23 '18 at 22:08









user401516user401516

92039




92039








  • 1




    $begingroup$
    Impossible that all the non-trivial elements in $;G;$ are of order $;p^2;$ since there exists a subgroup of order $;p;$ ...
    $endgroup$
    – DonAntonio
    Dec 23 '18 at 22:10










  • $begingroup$
    Sorry, I meant all $g in G - H$. I edited my question
    $endgroup$
    – user401516
    Dec 23 '18 at 22:18








  • 1




    $begingroup$
    Observe that the conclusion would be false when $p=2$. The quaternion group $Q_8$. In other words, the fact that $p$ is odd must come into play somehow.
    $endgroup$
    – Jyrki Lahtonen
    Dec 24 '18 at 6:36








  • 1




    $begingroup$
    The claim seems to follow from the observations made by the OP and the argument given in this old answer by Arturo Magidin.
    $endgroup$
    – Jyrki Lahtonen
    Dec 24 '18 at 7:21














  • 1




    $begingroup$
    Impossible that all the non-trivial elements in $;G;$ are of order $;p^2;$ since there exists a subgroup of order $;p;$ ...
    $endgroup$
    – DonAntonio
    Dec 23 '18 at 22:10










  • $begingroup$
    Sorry, I meant all $g in G - H$. I edited my question
    $endgroup$
    – user401516
    Dec 23 '18 at 22:18








  • 1




    $begingroup$
    Observe that the conclusion would be false when $p=2$. The quaternion group $Q_8$. In other words, the fact that $p$ is odd must come into play somehow.
    $endgroup$
    – Jyrki Lahtonen
    Dec 24 '18 at 6:36








  • 1




    $begingroup$
    The claim seems to follow from the observations made by the OP and the argument given in this old answer by Arturo Magidin.
    $endgroup$
    – Jyrki Lahtonen
    Dec 24 '18 at 7:21








1




1




$begingroup$
Impossible that all the non-trivial elements in $;G;$ are of order $;p^2;$ since there exists a subgroup of order $;p;$ ...
$endgroup$
– DonAntonio
Dec 23 '18 at 22:10




$begingroup$
Impossible that all the non-trivial elements in $;G;$ are of order $;p^2;$ since there exists a subgroup of order $;p;$ ...
$endgroup$
– DonAntonio
Dec 23 '18 at 22:10












$begingroup$
Sorry, I meant all $g in G - H$. I edited my question
$endgroup$
– user401516
Dec 23 '18 at 22:18






$begingroup$
Sorry, I meant all $g in G - H$. I edited my question
$endgroup$
– user401516
Dec 23 '18 at 22:18






1




1




$begingroup$
Observe that the conclusion would be false when $p=2$. The quaternion group $Q_8$. In other words, the fact that $p$ is odd must come into play somehow.
$endgroup$
– Jyrki Lahtonen
Dec 24 '18 at 6:36






$begingroup$
Observe that the conclusion would be false when $p=2$. The quaternion group $Q_8$. In other words, the fact that $p$ is odd must come into play somehow.
$endgroup$
– Jyrki Lahtonen
Dec 24 '18 at 6:36






1




1




$begingroup$
The claim seems to follow from the observations made by the OP and the argument given in this old answer by Arturo Magidin.
$endgroup$
– Jyrki Lahtonen
Dec 24 '18 at 7:21




$begingroup$
The claim seems to follow from the observations made by the OP and the argument given in this old answer by Arturo Magidin.
$endgroup$
– Jyrki Lahtonen
Dec 24 '18 at 7:21










1 Answer
1






active

oldest

votes


















3












$begingroup$

The result follows immediately if all elements of $G$ have order $1$ or $p$, so we can assume that there exists an element of order $p^2$. This generates a cyclic subgroup of index $p$, which is normal in $G$, so we can assume that $H$ is cyclic of order $p^2$.



So let $g in G setminus H$, and assume that $g$ has order $p^2$ (if it has order $p$ then we are done). So $g^p = h^p$ for some $h in H$. We claim that $(gh^{-1})^p=1$, and hence $gh^{-1}$ has order $p$ and we are done.



This is immediate if $G$ is abelian, so suppose not. Then $[G,G]=Z(G)$ has order $p$, so commutators are central, and $(xy^{-1})^p = x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ (where $[a,b]$ denotes the commutator $a^{-1}b^{-1}ab$).



Then, since $p$ is odd and $[y^{-1},x]$ has order $p$, we get $[y^{-1},x]^{p(p-1)/2}=1$ and the claim follows. (Note that this result is false when $p=2$, and the quaternion group $Q_8$ is a counterexample.)






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Thank you for your answer. Can you please explain why $(xy^{-1})^p=x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ ?
    $endgroup$
    – user401516
    Dec 24 '18 at 12:30








  • 1




    $begingroup$
    If $[G,G] le Z(G)$ then $(ab)^k = a^kb^k[b,a]^{k(k-1)/2}$ for all $a,b in G$ and all $k ge 0$. Prove it by induction on $k$, using $ba = ab[b,a]$.
    $endgroup$
    – Derek Holt
    Dec 24 '18 at 15:24












  • $begingroup$
    Where did you use the fact that $H$ is cyclic?
    $endgroup$
    – user401516
    Dec 24 '18 at 21:45










  • $begingroup$
    The existence of $h in H$ such that $g^p=h^p$.
    $endgroup$
    – Derek Holt
    Dec 24 '18 at 22:45











Your Answer





StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");

StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});

function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});


}
});














draft saved

draft discarded


















StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3050739%2fnon-cyclic-group-of-order-p3-satisfies-g-simeq-h-rtimes-thetak%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown

























1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









3












$begingroup$

The result follows immediately if all elements of $G$ have order $1$ or $p$, so we can assume that there exists an element of order $p^2$. This generates a cyclic subgroup of index $p$, which is normal in $G$, so we can assume that $H$ is cyclic of order $p^2$.



So let $g in G setminus H$, and assume that $g$ has order $p^2$ (if it has order $p$ then we are done). So $g^p = h^p$ for some $h in H$. We claim that $(gh^{-1})^p=1$, and hence $gh^{-1}$ has order $p$ and we are done.



This is immediate if $G$ is abelian, so suppose not. Then $[G,G]=Z(G)$ has order $p$, so commutators are central, and $(xy^{-1})^p = x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ (where $[a,b]$ denotes the commutator $a^{-1}b^{-1}ab$).



Then, since $p$ is odd and $[y^{-1},x]$ has order $p$, we get $[y^{-1},x]^{p(p-1)/2}=1$ and the claim follows. (Note that this result is false when $p=2$, and the quaternion group $Q_8$ is a counterexample.)






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Thank you for your answer. Can you please explain why $(xy^{-1})^p=x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ ?
    $endgroup$
    – user401516
    Dec 24 '18 at 12:30








  • 1




    $begingroup$
    If $[G,G] le Z(G)$ then $(ab)^k = a^kb^k[b,a]^{k(k-1)/2}$ for all $a,b in G$ and all $k ge 0$. Prove it by induction on $k$, using $ba = ab[b,a]$.
    $endgroup$
    – Derek Holt
    Dec 24 '18 at 15:24












  • $begingroup$
    Where did you use the fact that $H$ is cyclic?
    $endgroup$
    – user401516
    Dec 24 '18 at 21:45










  • $begingroup$
    The existence of $h in H$ such that $g^p=h^p$.
    $endgroup$
    – Derek Holt
    Dec 24 '18 at 22:45
















3












$begingroup$

The result follows immediately if all elements of $G$ have order $1$ or $p$, so we can assume that there exists an element of order $p^2$. This generates a cyclic subgroup of index $p$, which is normal in $G$, so we can assume that $H$ is cyclic of order $p^2$.



So let $g in G setminus H$, and assume that $g$ has order $p^2$ (if it has order $p$ then we are done). So $g^p = h^p$ for some $h in H$. We claim that $(gh^{-1})^p=1$, and hence $gh^{-1}$ has order $p$ and we are done.



This is immediate if $G$ is abelian, so suppose not. Then $[G,G]=Z(G)$ has order $p$, so commutators are central, and $(xy^{-1})^p = x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ (where $[a,b]$ denotes the commutator $a^{-1}b^{-1}ab$).



Then, since $p$ is odd and $[y^{-1},x]$ has order $p$, we get $[y^{-1},x]^{p(p-1)/2}=1$ and the claim follows. (Note that this result is false when $p=2$, and the quaternion group $Q_8$ is a counterexample.)






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Thank you for your answer. Can you please explain why $(xy^{-1})^p=x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ ?
    $endgroup$
    – user401516
    Dec 24 '18 at 12:30








  • 1




    $begingroup$
    If $[G,G] le Z(G)$ then $(ab)^k = a^kb^k[b,a]^{k(k-1)/2}$ for all $a,b in G$ and all $k ge 0$. Prove it by induction on $k$, using $ba = ab[b,a]$.
    $endgroup$
    – Derek Holt
    Dec 24 '18 at 15:24












  • $begingroup$
    Where did you use the fact that $H$ is cyclic?
    $endgroup$
    – user401516
    Dec 24 '18 at 21:45










  • $begingroup$
    The existence of $h in H$ such that $g^p=h^p$.
    $endgroup$
    – Derek Holt
    Dec 24 '18 at 22:45














3












3








3





$begingroup$

The result follows immediately if all elements of $G$ have order $1$ or $p$, so we can assume that there exists an element of order $p^2$. This generates a cyclic subgroup of index $p$, which is normal in $G$, so we can assume that $H$ is cyclic of order $p^2$.



So let $g in G setminus H$, and assume that $g$ has order $p^2$ (if it has order $p$ then we are done). So $g^p = h^p$ for some $h in H$. We claim that $(gh^{-1})^p=1$, and hence $gh^{-1}$ has order $p$ and we are done.



This is immediate if $G$ is abelian, so suppose not. Then $[G,G]=Z(G)$ has order $p$, so commutators are central, and $(xy^{-1})^p = x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ (where $[a,b]$ denotes the commutator $a^{-1}b^{-1}ab$).



Then, since $p$ is odd and $[y^{-1},x]$ has order $p$, we get $[y^{-1},x]^{p(p-1)/2}=1$ and the claim follows. (Note that this result is false when $p=2$, and the quaternion group $Q_8$ is a counterexample.)






share|cite|improve this answer











$endgroup$



The result follows immediately if all elements of $G$ have order $1$ or $p$, so we can assume that there exists an element of order $p^2$. This generates a cyclic subgroup of index $p$, which is normal in $G$, so we can assume that $H$ is cyclic of order $p^2$.



So let $g in G setminus H$, and assume that $g$ has order $p^2$ (if it has order $p$ then we are done). So $g^p = h^p$ for some $h in H$. We claim that $(gh^{-1})^p=1$, and hence $gh^{-1}$ has order $p$ and we are done.



This is immediate if $G$ is abelian, so suppose not. Then $[G,G]=Z(G)$ has order $p$, so commutators are central, and $(xy^{-1})^p = x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ (where $[a,b]$ denotes the commutator $a^{-1}b^{-1}ab$).



Then, since $p$ is odd and $[y^{-1},x]$ has order $p$, we get $[y^{-1},x]^{p(p-1)/2}=1$ and the claim follows. (Note that this result is false when $p=2$, and the quaternion group $Q_8$ is a counterexample.)







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Dec 24 '18 at 11:01

























answered Dec 24 '18 at 10:22









Derek HoltDerek Holt

53.7k53571




53.7k53571












  • $begingroup$
    Thank you for your answer. Can you please explain why $(xy^{-1})^p=x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ ?
    $endgroup$
    – user401516
    Dec 24 '18 at 12:30








  • 1




    $begingroup$
    If $[G,G] le Z(G)$ then $(ab)^k = a^kb^k[b,a]^{k(k-1)/2}$ for all $a,b in G$ and all $k ge 0$. Prove it by induction on $k$, using $ba = ab[b,a]$.
    $endgroup$
    – Derek Holt
    Dec 24 '18 at 15:24












  • $begingroup$
    Where did you use the fact that $H$ is cyclic?
    $endgroup$
    – user401516
    Dec 24 '18 at 21:45










  • $begingroup$
    The existence of $h in H$ such that $g^p=h^p$.
    $endgroup$
    – Derek Holt
    Dec 24 '18 at 22:45


















  • $begingroup$
    Thank you for your answer. Can you please explain why $(xy^{-1})^p=x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ ?
    $endgroup$
    – user401516
    Dec 24 '18 at 12:30








  • 1




    $begingroup$
    If $[G,G] le Z(G)$ then $(ab)^k = a^kb^k[b,a]^{k(k-1)/2}$ for all $a,b in G$ and all $k ge 0$. Prove it by induction on $k$, using $ba = ab[b,a]$.
    $endgroup$
    – Derek Holt
    Dec 24 '18 at 15:24












  • $begingroup$
    Where did you use the fact that $H$ is cyclic?
    $endgroup$
    – user401516
    Dec 24 '18 at 21:45










  • $begingroup$
    The existence of $h in H$ such that $g^p=h^p$.
    $endgroup$
    – Derek Holt
    Dec 24 '18 at 22:45
















$begingroup$
Thank you for your answer. Can you please explain why $(xy^{-1})^p=x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ ?
$endgroup$
– user401516
Dec 24 '18 at 12:30






$begingroup$
Thank you for your answer. Can you please explain why $(xy^{-1})^p=x^py^{-p}[y^{-1},x]^{p(p-1)/2}$ ?
$endgroup$
– user401516
Dec 24 '18 at 12:30






1




1




$begingroup$
If $[G,G] le Z(G)$ then $(ab)^k = a^kb^k[b,a]^{k(k-1)/2}$ for all $a,b in G$ and all $k ge 0$. Prove it by induction on $k$, using $ba = ab[b,a]$.
$endgroup$
– Derek Holt
Dec 24 '18 at 15:24






$begingroup$
If $[G,G] le Z(G)$ then $(ab)^k = a^kb^k[b,a]^{k(k-1)/2}$ for all $a,b in G$ and all $k ge 0$. Prove it by induction on $k$, using $ba = ab[b,a]$.
$endgroup$
– Derek Holt
Dec 24 '18 at 15:24














$begingroup$
Where did you use the fact that $H$ is cyclic?
$endgroup$
– user401516
Dec 24 '18 at 21:45




$begingroup$
Where did you use the fact that $H$ is cyclic?
$endgroup$
– user401516
Dec 24 '18 at 21:45












$begingroup$
The existence of $h in H$ such that $g^p=h^p$.
$endgroup$
– Derek Holt
Dec 24 '18 at 22:45




$begingroup$
The existence of $h in H$ such that $g^p=h^p$.
$endgroup$
– Derek Holt
Dec 24 '18 at 22:45


















draft saved

draft discarded




















































Thanks for contributing an answer to Mathematics Stack Exchange!


  • Please be sure to answer the question. Provide details and share your research!

But avoid



  • Asking for help, clarification, or responding to other answers.

  • Making statements based on opinion; back them up with references or personal experience.


Use MathJax to format equations. MathJax reference.


To learn more, see our tips on writing great answers.




draft saved


draft discarded














StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3050739%2fnon-cyclic-group-of-order-p3-satisfies-g-simeq-h-rtimes-thetak%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown





















































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown

































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown







Popular posts from this blog

Ellipse (mathématiques)

Quarter-circle Tiles

Mont Emei