Group mono-, endo-, iso-, homomorphism

  • MHB
  • Thread starter mathmari
  • Start date
  • Tags
    Group
In summary, the conversation discussed the definitions and properties of groups, including closure, associativity, identity, inverse, monomorphism, endomorphism, and isomorphism. It also explored the possible isomorphism between two groups, $(G, \#)$ and $(H, \square)$, and the necessary conditions for it to hold. The conversation concluded that the two groups, $\mathbb{Z}_4$ and $\mathbb{Z}_2 \times \mathbb{Z}_2$, are not isomorphic due to the difference in the order of their elements.
  • #1
mathmari
Gold Member
MHB
5,049
7
Hey! 😊

Let $(G, \#), \ (H, \square )$ be groups. Show:
  1. For $(g,h), (g',h')\in G\times H$ we define the operation $\star$ on $G\star H$ as follows:
    \begin{equation*}\star: (G\times H)\times (G\times H,\star), \ \left ((g,h), (g',h')\right )\mapsto (g\# g', h\square h')\end{equation*} Then $(G\times H, \star)$ is a group.
  2. The map $G\rightarrow G\times H, \ g\mapsto (g, e_H)$ is a monomorphism, where $e_H$ is the neutral element in $H$.
  3. The map $G\times H\rightarrow G, \ (g,h)\mapsto g$ is an endomorphism.
  4. Let $G=\mathbb{Z}/4\mathbb{Z}$ and $H=\mathbb{Z}/2\mathbb{Z}\times \mathbb{Z}/2\mathbb{Z}$. Does it hold that $G\sim H$ ?

For 1:
We have to show that the four axioms (closure, associativity, identity, inverse) are satisfied. For 2:
Let $\phi$ be the name of that map. Then we have that $\phi (g\#g')=(g\#g', e_H)$. How could we continue? For 3:
Let $\psi$ be the name of that map. Then we have that $\psi ((g,h)\star (g',h'))=(g\#g', h\square h')$. How could we continue? For 4:
We have to define $f:G\rightarrow H$ and check if this map is bijective, right? :unsure:
 
Physics news on Phys.org
  • #2
mathmari said:
For 1:
We have to show that the four axioms (closure, associativity, identity, inverse) are satisfied.

Hey mathmari!

Yep. (Nod)

Btw, shouldn't it be:
  • For $(g,h), (g',h')\in G\times H$ we define the operation $\star$ on $G\times H$ as follows:
    \begin{equation*}\star: (G\times H)\times (G\times H)\to (G\times H), \ \left ((g,h), (g',h')\right )\mapsto (g\# g', h\square h')\end{equation*} Then $(G\times H, \star)$ is a group.
? (Wondering)

mathmari said:
For 2:
Let $\phi$ be the name of that map. Then we have that $\phi (g\#g')=(g\#g', e_H)$. How could we continue?

Shouldn't we check if it is equal to $\phi(g)\star \phi(g')$? 🤔

mathmari said:
For 3:
Let $\psi$ be the name of that map. Then we have that $\psi ((g,h)\star (g',h'))=(g\#g', h\square h')$. How could we continue?

Shouldn't we check if it is equal to $\psi((g,h))\star\psi((g',h'))$? 🤔

mathmari said:
For 4:
We have to define $f:G\rightarrow H$ and check if this map is bijective, right?

Shouldn't we check if the map is a homomorphism as well? 🤔
 
  • #3
Klaas van Aarsen said:
Btw, shouldn't it be:
  • For $(g,h), (g',h')\in G\times H$ we define the operation $\star$ on $G\times H$ as follows:
    \begin{equation*}\star: (G\times H)\times (G\times H)\to (G\times H), \ \left ((g,h), (g',h')\right )\mapsto (g\# g', h\square h')\end{equation*} Then $(G\times H, \star)$ is a group.
? (Wondering)

Ahh yes!

It is closed since the result of $(g,h)\star (g',h')$ with $(g,h), (g',h')\in G\times H$ is again in $G\times H$.

We have that \begin{align*}&[(g,h)\star (g',h')]\star (\tilde{g},\tilde{h})=(g\# g', h\square h')\star (\tilde{g},\tilde{h})=(g\# g'\#\tilde{g}, h\square h'\square \tilde{h}) \\ &(g,h)\star [(g',h')\star (\tilde{g},\tilde{h})]=(g,h)\star (g'\# \tilde{g},h'\square \tilde{h})= (g\#g'\# \tilde{g},h\square h'\square \tilde{h})\end{align*}
So the associativity follows.

The identity is $(e_G,e_H)$ since $(g,h)\star (e_G,e_H)=(g\# e_G, h\square e_H)=(g,h)$ and $ (e_G,e_H)\star (g,h)=(e_G\# g, e_H\square h)=(g,h)$.

Since $G$ and $H$ are groups, they have closed by inverses. So $g^{-1}\in G$ and $h^{-1}\in H$ the inverses of $g$ and $h$ respectively. Then the inverse of $(g,h)\in G\times H$ is $(g^{-1}, h^{-1})$ since $(g,h)\star (g^{-1}, h^{-1})=(g\# g^{-1}, h\square h^{-1})=(e_G,e_H)$ and $ (g^{-1},h^{-1})\star (g,h)=(g^{-1}\# g, h^{-1}\square h)=(e_G,e_H)$.

So it follows that $(G\times H, \star)$ is a group.
Klaas van Aarsen said:
Shouldn't we check if it is equal to $\phi(g)\star \phi(g')$? 🤔

We have that $\phi(g)\star \phi(g')=(g,e_H)\star (g',e_H)=(g\#g', e_H\square e_H)$. It holds that $e_H\square e_H=e_H$ because $e_H$ is the neutral element of $H$, right?
Since $\phi (g\#g')=(g\#g', e_H)$ and so $\phi(g)\star \phi(g')=\phi (g\#g')$

Further we have to show that $\phi$ is injective. Let $\phi(g)= \phi(g')$. Then we get $(g,e_H)=(g',e_H)$ and it follows that $g=g'$.
Therefore $\phi$ is injective and so it is a monmorphism.
Klaas van Aarsen said:
Shouldn't we check if it is equal to $\psi((g,h))\star\psi((g',h'))$? 🤔

We have that $\psi((g,h))\star\psi((g',h'))=g\star g'$. Is this correct? Isn't the operation $\star$ defined at elements of $G\times H$ but we have elements of $G$. :unsure:
Klaas van Aarsen said:
Shouldn't we check if the map is a homomorphism as well? 🤔

Ok.. But how is the map defined? :unsure:
 
  • #4
mathmari said:
We have that $\psi((g,h))\star\psi((g',h'))=g\star g'$. Is this correct? Isn't the operation $\star$ defined at elements of $G\times H$ but we have elements of $G$.

Ah, I intended $\psi((g,h))\#\psi((g',h'))$. (Blush)
mathmari said:
Ok.. But how is the map defined?
I think we have to come up with a map ourselves, don't we?
Or otherwise prove that there is no such map.
We might check what the order of the elements are to see if there is such a map. 🤔
 
  • #5
Klaas van Aarsen said:
Ah, I intended $\psi((g,h))\#\psi((g',h'))$. (Blush)

Ahh! So we have that $\psi((g,h))\#\psi((g',h'))=g\#g$. We also have that $\psi ((g,h)\star (g',h'))=\psi((g\#g', h\square h'))=g\#g'$.
Therefore $\psi$ is an homomorphism.

It is left to show that $\psi$ is surjective. For each $g\in G$ there is an element $(g,h)\in G\times H$ for some $h\in H$ such that $\psi ((g,h))=g$.
Therefore $\psi$ is an endomorphism.

Is that correct? :unsure:
Klaas van Aarsen said:
I think we have to come up with a map ourselves, don't we?
Or otherwise prove that there is no such map.
We might check what the order of the elements are to see if there is such a map. 🤔

We have that the orders of elements of $\mathbb{Z}_2 \times \mathbb{Z}_2$ are $|(0, 0)| = 1, |(0, 1)| = 2, |(1, 0)| = 2, |(1, 1)| = 2$, while the orders of the elements of $\mathbb{Z}_4$ are $|0| = 1, |1| = 4, |2| = 2$, and $|3| = 4$. Since at $\mathbb{Z}_4$ there is an element of order 4 but not in $\mathbb{Z}_2 \times \mathbb{Z}_2$, these cannot be isomorphic, right? :unsure:
 
  • #6
mathmari said:
Therefore $\psi$ is an endomorphism.

Is that correct?

It's not an endomorphism is it?
An endomorphism is a homomorphism from a group to the same group. (Worried)

mathmari said:
these cannot be isomorphic, right?
Indeed. :)
 
  • #7
Klaas van Aarsen said:
It's not an endomorphism is it?
An endomorphism is a homomorphism from a group to the same group. (Worried)

Oh I meant epimorphism, it was typo...Sorry!
Klaas van Aarsen said:
Indeed. :)

In this case, for these $G$ and $H$ do we consider the operations as above or the addition? :unsure:
 
  • #8
mathmari said:
Oh I meant epimorphism, it was typo...Sorry!

Then it is correct. :)
mathmari said:
In this case, for these $G$ and $H$ do we consider the operations as above or the addition?

For $G$ it is indeed addition.
For $H$ it would be the $\star$ as above where each individual operation is addition, wouldn't it? 🤔
 
  • #9
Klaas van Aarsen said:
For $G$ it is indeed addition.
For $H$ it would be the $\star$ as above where each individual operation is addition, wouldn't it? 🤔

So in this case we consider $\#$ to be $+$.
But at $H$ above we have $\square$, why do we consider here $\star$ ? I got stuck right now. Or do you mean at $G\star H$ to consider $\star$ ? :unsure:
 
  • #10
mathmari said:
So in this case we consider $\#$ to be $+$.
But at $H$ above we have $\square$, why do we consider here $\star$ ? I got stuck right now. Or do you mean at $G\star H$ to consider $\star$ ?
The symbols $G$ and $H$ are confusing here since we had the group $(G\times H,\star)$ above, which is not what we have in this question 5.
Let's rename the groups in question 5 to $\tilde G = (\mathbb Z/4\mathbb Z,+)$ and $\tilde H=(\mathbb Z/2\mathbb Z\times \mathbb Z/2\mathbb Z,\star)$, where $(a,b)\star(c,d)=(a+b,c+d)$.
So the operation for $\tilde G$ is simply $+$, which is the default operation on $\mathbb Z/n\mathbb Z$. No need to involve $\#$ here.
And both $\#$ and $\square$ are $+$ for $\tilde H$. Instead of $\star$, we might also simply use $+$.

Does it make sense now? (Wondering)
 
Last edited:

FAQ: Group mono-, endo-, iso-, homomorphism

1. What is a group homomorphism?

A group homomorphism is a function that preserves the structure of a group. In other words, it maps elements from one group to another in a way that maintains the group operation. This means that the result of applying the group operation to two elements in the first group will be the same as applying the group operation to the corresponding elements in the second group.

2. What is the difference between a monomorphism and an endomorphism?

A monomorphism is a homomorphism that is injective, meaning that each element in the first group maps to a unique element in the second group. An endomorphism is a homomorphism where the first and second group are the same. In other words, it maps elements from a group to itself.

3. How is an isomorphism different from a homomorphism?

An isomorphism is a bijective homomorphism, meaning that it is both injective and surjective. This means that not only does each element in the first group map to a unique element in the second group, but also that every element in the second group has a corresponding element in the first group.

4. What does it mean for a homomorphism to be a group action?

A group action is a special type of homomorphism where the first group is a group of transformations and the second group is a set of objects on which the transformations act. This means that the homomorphism maps each transformation to a function that acts on the objects in the second group.

5. How are group homomorphisms used in mathematics?

Group homomorphisms are used to study the relationship between different groups and their structures. They can be used to show that two groups are isomorphic, which means that they have the same structure. They are also used in abstract algebra, topology, and other areas of mathematics to prove theorems and solve problems.

Similar threads

Replies
19
Views
2K
Replies
1
Views
1K
Replies
9
Views
2K
Replies
5
Views
2K
Replies
1
Views
1K
Replies
2
Views
2K
Replies
6
Views
1K
Replies
16
Views
1K
Back
Top