finding the order of an element in a group presentation
Clash Royale CLAN TAG#URR8PPP
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4
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Let G be a group given by
$G = langle x, y | x^4 = y^4=1, yx = x^2 y^2 rangle$
I found $ G/G'$ to be isomorphic to C4
What can we say about the order of $ x$ in G?
The final answer is 4
But $ x^4 = 1$ in G means that x belong to the normal closure of R where R is the relations in G.
I can't see the connection. Can any one help
group-theory combinatorial-group-theory
 |Â
show 1 more comment
up vote
4
down vote
favorite
Let G be a group given by
$G = langle x, y | x^4 = y^4=1, yx = x^2 y^2 rangle$
I found $ G/G'$ to be isomorphic to C4
What can we say about the order of $ x$ in G?
The final answer is 4
But $ x^4 = 1$ in G means that x belong to the normal closure of R where R is the relations in G.
I can't see the connection. Can any one help
group-theory combinatorial-group-theory
I'm just curious: how did you find G/G' isomorphic to C4?
– Yip Jung Hon
Jul 29 at 7:53
@YipJungHon: If $G$ is presented by $langle X;|;Rrangle$, then $G/G'$ can be presented by $langle X;|;Rcup Crangle$, where $C = [x_i,x_j];$. From this it is not hard to see that, for the group $G$ of this problem, $G/G'$ contains only the elements $G', xG', (xG')^2, (xG')^3=yG'$. To see that these four elements must be distinct in the presented group, it suffices to observe that there exists a group, namely $C_4=1,g,g^2,g^3$, which has an element $x:=g$ of order $4$ and another element $y:=g^-1$ which satisfies all relations in the presentation.
– Keith Kearnes
Jul 29 at 10:01
Just curious, if I define the group slightly differently, with it being: $G=<x,y | x^4=y^4=5>,$ it will still be $cong$ to $C_4$ right?
– Yip Jung Hon
Jul 29 at 11:06
@Yip Jung Hon by adding the relation xy=yx the group become abelian and hence from the last relation you have x=y and simply remove the generator y you left with x^4=1
– Rosa1
Jul 29 at 14:08
In G/G' you only make the generators abelian
– Rosa1
Jul 29 at 14:09
 |Â
show 1 more comment
up vote
4
down vote
favorite
up vote
4
down vote
favorite
Let G be a group given by
$G = langle x, y | x^4 = y^4=1, yx = x^2 y^2 rangle$
I found $ G/G'$ to be isomorphic to C4
What can we say about the order of $ x$ in G?
The final answer is 4
But $ x^4 = 1$ in G means that x belong to the normal closure of R where R is the relations in G.
I can't see the connection. Can any one help
group-theory combinatorial-group-theory
Let G be a group given by
$G = langle x, y | x^4 = y^4=1, yx = x^2 y^2 rangle$
I found $ G/G'$ to be isomorphic to C4
What can we say about the order of $ x$ in G?
The final answer is 4
But $ x^4 = 1$ in G means that x belong to the normal closure of R where R is the relations in G.
I can't see the connection. Can any one help
group-theory combinatorial-group-theory
edited Jul 29 at 7:29
asked Jul 29 at 7:22
Rosa1
405
405
I'm just curious: how did you find G/G' isomorphic to C4?
– Yip Jung Hon
Jul 29 at 7:53
@YipJungHon: If $G$ is presented by $langle X;|;Rrangle$, then $G/G'$ can be presented by $langle X;|;Rcup Crangle$, where $C = [x_i,x_j];$. From this it is not hard to see that, for the group $G$ of this problem, $G/G'$ contains only the elements $G', xG', (xG')^2, (xG')^3=yG'$. To see that these four elements must be distinct in the presented group, it suffices to observe that there exists a group, namely $C_4=1,g,g^2,g^3$, which has an element $x:=g$ of order $4$ and another element $y:=g^-1$ which satisfies all relations in the presentation.
– Keith Kearnes
Jul 29 at 10:01
Just curious, if I define the group slightly differently, with it being: $G=<x,y | x^4=y^4=5>,$ it will still be $cong$ to $C_4$ right?
– Yip Jung Hon
Jul 29 at 11:06
@Yip Jung Hon by adding the relation xy=yx the group become abelian and hence from the last relation you have x=y and simply remove the generator y you left with x^4=1
– Rosa1
Jul 29 at 14:08
In G/G' you only make the generators abelian
– Rosa1
Jul 29 at 14:09
 |Â
show 1 more comment
I'm just curious: how did you find G/G' isomorphic to C4?
– Yip Jung Hon
Jul 29 at 7:53
@YipJungHon: If $G$ is presented by $langle X;|;Rrangle$, then $G/G'$ can be presented by $langle X;|;Rcup Crangle$, where $C = [x_i,x_j];$. From this it is not hard to see that, for the group $G$ of this problem, $G/G'$ contains only the elements $G', xG', (xG')^2, (xG')^3=yG'$. To see that these four elements must be distinct in the presented group, it suffices to observe that there exists a group, namely $C_4=1,g,g^2,g^3$, which has an element $x:=g$ of order $4$ and another element $y:=g^-1$ which satisfies all relations in the presentation.
– Keith Kearnes
Jul 29 at 10:01
Just curious, if I define the group slightly differently, with it being: $G=<x,y | x^4=y^4=5>,$ it will still be $cong$ to $C_4$ right?
– Yip Jung Hon
Jul 29 at 11:06
@Yip Jung Hon by adding the relation xy=yx the group become abelian and hence from the last relation you have x=y and simply remove the generator y you left with x^4=1
– Rosa1
Jul 29 at 14:08
In G/G' you only make the generators abelian
– Rosa1
Jul 29 at 14:09
I'm just curious: how did you find G/G' isomorphic to C4?
– Yip Jung Hon
Jul 29 at 7:53
I'm just curious: how did you find G/G' isomorphic to C4?
– Yip Jung Hon
Jul 29 at 7:53
@YipJungHon: If $G$ is presented by $langle X;|;Rrangle$, then $G/G'$ can be presented by $langle X;|;Rcup Crangle$, where $C = [x_i,x_j];$. From this it is not hard to see that, for the group $G$ of this problem, $G/G'$ contains only the elements $G', xG', (xG')^2, (xG')^3=yG'$. To see that these four elements must be distinct in the presented group, it suffices to observe that there exists a group, namely $C_4=1,g,g^2,g^3$, which has an element $x:=g$ of order $4$ and another element $y:=g^-1$ which satisfies all relations in the presentation.
– Keith Kearnes
Jul 29 at 10:01
@YipJungHon: If $G$ is presented by $langle X;|;Rrangle$, then $G/G'$ can be presented by $langle X;|;Rcup Crangle$, where $C = [x_i,x_j];$. From this it is not hard to see that, for the group $G$ of this problem, $G/G'$ contains only the elements $G', xG', (xG')^2, (xG')^3=yG'$. To see that these four elements must be distinct in the presented group, it suffices to observe that there exists a group, namely $C_4=1,g,g^2,g^3$, which has an element $x:=g$ of order $4$ and another element $y:=g^-1$ which satisfies all relations in the presentation.
– Keith Kearnes
Jul 29 at 10:01
Just curious, if I define the group slightly differently, with it being: $G=<x,y | x^4=y^4=5>,$ it will still be $cong$ to $C_4$ right?
– Yip Jung Hon
Jul 29 at 11:06
Just curious, if I define the group slightly differently, with it being: $G=<x,y | x^4=y^4=5>,$ it will still be $cong$ to $C_4$ right?
– Yip Jung Hon
Jul 29 at 11:06
@Yip Jung Hon by adding the relation xy=yx the group become abelian and hence from the last relation you have x=y and simply remove the generator y you left with x^4=1
– Rosa1
Jul 29 at 14:08
@Yip Jung Hon by adding the relation xy=yx the group become abelian and hence from the last relation you have x=y and simply remove the generator y you left with x^4=1
– Rosa1
Jul 29 at 14:08
In G/G' you only make the generators abelian
– Rosa1
Jul 29 at 14:09
In G/G' you only make the generators abelian
– Rosa1
Jul 29 at 14:09
 |Â
show 1 more comment
2 Answers
2
active
oldest
votes
up vote
1
down vote
accepted
The relation $x^4=1$ guarantees that the order of $x$ in $G$ is at most $4$. The fact that $xG'$ generates $G/G'cong C_4$ guarantees that the order of $x$ in $G$ is at least $4$.
Thank you . @Keith Kearnes The relation you mentioned is saying that $ x^4 $ belong to the normal closer of R so $ x^4R = R$ Where can we say that the order must be at least 4
– Rosa1
Jul 29 at 10:05
1
The answer explains exactly why the order is at least $4$.
– Derek Holt
Jul 29 at 12:27
@Derek Holt but he talked about the order of x in G/G'. How this is connected to the order of x in G
– Rosa1
Jul 29 at 14:27
1
The order of $xG'$ in $G/G'$ is $4$, so the order of $x$ in $G$ is at least $4$.
– Derek Holt
Jul 29 at 15:15
add a comment |Â
up vote
1
down vote
Well, the normal closure is even bigger than the subgroup generated by the relation. The only thing that has to be shown is that the order is, in fact, 4 and not less. Since the order divides 4, this boils down to showing that $x^2$ is not in the normal closure. Let $N$ be this normal closure, and let $R le N$ be the subgroup generated by the relations. By considering the powers of $x$ and $y$ occuring in any element of $R$, we find that $x^2 notin R$. But the normal closure is precisely the union of $fRf^-1$ ($f in Flangle x, y rangle$, the free group over $x$ and $y$), and we see that $x^2$ is not in any of these sets. Thus, $x^2$ is nonzero in your group $G$.
Thank you . But why you are saying that $ x^2$ is not in R can you clarify
– Rosa1
Jul 29 at 9:57
can you clarify
– Rosa1
Jul 29 at 10:09
add a comment |Â
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
1
down vote
accepted
The relation $x^4=1$ guarantees that the order of $x$ in $G$ is at most $4$. The fact that $xG'$ generates $G/G'cong C_4$ guarantees that the order of $x$ in $G$ is at least $4$.
Thank you . @Keith Kearnes The relation you mentioned is saying that $ x^4 $ belong to the normal closer of R so $ x^4R = R$ Where can we say that the order must be at least 4
– Rosa1
Jul 29 at 10:05
1
The answer explains exactly why the order is at least $4$.
– Derek Holt
Jul 29 at 12:27
@Derek Holt but he talked about the order of x in G/G'. How this is connected to the order of x in G
– Rosa1
Jul 29 at 14:27
1
The order of $xG'$ in $G/G'$ is $4$, so the order of $x$ in $G$ is at least $4$.
– Derek Holt
Jul 29 at 15:15
add a comment |Â
up vote
1
down vote
accepted
The relation $x^4=1$ guarantees that the order of $x$ in $G$ is at most $4$. The fact that $xG'$ generates $G/G'cong C_4$ guarantees that the order of $x$ in $G$ is at least $4$.
Thank you . @Keith Kearnes The relation you mentioned is saying that $ x^4 $ belong to the normal closer of R so $ x^4R = R$ Where can we say that the order must be at least 4
– Rosa1
Jul 29 at 10:05
1
The answer explains exactly why the order is at least $4$.
– Derek Holt
Jul 29 at 12:27
@Derek Holt but he talked about the order of x in G/G'. How this is connected to the order of x in G
– Rosa1
Jul 29 at 14:27
1
The order of $xG'$ in $G/G'$ is $4$, so the order of $x$ in $G$ is at least $4$.
– Derek Holt
Jul 29 at 15:15
add a comment |Â
up vote
1
down vote
accepted
up vote
1
down vote
accepted
The relation $x^4=1$ guarantees that the order of $x$ in $G$ is at most $4$. The fact that $xG'$ generates $G/G'cong C_4$ guarantees that the order of $x$ in $G$ is at least $4$.
The relation $x^4=1$ guarantees that the order of $x$ in $G$ is at most $4$. The fact that $xG'$ generates $G/G'cong C_4$ guarantees that the order of $x$ in $G$ is at least $4$.
answered Jul 29 at 7:46
Keith Kearnes
5,1691626
5,1691626
Thank you . @Keith Kearnes The relation you mentioned is saying that $ x^4 $ belong to the normal closer of R so $ x^4R = R$ Where can we say that the order must be at least 4
– Rosa1
Jul 29 at 10:05
1
The answer explains exactly why the order is at least $4$.
– Derek Holt
Jul 29 at 12:27
@Derek Holt but he talked about the order of x in G/G'. How this is connected to the order of x in G
– Rosa1
Jul 29 at 14:27
1
The order of $xG'$ in $G/G'$ is $4$, so the order of $x$ in $G$ is at least $4$.
– Derek Holt
Jul 29 at 15:15
add a comment |Â
Thank you . @Keith Kearnes The relation you mentioned is saying that $ x^4 $ belong to the normal closer of R so $ x^4R = R$ Where can we say that the order must be at least 4
– Rosa1
Jul 29 at 10:05
1
The answer explains exactly why the order is at least $4$.
– Derek Holt
Jul 29 at 12:27
@Derek Holt but he talked about the order of x in G/G'. How this is connected to the order of x in G
– Rosa1
Jul 29 at 14:27
1
The order of $xG'$ in $G/G'$ is $4$, so the order of $x$ in $G$ is at least $4$.
– Derek Holt
Jul 29 at 15:15
Thank you . @Keith Kearnes The relation you mentioned is saying that $ x^4 $ belong to the normal closer of R so $ x^4R = R$ Where can we say that the order must be at least 4
– Rosa1
Jul 29 at 10:05
Thank you . @Keith Kearnes The relation you mentioned is saying that $ x^4 $ belong to the normal closer of R so $ x^4R = R$ Where can we say that the order must be at least 4
– Rosa1
Jul 29 at 10:05
1
1
The answer explains exactly why the order is at least $4$.
– Derek Holt
Jul 29 at 12:27
The answer explains exactly why the order is at least $4$.
– Derek Holt
Jul 29 at 12:27
@Derek Holt but he talked about the order of x in G/G'. How this is connected to the order of x in G
– Rosa1
Jul 29 at 14:27
@Derek Holt but he talked about the order of x in G/G'. How this is connected to the order of x in G
– Rosa1
Jul 29 at 14:27
1
1
The order of $xG'$ in $G/G'$ is $4$, so the order of $x$ in $G$ is at least $4$.
– Derek Holt
Jul 29 at 15:15
The order of $xG'$ in $G/G'$ is $4$, so the order of $x$ in $G$ is at least $4$.
– Derek Holt
Jul 29 at 15:15
add a comment |Â
up vote
1
down vote
Well, the normal closure is even bigger than the subgroup generated by the relation. The only thing that has to be shown is that the order is, in fact, 4 and not less. Since the order divides 4, this boils down to showing that $x^2$ is not in the normal closure. Let $N$ be this normal closure, and let $R le N$ be the subgroup generated by the relations. By considering the powers of $x$ and $y$ occuring in any element of $R$, we find that $x^2 notin R$. But the normal closure is precisely the union of $fRf^-1$ ($f in Flangle x, y rangle$, the free group over $x$ and $y$), and we see that $x^2$ is not in any of these sets. Thus, $x^2$ is nonzero in your group $G$.
Thank you . But why you are saying that $ x^2$ is not in R can you clarify
– Rosa1
Jul 29 at 9:57
can you clarify
– Rosa1
Jul 29 at 10:09
add a comment |Â
up vote
1
down vote
Well, the normal closure is even bigger than the subgroup generated by the relation. The only thing that has to be shown is that the order is, in fact, 4 and not less. Since the order divides 4, this boils down to showing that $x^2$ is not in the normal closure. Let $N$ be this normal closure, and let $R le N$ be the subgroup generated by the relations. By considering the powers of $x$ and $y$ occuring in any element of $R$, we find that $x^2 notin R$. But the normal closure is precisely the union of $fRf^-1$ ($f in Flangle x, y rangle$, the free group over $x$ and $y$), and we see that $x^2$ is not in any of these sets. Thus, $x^2$ is nonzero in your group $G$.
Thank you . But why you are saying that $ x^2$ is not in R can you clarify
– Rosa1
Jul 29 at 9:57
can you clarify
– Rosa1
Jul 29 at 10:09
add a comment |Â
up vote
1
down vote
up vote
1
down vote
Well, the normal closure is even bigger than the subgroup generated by the relation. The only thing that has to be shown is that the order is, in fact, 4 and not less. Since the order divides 4, this boils down to showing that $x^2$ is not in the normal closure. Let $N$ be this normal closure, and let $R le N$ be the subgroup generated by the relations. By considering the powers of $x$ and $y$ occuring in any element of $R$, we find that $x^2 notin R$. But the normal closure is precisely the union of $fRf^-1$ ($f in Flangle x, y rangle$, the free group over $x$ and $y$), and we see that $x^2$ is not in any of these sets. Thus, $x^2$ is nonzero in your group $G$.
Well, the normal closure is even bigger than the subgroup generated by the relation. The only thing that has to be shown is that the order is, in fact, 4 and not less. Since the order divides 4, this boils down to showing that $x^2$ is not in the normal closure. Let $N$ be this normal closure, and let $R le N$ be the subgroup generated by the relations. By considering the powers of $x$ and $y$ occuring in any element of $R$, we find that $x^2 notin R$. But the normal closure is precisely the union of $fRf^-1$ ($f in Flangle x, y rangle$, the free group over $x$ and $y$), and we see that $x^2$ is not in any of these sets. Thus, $x^2$ is nonzero in your group $G$.
answered Jul 29 at 7:39
AlgebraicsAnonymous
66611
66611
Thank you . But why you are saying that $ x^2$ is not in R can you clarify
– Rosa1
Jul 29 at 9:57
can you clarify
– Rosa1
Jul 29 at 10:09
add a comment |Â
Thank you . But why you are saying that $ x^2$ is not in R can you clarify
– Rosa1
Jul 29 at 9:57
can you clarify
– Rosa1
Jul 29 at 10:09
Thank you . But why you are saying that $ x^2$ is not in R can you clarify
– Rosa1
Jul 29 at 9:57
Thank you . But why you are saying that $ x^2$ is not in R can you clarify
– Rosa1
Jul 29 at 9:57
can you clarify
– Rosa1
Jul 29 at 10:09
can you clarify
– Rosa1
Jul 29 at 10:09
add a comment |Â
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I'm just curious: how did you find G/G' isomorphic to C4?
– Yip Jung Hon
Jul 29 at 7:53
@YipJungHon: If $G$ is presented by $langle X;|;Rrangle$, then $G/G'$ can be presented by $langle X;|;Rcup Crangle$, where $C = [x_i,x_j];$. From this it is not hard to see that, for the group $G$ of this problem, $G/G'$ contains only the elements $G', xG', (xG')^2, (xG')^3=yG'$. To see that these four elements must be distinct in the presented group, it suffices to observe that there exists a group, namely $C_4=1,g,g^2,g^3$, which has an element $x:=g$ of order $4$ and another element $y:=g^-1$ which satisfies all relations in the presentation.
– Keith Kearnes
Jul 29 at 10:01
Just curious, if I define the group slightly differently, with it being: $G=<x,y | x^4=y^4=5>,$ it will still be $cong$ to $C_4$ right?
– Yip Jung Hon
Jul 29 at 11:06
@Yip Jung Hon by adding the relation xy=yx the group become abelian and hence from the last relation you have x=y and simply remove the generator y you left with x^4=1
– Rosa1
Jul 29 at 14:08
In G/G' you only make the generators abelian
– Rosa1
Jul 29 at 14:09