Each group of order 8 has a subgroup of order 2 and a subgroup of order 4.
$begingroup$
So, I was trying to prove the following theorem:
Let $G$ be a group of order $8$. So $G$ has a subgroup of order $2$ and a subgroup of order $4$.
First I proved that if a group has a finite even order, it has an element $g_0$ of order $2$. So $H={g_0,e}$ is a group of order $2$. Now I'm trying to find the group of order $4$. By Lagrange theorem I can understand that:
$$|G|=|H|cdot |G,:,H| Rightarrow |G,:,H|=4$$
So we have $4$ Cosets. But how to continue from here? I feel like I miss the last one-two lines of the proof.
abstract-algebra group-theory
$endgroup$
add a comment |
$begingroup$
So, I was trying to prove the following theorem:
Let $G$ be a group of order $8$. So $G$ has a subgroup of order $2$ and a subgroup of order $4$.
First I proved that if a group has a finite even order, it has an element $g_0$ of order $2$. So $H={g_0,e}$ is a group of order $2$. Now I'm trying to find the group of order $4$. By Lagrange theorem I can understand that:
$$|G|=|H|cdot |G,:,H| Rightarrow |G,:,H|=4$$
So we have $4$ Cosets. But how to continue from here? I feel like I miss the last one-two lines of the proof.
abstract-algebra group-theory
$endgroup$
1
$begingroup$
Do you know that nilpotent groups (and in particular $p$-groups) have non-trivial centres? If yes, it would be better and more instructive to prove that nilpotent groups (and in particular $p$-groups) have subgroups of each possible order. (In fact, this holds for a broader class of groups.)
$endgroup$
– the_fox
Feb 15 at 17:37
add a comment |
$begingroup$
So, I was trying to prove the following theorem:
Let $G$ be a group of order $8$. So $G$ has a subgroup of order $2$ and a subgroup of order $4$.
First I proved that if a group has a finite even order, it has an element $g_0$ of order $2$. So $H={g_0,e}$ is a group of order $2$. Now I'm trying to find the group of order $4$. By Lagrange theorem I can understand that:
$$|G|=|H|cdot |G,:,H| Rightarrow |G,:,H|=4$$
So we have $4$ Cosets. But how to continue from here? I feel like I miss the last one-two lines of the proof.
abstract-algebra group-theory
$endgroup$
So, I was trying to prove the following theorem:
Let $G$ be a group of order $8$. So $G$ has a subgroup of order $2$ and a subgroup of order $4$.
First I proved that if a group has a finite even order, it has an element $g_0$ of order $2$. So $H={g_0,e}$ is a group of order $2$. Now I'm trying to find the group of order $4$. By Lagrange theorem I can understand that:
$$|G|=|H|cdot |G,:,H| Rightarrow |G,:,H|=4$$
So we have $4$ Cosets. But how to continue from here? I feel like I miss the last one-two lines of the proof.
abstract-algebra group-theory
abstract-algebra group-theory
edited Feb 15 at 17:58
Shaun
9,268113684
9,268113684
asked Feb 15 at 17:23
BadukBaduk
493
493
1
$begingroup$
Do you know that nilpotent groups (and in particular $p$-groups) have non-trivial centres? If yes, it would be better and more instructive to prove that nilpotent groups (and in particular $p$-groups) have subgroups of each possible order. (In fact, this holds for a broader class of groups.)
$endgroup$
– the_fox
Feb 15 at 17:37
add a comment |
1
$begingroup$
Do you know that nilpotent groups (and in particular $p$-groups) have non-trivial centres? If yes, it would be better and more instructive to prove that nilpotent groups (and in particular $p$-groups) have subgroups of each possible order. (In fact, this holds for a broader class of groups.)
$endgroup$
– the_fox
Feb 15 at 17:37
1
1
$begingroup$
Do you know that nilpotent groups (and in particular $p$-groups) have non-trivial centres? If yes, it would be better and more instructive to prove that nilpotent groups (and in particular $p$-groups) have subgroups of each possible order. (In fact, this holds for a broader class of groups.)
$endgroup$
– the_fox
Feb 15 at 17:37
$begingroup$
Do you know that nilpotent groups (and in particular $p$-groups) have non-trivial centres? If yes, it would be better and more instructive to prove that nilpotent groups (and in particular $p$-groups) have subgroups of each possible order. (In fact, this holds for a broader class of groups.)
$endgroup$
– the_fox
Feb 15 at 17:37
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Well, you can show that $G$ has a subgroup of order 4 directly. If $G$ has an element of order 4 or 8 then we are already done [why?]
Otherwise let $pi$ and $alpha$ be two distinct elements in $G$ of order 2. So $pi = pi^{-1}$ and $alpha = alpha^{-1}$. Then $pialpha$ must have order 2 as well, lest there be an element of order 4 or 8 which would imply that we are done. This implies that $pi alpha = alpha pi$. [Indeed, for any elements $a,b,c$ in any group $G'$, if $ab=ac$ then $b$ must equal $c$. But here $(pi alpha)(pi alpha) = 1$, while $(pi alpha)(alpha pi) = (pi alpha)(alpha^{-1}pi^{-1}) = 1$. So indeed, $pi alpha = alpha pi$.] This implies that $H doteq {1,alpha, pi, pialpha }$ is closed under composition, and as every element in $H$ has its inverse in $H$, it follows that $H$ is a subgroup, and has order 4.
$endgroup$
add a comment |
$begingroup$
If $G$ has no subgroup of order $4$, then every $gne e$ has order $2$. Then for any $g,h$,
$$
e=(gh)^2=ghgh=ghg^{-1}h^{-1},
$$i.e. $G$ is abelian. Then ${e,g,h,gh}$ would be a subgroup of order $4$.
$endgroup$
$begingroup$
why every $gneq e$ has order $2$ if $G$ does not have subgroup of order $4$? Also why $ghgh=ghg^{-1}h^{-1}$?
$endgroup$
– Baduk
Feb 15 at 18:21
$begingroup$
@Baduk: Apply Lagrange's Theorem. Also, we have $x^{-1}=x$ iff $x^2=e$. (Why?)
$endgroup$
– Shaun
Feb 15 at 18:50
add a comment |
$begingroup$
Hint: No group has exactly two elements of order two.
Assume no subgroup of order four exists. Apply Lagrange's Theorem. Apply the hint. See @Mike's answer.
Reference for the lemma that is the hint:
Exercise 4.61 of Gallian's "Contemporary Abstract Algebra (Eighth Edition)"
$endgroup$
$begingroup$
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review
$endgroup$
– Gibbs
Feb 15 at 17:58
1
$begingroup$
@Gibbs, I've included a reference.
$endgroup$
– Shaun
Feb 15 at 18:01
add a comment |
Your Answer
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Well, you can show that $G$ has a subgroup of order 4 directly. If $G$ has an element of order 4 or 8 then we are already done [why?]
Otherwise let $pi$ and $alpha$ be two distinct elements in $G$ of order 2. So $pi = pi^{-1}$ and $alpha = alpha^{-1}$. Then $pialpha$ must have order 2 as well, lest there be an element of order 4 or 8 which would imply that we are done. This implies that $pi alpha = alpha pi$. [Indeed, for any elements $a,b,c$ in any group $G'$, if $ab=ac$ then $b$ must equal $c$. But here $(pi alpha)(pi alpha) = 1$, while $(pi alpha)(alpha pi) = (pi alpha)(alpha^{-1}pi^{-1}) = 1$. So indeed, $pi alpha = alpha pi$.] This implies that $H doteq {1,alpha, pi, pialpha }$ is closed under composition, and as every element in $H$ has its inverse in $H$, it follows that $H$ is a subgroup, and has order 4.
$endgroup$
add a comment |
$begingroup$
Well, you can show that $G$ has a subgroup of order 4 directly. If $G$ has an element of order 4 or 8 then we are already done [why?]
Otherwise let $pi$ and $alpha$ be two distinct elements in $G$ of order 2. So $pi = pi^{-1}$ and $alpha = alpha^{-1}$. Then $pialpha$ must have order 2 as well, lest there be an element of order 4 or 8 which would imply that we are done. This implies that $pi alpha = alpha pi$. [Indeed, for any elements $a,b,c$ in any group $G'$, if $ab=ac$ then $b$ must equal $c$. But here $(pi alpha)(pi alpha) = 1$, while $(pi alpha)(alpha pi) = (pi alpha)(alpha^{-1}pi^{-1}) = 1$. So indeed, $pi alpha = alpha pi$.] This implies that $H doteq {1,alpha, pi, pialpha }$ is closed under composition, and as every element in $H$ has its inverse in $H$, it follows that $H$ is a subgroup, and has order 4.
$endgroup$
add a comment |
$begingroup$
Well, you can show that $G$ has a subgroup of order 4 directly. If $G$ has an element of order 4 or 8 then we are already done [why?]
Otherwise let $pi$ and $alpha$ be two distinct elements in $G$ of order 2. So $pi = pi^{-1}$ and $alpha = alpha^{-1}$. Then $pialpha$ must have order 2 as well, lest there be an element of order 4 or 8 which would imply that we are done. This implies that $pi alpha = alpha pi$. [Indeed, for any elements $a,b,c$ in any group $G'$, if $ab=ac$ then $b$ must equal $c$. But here $(pi alpha)(pi alpha) = 1$, while $(pi alpha)(alpha pi) = (pi alpha)(alpha^{-1}pi^{-1}) = 1$. So indeed, $pi alpha = alpha pi$.] This implies that $H doteq {1,alpha, pi, pialpha }$ is closed under composition, and as every element in $H$ has its inverse in $H$, it follows that $H$ is a subgroup, and has order 4.
$endgroup$
Well, you can show that $G$ has a subgroup of order 4 directly. If $G$ has an element of order 4 or 8 then we are already done [why?]
Otherwise let $pi$ and $alpha$ be two distinct elements in $G$ of order 2. So $pi = pi^{-1}$ and $alpha = alpha^{-1}$. Then $pialpha$ must have order 2 as well, lest there be an element of order 4 or 8 which would imply that we are done. This implies that $pi alpha = alpha pi$. [Indeed, for any elements $a,b,c$ in any group $G'$, if $ab=ac$ then $b$ must equal $c$. But here $(pi alpha)(pi alpha) = 1$, while $(pi alpha)(alpha pi) = (pi alpha)(alpha^{-1}pi^{-1}) = 1$. So indeed, $pi alpha = alpha pi$.] This implies that $H doteq {1,alpha, pi, pialpha }$ is closed under composition, and as every element in $H$ has its inverse in $H$, it follows that $H$ is a subgroup, and has order 4.
edited Feb 15 at 17:43
answered Feb 15 at 17:35
MikeMike
4,171412
4,171412
add a comment |
add a comment |
$begingroup$
If $G$ has no subgroup of order $4$, then every $gne e$ has order $2$. Then for any $g,h$,
$$
e=(gh)^2=ghgh=ghg^{-1}h^{-1},
$$i.e. $G$ is abelian. Then ${e,g,h,gh}$ would be a subgroup of order $4$.
$endgroup$
$begingroup$
why every $gneq e$ has order $2$ if $G$ does not have subgroup of order $4$? Also why $ghgh=ghg^{-1}h^{-1}$?
$endgroup$
– Baduk
Feb 15 at 18:21
$begingroup$
@Baduk: Apply Lagrange's Theorem. Also, we have $x^{-1}=x$ iff $x^2=e$. (Why?)
$endgroup$
– Shaun
Feb 15 at 18:50
add a comment |
$begingroup$
If $G$ has no subgroup of order $4$, then every $gne e$ has order $2$. Then for any $g,h$,
$$
e=(gh)^2=ghgh=ghg^{-1}h^{-1},
$$i.e. $G$ is abelian. Then ${e,g,h,gh}$ would be a subgroup of order $4$.
$endgroup$
$begingroup$
why every $gneq e$ has order $2$ if $G$ does not have subgroup of order $4$? Also why $ghgh=ghg^{-1}h^{-1}$?
$endgroup$
– Baduk
Feb 15 at 18:21
$begingroup$
@Baduk: Apply Lagrange's Theorem. Also, we have $x^{-1}=x$ iff $x^2=e$. (Why?)
$endgroup$
– Shaun
Feb 15 at 18:50
add a comment |
$begingroup$
If $G$ has no subgroup of order $4$, then every $gne e$ has order $2$. Then for any $g,h$,
$$
e=(gh)^2=ghgh=ghg^{-1}h^{-1},
$$i.e. $G$ is abelian. Then ${e,g,h,gh}$ would be a subgroup of order $4$.
$endgroup$
If $G$ has no subgroup of order $4$, then every $gne e$ has order $2$. Then for any $g,h$,
$$
e=(gh)^2=ghgh=ghg^{-1}h^{-1},
$$i.e. $G$ is abelian. Then ${e,g,h,gh}$ would be a subgroup of order $4$.
answered Feb 15 at 17:33
SongSong
14.4k1635
14.4k1635
$begingroup$
why every $gneq e$ has order $2$ if $G$ does not have subgroup of order $4$? Also why $ghgh=ghg^{-1}h^{-1}$?
$endgroup$
– Baduk
Feb 15 at 18:21
$begingroup$
@Baduk: Apply Lagrange's Theorem. Also, we have $x^{-1}=x$ iff $x^2=e$. (Why?)
$endgroup$
– Shaun
Feb 15 at 18:50
add a comment |
$begingroup$
why every $gneq e$ has order $2$ if $G$ does not have subgroup of order $4$? Also why $ghgh=ghg^{-1}h^{-1}$?
$endgroup$
– Baduk
Feb 15 at 18:21
$begingroup$
@Baduk: Apply Lagrange's Theorem. Also, we have $x^{-1}=x$ iff $x^2=e$. (Why?)
$endgroup$
– Shaun
Feb 15 at 18:50
$begingroup$
why every $gneq e$ has order $2$ if $G$ does not have subgroup of order $4$? Also why $ghgh=ghg^{-1}h^{-1}$?
$endgroup$
– Baduk
Feb 15 at 18:21
$begingroup$
why every $gneq e$ has order $2$ if $G$ does not have subgroup of order $4$? Also why $ghgh=ghg^{-1}h^{-1}$?
$endgroup$
– Baduk
Feb 15 at 18:21
$begingroup$
@Baduk: Apply Lagrange's Theorem. Also, we have $x^{-1}=x$ iff $x^2=e$. (Why?)
$endgroup$
– Shaun
Feb 15 at 18:50
$begingroup$
@Baduk: Apply Lagrange's Theorem. Also, we have $x^{-1}=x$ iff $x^2=e$. (Why?)
$endgroup$
– Shaun
Feb 15 at 18:50
add a comment |
$begingroup$
Hint: No group has exactly two elements of order two.
Assume no subgroup of order four exists. Apply Lagrange's Theorem. Apply the hint. See @Mike's answer.
Reference for the lemma that is the hint:
Exercise 4.61 of Gallian's "Contemporary Abstract Algebra (Eighth Edition)"
$endgroup$
$begingroup$
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review
$endgroup$
– Gibbs
Feb 15 at 17:58
1
$begingroup$
@Gibbs, I've included a reference.
$endgroup$
– Shaun
Feb 15 at 18:01
add a comment |
$begingroup$
Hint: No group has exactly two elements of order two.
Assume no subgroup of order four exists. Apply Lagrange's Theorem. Apply the hint. See @Mike's answer.
Reference for the lemma that is the hint:
Exercise 4.61 of Gallian's "Contemporary Abstract Algebra (Eighth Edition)"
$endgroup$
$begingroup$
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review
$endgroup$
– Gibbs
Feb 15 at 17:58
1
$begingroup$
@Gibbs, I've included a reference.
$endgroup$
– Shaun
Feb 15 at 18:01
add a comment |
$begingroup$
Hint: No group has exactly two elements of order two.
Assume no subgroup of order four exists. Apply Lagrange's Theorem. Apply the hint. See @Mike's answer.
Reference for the lemma that is the hint:
Exercise 4.61 of Gallian's "Contemporary Abstract Algebra (Eighth Edition)"
$endgroup$
Hint: No group has exactly two elements of order two.
Assume no subgroup of order four exists. Apply Lagrange's Theorem. Apply the hint. See @Mike's answer.
Reference for the lemma that is the hint:
Exercise 4.61 of Gallian's "Contemporary Abstract Algebra (Eighth Edition)"
edited Feb 15 at 18:09
answered Feb 15 at 17:31
ShaunShaun
9,268113684
9,268113684
$begingroup$
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review
$endgroup$
– Gibbs
Feb 15 at 17:58
1
$begingroup$
@Gibbs, I've included a reference.
$endgroup$
– Shaun
Feb 15 at 18:01
add a comment |
$begingroup$
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review
$endgroup$
– Gibbs
Feb 15 at 17:58
1
$begingroup$
@Gibbs, I've included a reference.
$endgroup$
– Shaun
Feb 15 at 18:01
$begingroup$
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review
$endgroup$
– Gibbs
Feb 15 at 17:58
$begingroup$
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review
$endgroup$
– Gibbs
Feb 15 at 17:58
1
1
$begingroup$
@Gibbs, I've included a reference.
$endgroup$
– Shaun
Feb 15 at 18:01
$begingroup$
@Gibbs, I've included a reference.
$endgroup$
– Shaun
Feb 15 at 18:01
add a comment |
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$begingroup$
Do you know that nilpotent groups (and in particular $p$-groups) have non-trivial centres? If yes, it would be better and more instructive to prove that nilpotent groups (and in particular $p$-groups) have subgroups of each possible order. (In fact, this holds for a broader class of groups.)
$endgroup$
– the_fox
Feb 15 at 17:37