Are Normal Subgroups and p-Groups Key to Understanding Finite Groups?

In summary, we are given a finite group G and a normal subgroup N which is a p-group for some prime p. Using the Sylow theorems, we can show that there exists a subgroup H of G containing N such that p does not divide the index [G:H]. Additionally, for any p-subgroup of G, N is a subgroup of it. This is because N is the only p-subgroup of G, as all p-subgroups are conjugate. However, this does not mean that N is the only sylow p-subgroup, as there can be multiple sylow p-subgroups of the same order.
  • #1
Poirot1
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Let G be a finite group and N a normal subgroup of G. Assume further that N is a p -group for some prime p.
1) By considering G/N, show that there is a subgroup H of G contaning N such that p does not divide [G:H].

2) Show that N is a subgroup of all p-subgroups of G.


My thoughts: for 1) maybe use sylow theorems. for 2) I don't understand because if N is a normal p-subgroup, then we have gNg^-1=N for all g. But by sylow, every p-subgroup is conjugate so we must conclude that N is the only p-subgroup of G.
 
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  • #2
your reasoning for #2 is fine:

if N is a subgroup of a sylow p-subgroup P, then certainly gNg-1 is a subgroup of gPg-1.

you're on the right track for #1: let H be a sylow p-subgroup of G.

then |G| = |H|m, where p does not divide m, and [G:H] = |G|/|H| = |H|m/|H| = m.
 
  • #3
Poirot said:
Let G be a finite group and N a normal subgroup of G. Assume further that N is a p -group for some prime p.
1) By considering G/N, show that there is a subgroup H of G contaning N such that p does not divide [G:H].

2) Show that N is a subgroup of all p-subgroups of G.


My thoughts: for 1) maybe use sylow theorems. for 2) I don't understand because if N is a normal p-subgroup, then we have gNg^-1=N for all g. But by sylow, every p-subgroup is conjugate so we must conclude that N is the only p-subgroup of G.
I think use of sylow theorem in (1) helps.
Let where doesn't divide . Let . Let be a sylow p-subgroup of . So . Then we know that (by what is probably called the fourth isomorphism theorem) such that and . It follows that . Thus we have shown that is contained in which is a sylow p-subgroup of and this is enough to prove (1).
 
  • #4
what is the point of question 2) when N is the only p-subgroup? Wait, there's no difference between a sylow p-subgroup and a p-subgroup is there?
 
  • #5
Poirot said:
what is the point of question 2) when N is the only p-subgroup? Wait, there's no difference between a sylow p-subgroup and a p-subgroup is there?
Yeah I think there ain't any difference between a sylow p-subgroup and a p-subgroup. But p-subgroups and p-groups are different. So (2) ain't exactly trivial.
We are given a p-group N, which is a normal subgroup of G. We need to prove that it is a subgroup of all the p-subgroups.
Using (1) this is easy.
 
  • #6
caffeinemachine said:
Yeah I think there ain't any difference between a sylow p-subgroup and a p-subgroup. But p-subgroups and p-groups are different. So (2) ain't exactly trivial.
We are given a p-group N, which is a normal subgroup of G. We need to prove that it is a subgroup of all the p-subgroups.
Using (1) this is easy.

What is the difference ? Is it that (sylow) p subgroups have order p^m where p^m is the highest power of p dividing o(G), whearas a p-group has order p^r where r is between 1 and m? That's the only way I can make sense of it.
 
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  • #7
Poirot said:
What is the difference ? Is it that (sylow) p subgroups have order p^m where p^m is the highest power of p dividing o(G), whearas a p-group has order p^r where r is between 1 and m? That's the only way I can make sense of it.
This is the way I have made sense of it too.
 
  • #8
sylow p-subgroups are maximal p-subgroups.
 
  • #9
by sylow theorems, if we let t represent the number of sylow p-subgroups of order say p^n, then there exist m,k such that and =

How do you know that the order of G is when m does not divide p?
 
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  • #10
Poirot said:
by sylow theorems, if we let t represent the number of sylow p-subgroups of order say p^n, then there exist m,k such that and =

How do you know that the order of G is when m does not divide p?

because EVERY positive integer is of that form (although n might be 0). although if we have a p-subgroup N, by lagrange n can't be 0.
 

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