Few group theory questions
$begingroup$
I am trying to solve the following;
First, given $G$ is a group and $H$ a subgroup of $G$, what can we say about the relation $a cong b$ if $b^{-1}a in H$
I can show that it is reflexive as the identity is always in the subgroup.
if $a cong b$ then $b^{-1}a in H$ and so $(b^{-1}a)^{-1}=a^{-1}b in H$ so $b cong c$.
Now I must determine if $G$ being abelian is required for this to be transitive.
My thought would be that if $ a cong b $ then $b^{-1}a in H $ and if $b cong c$ then $c^{-1}b in H$ and by properties of subgroups so to is $c^{-1}bb^{-1}a=c^{-1}a$ which would seem to imply $a cong c$, but I don't know if I am somehow doing something that couldn't be done if $G$ was not abelian. What do you guys think?
And second part asks to discuss the possible homomorphism (group) from $mathbb{Z}/nmathbb{Z}$ to $mathbb{Z}$ and it says for $n ge 1$, is every homomorphism the zero one. Either yes, no, or it depends on $n$.
My thoughts are that only the zero homomorphism is possible, since if $phi$ was a homomorphism then $phi(ab)=phi(a)phi(b)$ and this would quickly result in having $ab=n=0$ but $phi(a)$ and $phi(b)$ not being $0$.
If $n$ was prime, then I am not so sure, but wouldn't this still end up mapping many things to the identity etc?
Thank you all
group-theory relations group-homomorphism
$endgroup$
add a comment |
$begingroup$
I am trying to solve the following;
First, given $G$ is a group and $H$ a subgroup of $G$, what can we say about the relation $a cong b$ if $b^{-1}a in H$
I can show that it is reflexive as the identity is always in the subgroup.
if $a cong b$ then $b^{-1}a in H$ and so $(b^{-1}a)^{-1}=a^{-1}b in H$ so $b cong c$.
Now I must determine if $G$ being abelian is required for this to be transitive.
My thought would be that if $ a cong b $ then $b^{-1}a in H $ and if $b cong c$ then $c^{-1}b in H$ and by properties of subgroups so to is $c^{-1}bb^{-1}a=c^{-1}a$ which would seem to imply $a cong c$, but I don't know if I am somehow doing something that couldn't be done if $G$ was not abelian. What do you guys think?
And second part asks to discuss the possible homomorphism (group) from $mathbb{Z}/nmathbb{Z}$ to $mathbb{Z}$ and it says for $n ge 1$, is every homomorphism the zero one. Either yes, no, or it depends on $n$.
My thoughts are that only the zero homomorphism is possible, since if $phi$ was a homomorphism then $phi(ab)=phi(a)phi(b)$ and this would quickly result in having $ab=n=0$ but $phi(a)$ and $phi(b)$ not being $0$.
If $n$ was prime, then I am not so sure, but wouldn't this still end up mapping many things to the identity etc?
Thank you all
group-theory relations group-homomorphism
$endgroup$
$begingroup$
I see nothing wrong with your transitivity proof and it doesn't require commutativity. In part 2 you seem to be confabulating addition with multiplication. This is group, not field theory. ab = n = e means a and b are inverses. Not that one or the other is 0.
$endgroup$
– fleablood
Dec 8 '15 at 21:32
add a comment |
$begingroup$
I am trying to solve the following;
First, given $G$ is a group and $H$ a subgroup of $G$, what can we say about the relation $a cong b$ if $b^{-1}a in H$
I can show that it is reflexive as the identity is always in the subgroup.
if $a cong b$ then $b^{-1}a in H$ and so $(b^{-1}a)^{-1}=a^{-1}b in H$ so $b cong c$.
Now I must determine if $G$ being abelian is required for this to be transitive.
My thought would be that if $ a cong b $ then $b^{-1}a in H $ and if $b cong c$ then $c^{-1}b in H$ and by properties of subgroups so to is $c^{-1}bb^{-1}a=c^{-1}a$ which would seem to imply $a cong c$, but I don't know if I am somehow doing something that couldn't be done if $G$ was not abelian. What do you guys think?
And second part asks to discuss the possible homomorphism (group) from $mathbb{Z}/nmathbb{Z}$ to $mathbb{Z}$ and it says for $n ge 1$, is every homomorphism the zero one. Either yes, no, or it depends on $n$.
My thoughts are that only the zero homomorphism is possible, since if $phi$ was a homomorphism then $phi(ab)=phi(a)phi(b)$ and this would quickly result in having $ab=n=0$ but $phi(a)$ and $phi(b)$ not being $0$.
If $n$ was prime, then I am not so sure, but wouldn't this still end up mapping many things to the identity etc?
Thank you all
group-theory relations group-homomorphism
$endgroup$
I am trying to solve the following;
First, given $G$ is a group and $H$ a subgroup of $G$, what can we say about the relation $a cong b$ if $b^{-1}a in H$
I can show that it is reflexive as the identity is always in the subgroup.
if $a cong b$ then $b^{-1}a in H$ and so $(b^{-1}a)^{-1}=a^{-1}b in H$ so $b cong c$.
Now I must determine if $G$ being abelian is required for this to be transitive.
My thought would be that if $ a cong b $ then $b^{-1}a in H $ and if $b cong c$ then $c^{-1}b in H$ and by properties of subgroups so to is $c^{-1}bb^{-1}a=c^{-1}a$ which would seem to imply $a cong c$, but I don't know if I am somehow doing something that couldn't be done if $G$ was not abelian. What do you guys think?
And second part asks to discuss the possible homomorphism (group) from $mathbb{Z}/nmathbb{Z}$ to $mathbb{Z}$ and it says for $n ge 1$, is every homomorphism the zero one. Either yes, no, or it depends on $n$.
My thoughts are that only the zero homomorphism is possible, since if $phi$ was a homomorphism then $phi(ab)=phi(a)phi(b)$ and this would quickly result in having $ab=n=0$ but $phi(a)$ and $phi(b)$ not being $0$.
If $n$ was prime, then I am not so sure, but wouldn't this still end up mapping many things to the identity etc?
Thank you all
group-theory relations group-homomorphism
group-theory relations group-homomorphism
edited Nov 28 '18 at 22:43
the_fox
2,57711432
2,57711432
asked Dec 8 '15 at 21:17
PersonaAPersonaA
329617
329617
$begingroup$
I see nothing wrong with your transitivity proof and it doesn't require commutativity. In part 2 you seem to be confabulating addition with multiplication. This is group, not field theory. ab = n = e means a and b are inverses. Not that one or the other is 0.
$endgroup$
– fleablood
Dec 8 '15 at 21:32
add a comment |
$begingroup$
I see nothing wrong with your transitivity proof and it doesn't require commutativity. In part 2 you seem to be confabulating addition with multiplication. This is group, not field theory. ab = n = e means a and b are inverses. Not that one or the other is 0.
$endgroup$
– fleablood
Dec 8 '15 at 21:32
$begingroup$
I see nothing wrong with your transitivity proof and it doesn't require commutativity. In part 2 you seem to be confabulating addition with multiplication. This is group, not field theory. ab = n = e means a and b are inverses. Not that one or the other is 0.
$endgroup$
– fleablood
Dec 8 '15 at 21:32
$begingroup$
I see nothing wrong with your transitivity proof and it doesn't require commutativity. In part 2 you seem to be confabulating addition with multiplication. This is group, not field theory. ab = n = e means a and b are inverses. Not that one or the other is 0.
$endgroup$
– fleablood
Dec 8 '15 at 21:32
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Let $phi:Bbb Z/nto Bbb Z$ be a homomorphism.
So $phi(a + b) = phi(a) + phi(b)$ so $phi(a) = phi(0 + a) = phi(0) + phi(a)$ so $phi(0) = 0$.
Now $$begin{align}phi(am) &= phileft(underbrace{a + a + dots + a}_{mtext{ times.}}right) \ &= underbrace{phi(a) + phi(a) + dots + phi(a)}_{mtext{ times.}} \ &= phi(a)times m.end{align}$$
And $an = 0$ for all $a$ in $Bbb Z/n$.
So $phi(an) = phi(a)times n = 0$ for all $a$ in $Bbb Z/n$. But in $phi(a) in Bbb Z$. $phi(a)times n = 0 implies phi(a) = 0$. This is true for all $a$ in $Bbb Z/n$.
So $phi$ is the zero homomorphism. It's the only possible homomorphism.
$endgroup$
add a comment |
$begingroup$
- Correct, commutativity is not needed and not used in your proof.
- The group operation is rather addition.
Hint: Where can the equivalence class $[1]_{nBbb Z}$ be mapped by a homomorphism to $Bbb Z$?
$endgroup$
$begingroup$
So if it is addition, then the identity element is zero correct? so it would have to be mapped to zero in Z, no?
$endgroup$
– PersonaA
Dec 8 '15 at 21:35
$begingroup$
The identity gets mapped to the identity under all homomorphisms to any group. Were can 1, the generator of Z/n, get mapped to, is what is being asked. Hint: 1n = 0 in Z/n. So which a $in$ Z have the property an = 0?
$endgroup$
– fleablood
Dec 8 '15 at 21:43
$begingroup$
Wouldn't it be all of them relatively ptime?
$endgroup$
– PersonaA
Dec 8 '15 at 21:47
$begingroup$
But what elements a in Z (not Z/n) have the property an = 0? In Z. Not Z/n
$endgroup$
– fleablood
Dec 8 '15 at 21:48
$begingroup$
Only a=0 then ?
$endgroup$
– PersonaA
Dec 8 '15 at 21:48
|
show 1 more comment
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
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active
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active
oldest
votes
$begingroup$
Let $phi:Bbb Z/nto Bbb Z$ be a homomorphism.
So $phi(a + b) = phi(a) + phi(b)$ so $phi(a) = phi(0 + a) = phi(0) + phi(a)$ so $phi(0) = 0$.
Now $$begin{align}phi(am) &= phileft(underbrace{a + a + dots + a}_{mtext{ times.}}right) \ &= underbrace{phi(a) + phi(a) + dots + phi(a)}_{mtext{ times.}} \ &= phi(a)times m.end{align}$$
And $an = 0$ for all $a$ in $Bbb Z/n$.
So $phi(an) = phi(a)times n = 0$ for all $a$ in $Bbb Z/n$. But in $phi(a) in Bbb Z$. $phi(a)times n = 0 implies phi(a) = 0$. This is true for all $a$ in $Bbb Z/n$.
So $phi$ is the zero homomorphism. It's the only possible homomorphism.
$endgroup$
add a comment |
$begingroup$
Let $phi:Bbb Z/nto Bbb Z$ be a homomorphism.
So $phi(a + b) = phi(a) + phi(b)$ so $phi(a) = phi(0 + a) = phi(0) + phi(a)$ so $phi(0) = 0$.
Now $$begin{align}phi(am) &= phileft(underbrace{a + a + dots + a}_{mtext{ times.}}right) \ &= underbrace{phi(a) + phi(a) + dots + phi(a)}_{mtext{ times.}} \ &= phi(a)times m.end{align}$$
And $an = 0$ for all $a$ in $Bbb Z/n$.
So $phi(an) = phi(a)times n = 0$ for all $a$ in $Bbb Z/n$. But in $phi(a) in Bbb Z$. $phi(a)times n = 0 implies phi(a) = 0$. This is true for all $a$ in $Bbb Z/n$.
So $phi$ is the zero homomorphism. It's the only possible homomorphism.
$endgroup$
add a comment |
$begingroup$
Let $phi:Bbb Z/nto Bbb Z$ be a homomorphism.
So $phi(a + b) = phi(a) + phi(b)$ so $phi(a) = phi(0 + a) = phi(0) + phi(a)$ so $phi(0) = 0$.
Now $$begin{align}phi(am) &= phileft(underbrace{a + a + dots + a}_{mtext{ times.}}right) \ &= underbrace{phi(a) + phi(a) + dots + phi(a)}_{mtext{ times.}} \ &= phi(a)times m.end{align}$$
And $an = 0$ for all $a$ in $Bbb Z/n$.
So $phi(an) = phi(a)times n = 0$ for all $a$ in $Bbb Z/n$. But in $phi(a) in Bbb Z$. $phi(a)times n = 0 implies phi(a) = 0$. This is true for all $a$ in $Bbb Z/n$.
So $phi$ is the zero homomorphism. It's the only possible homomorphism.
$endgroup$
Let $phi:Bbb Z/nto Bbb Z$ be a homomorphism.
So $phi(a + b) = phi(a) + phi(b)$ so $phi(a) = phi(0 + a) = phi(0) + phi(a)$ so $phi(0) = 0$.
Now $$begin{align}phi(am) &= phileft(underbrace{a + a + dots + a}_{mtext{ times.}}right) \ &= underbrace{phi(a) + phi(a) + dots + phi(a)}_{mtext{ times.}} \ &= phi(a)times m.end{align}$$
And $an = 0$ for all $a$ in $Bbb Z/n$.
So $phi(an) = phi(a)times n = 0$ for all $a$ in $Bbb Z/n$. But in $phi(a) in Bbb Z$. $phi(a)times n = 0 implies phi(a) = 0$. This is true for all $a$ in $Bbb Z/n$.
So $phi$ is the zero homomorphism. It's the only possible homomorphism.
edited Nov 28 '18 at 22:21
Shaun
8,832113681
8,832113681
answered Dec 8 '15 at 21:53
fleabloodfleablood
68.7k22685
68.7k22685
add a comment |
add a comment |
$begingroup$
- Correct, commutativity is not needed and not used in your proof.
- The group operation is rather addition.
Hint: Where can the equivalence class $[1]_{nBbb Z}$ be mapped by a homomorphism to $Bbb Z$?
$endgroup$
$begingroup$
So if it is addition, then the identity element is zero correct? so it would have to be mapped to zero in Z, no?
$endgroup$
– PersonaA
Dec 8 '15 at 21:35
$begingroup$
The identity gets mapped to the identity under all homomorphisms to any group. Were can 1, the generator of Z/n, get mapped to, is what is being asked. Hint: 1n = 0 in Z/n. So which a $in$ Z have the property an = 0?
$endgroup$
– fleablood
Dec 8 '15 at 21:43
$begingroup$
Wouldn't it be all of them relatively ptime?
$endgroup$
– PersonaA
Dec 8 '15 at 21:47
$begingroup$
But what elements a in Z (not Z/n) have the property an = 0? In Z. Not Z/n
$endgroup$
– fleablood
Dec 8 '15 at 21:48
$begingroup$
Only a=0 then ?
$endgroup$
– PersonaA
Dec 8 '15 at 21:48
|
show 1 more comment
$begingroup$
- Correct, commutativity is not needed and not used in your proof.
- The group operation is rather addition.
Hint: Where can the equivalence class $[1]_{nBbb Z}$ be mapped by a homomorphism to $Bbb Z$?
$endgroup$
$begingroup$
So if it is addition, then the identity element is zero correct? so it would have to be mapped to zero in Z, no?
$endgroup$
– PersonaA
Dec 8 '15 at 21:35
$begingroup$
The identity gets mapped to the identity under all homomorphisms to any group. Were can 1, the generator of Z/n, get mapped to, is what is being asked. Hint: 1n = 0 in Z/n. So which a $in$ Z have the property an = 0?
$endgroup$
– fleablood
Dec 8 '15 at 21:43
$begingroup$
Wouldn't it be all of them relatively ptime?
$endgroup$
– PersonaA
Dec 8 '15 at 21:47
$begingroup$
But what elements a in Z (not Z/n) have the property an = 0? In Z. Not Z/n
$endgroup$
– fleablood
Dec 8 '15 at 21:48
$begingroup$
Only a=0 then ?
$endgroup$
– PersonaA
Dec 8 '15 at 21:48
|
show 1 more comment
$begingroup$
- Correct, commutativity is not needed and not used in your proof.
- The group operation is rather addition.
Hint: Where can the equivalence class $[1]_{nBbb Z}$ be mapped by a homomorphism to $Bbb Z$?
$endgroup$
- Correct, commutativity is not needed and not used in your proof.
- The group operation is rather addition.
Hint: Where can the equivalence class $[1]_{nBbb Z}$ be mapped by a homomorphism to $Bbb Z$?
answered Dec 8 '15 at 21:23
BerciBerci
60k23672
60k23672
$begingroup$
So if it is addition, then the identity element is zero correct? so it would have to be mapped to zero in Z, no?
$endgroup$
– PersonaA
Dec 8 '15 at 21:35
$begingroup$
The identity gets mapped to the identity under all homomorphisms to any group. Were can 1, the generator of Z/n, get mapped to, is what is being asked. Hint: 1n = 0 in Z/n. So which a $in$ Z have the property an = 0?
$endgroup$
– fleablood
Dec 8 '15 at 21:43
$begingroup$
Wouldn't it be all of them relatively ptime?
$endgroup$
– PersonaA
Dec 8 '15 at 21:47
$begingroup$
But what elements a in Z (not Z/n) have the property an = 0? In Z. Not Z/n
$endgroup$
– fleablood
Dec 8 '15 at 21:48
$begingroup$
Only a=0 then ?
$endgroup$
– PersonaA
Dec 8 '15 at 21:48
|
show 1 more comment
$begingroup$
So if it is addition, then the identity element is zero correct? so it would have to be mapped to zero in Z, no?
$endgroup$
– PersonaA
Dec 8 '15 at 21:35
$begingroup$
The identity gets mapped to the identity under all homomorphisms to any group. Were can 1, the generator of Z/n, get mapped to, is what is being asked. Hint: 1n = 0 in Z/n. So which a $in$ Z have the property an = 0?
$endgroup$
– fleablood
Dec 8 '15 at 21:43
$begingroup$
Wouldn't it be all of them relatively ptime?
$endgroup$
– PersonaA
Dec 8 '15 at 21:47
$begingroup$
But what elements a in Z (not Z/n) have the property an = 0? In Z. Not Z/n
$endgroup$
– fleablood
Dec 8 '15 at 21:48
$begingroup$
Only a=0 then ?
$endgroup$
– PersonaA
Dec 8 '15 at 21:48
$begingroup$
So if it is addition, then the identity element is zero correct? so it would have to be mapped to zero in Z, no?
$endgroup$
– PersonaA
Dec 8 '15 at 21:35
$begingroup$
So if it is addition, then the identity element is zero correct? so it would have to be mapped to zero in Z, no?
$endgroup$
– PersonaA
Dec 8 '15 at 21:35
$begingroup$
The identity gets mapped to the identity under all homomorphisms to any group. Were can 1, the generator of Z/n, get mapped to, is what is being asked. Hint: 1n = 0 in Z/n. So which a $in$ Z have the property an = 0?
$endgroup$
– fleablood
Dec 8 '15 at 21:43
$begingroup$
The identity gets mapped to the identity under all homomorphisms to any group. Were can 1, the generator of Z/n, get mapped to, is what is being asked. Hint: 1n = 0 in Z/n. So which a $in$ Z have the property an = 0?
$endgroup$
– fleablood
Dec 8 '15 at 21:43
$begingroup$
Wouldn't it be all of them relatively ptime?
$endgroup$
– PersonaA
Dec 8 '15 at 21:47
$begingroup$
Wouldn't it be all of them relatively ptime?
$endgroup$
– PersonaA
Dec 8 '15 at 21:47
$begingroup$
But what elements a in Z (not Z/n) have the property an = 0? In Z. Not Z/n
$endgroup$
– fleablood
Dec 8 '15 at 21:48
$begingroup$
But what elements a in Z (not Z/n) have the property an = 0? In Z. Not Z/n
$endgroup$
– fleablood
Dec 8 '15 at 21:48
$begingroup$
Only a=0 then ?
$endgroup$
– PersonaA
Dec 8 '15 at 21:48
$begingroup$
Only a=0 then ?
$endgroup$
– PersonaA
Dec 8 '15 at 21:48
|
show 1 more comment
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$begingroup$
I see nothing wrong with your transitivity proof and it doesn't require commutativity. In part 2 you seem to be confabulating addition with multiplication. This is group, not field theory. ab = n = e means a and b are inverses. Not that one or the other is 0.
$endgroup$
– fleablood
Dec 8 '15 at 21:32