Is There a Normal Subgroup K in Groups G and H with Index (G:K) ≤ n!?

In summary, for groups G and H (finite or infinite), if the index of H in G is n, then there exists a normal subgroup K of G such that the index of K in G is less than or equal to n!. This can be proven by considering a homomorphism from G to the group of permutations of the cosets of H and the kernel of this homomorphism. It is also worth noting that (G:K) divides n!.
  • #1
emptyboat
28
1
G,H be groups(finite or infinite)
Prove that if (G:H)=n, then there exist some normal subgroup K of G (G:K)≤n!
example) let G=A5, H=A4 then (G:H)=5, then K={id} exists, (G:K)≤5!
 
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  • #2
This is a standard result. If you think how elements of G can act on the left (or right) cosets of H you should come up with a homomorphism of G into the group of permutations of the cosets. Then think about the kernel of the homomorphism.
 
  • #3
Actually you have (G:K)|n! which is sometimes more useful.
 
  • #4
Thanks a lot Martin. I understand it.
 
  • #5


I can confirm that the existence of a normal subgroup K in groups G and H, where (G:H)=n, is a well-established mathematical concept. This result is known as the "Correspondence Theorem" in group theory.

To prove this, we can use the fact that the index of a subgroup, (G:H), represents the number of distinct cosets of H in G. Since we know that H is a subgroup of G, it follows that the identity element, denoted as id, is contained in H. Therefore, the coset H=idH is a subgroup of G, and its index is equal to 1.

Now, let us consider the subgroup K=id. Since K is a subgroup of H, it follows that the coset K=idK is also a subgroup of G. Therefore, the index of K in G, denoted as (G:K), is equal to the index of H in G, denoted as (G:H), which is equal to n.

Furthermore, since K is a subgroup of H, it follows that K is a normal subgroup of G. This is because for any element g∈G and k∈K, we have that gkg^-1∈K, since K=id and g^-1∈H. Therefore, K is a normal subgroup of G, and we have proven that (G:K)≤n!

In the example given, where G=A5 and H=A4, we can see that (G:H)=5. Using the above proof, we can conclude that K={id} exists as a normal subgroup of G, and (G:K)≤5!, which is equal to 120. This means that there are at most 120 cosets of K in G.

In conclusion, the existence of a normal subgroup K in groups G and H, where (G:H)=n, is a proven result in group theory. This result has many applications in different areas of mathematics, including group theory, abstract algebra, and number theory.
 

Related to Is There a Normal Subgroup K in Groups G and H with Index (G:K) ≤ n!?

Question 1: What is a normal subgroup?

A normal subgroup is a subset of a group that is invariant under conjugation by elements of the group. This means that if an element of the group is conjugated with an element of the normal subgroup, the result will still be in the normal subgroup.

Question 2: How do you determine if a subgroup is normal?

There are a few different ways to determine if a subgroup is normal. One way is to check if every element of the subgroup commutes with every element of the larger group. Another way is to check if the left and right cosets of the subgroup are equal. Finally, if the subgroup is the kernel of a homomorphism, it is automatically a normal subgroup.

Question 3: Can every group have a normal subgroup?

No, not every group has a normal subgroup. For example, a group with prime order does not have any non-trivial normal subgroups.

Question 4: What is the significance of a normal subgroup?

A normal subgroup is significant because it allows us to define quotient groups, which are an important concept in abstract algebra. Normal subgroups also play a role in the structure of a group, as they can be used to classify groups into different types.

Question 5: Can a group have more than one normal subgroup?

Yes, a group can have more than one normal subgroup. In fact, a group can have multiple normal subgroups of different orders. However, if a group has only one normal subgroup, it is called a simple group and has a very special structure.

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