# Proof of first Sylow theorem concerning conjugation of two coset/two orbits of length 1

The proof that this question concerns is from the Sylow's theorems.

Let $G$ be a finite group of order $p^rm$ where $p$ is prime, $r\geq1$ and $p\nmid m$. Proof the existence of at least one subgroup $P$ of order $p^r$.

I am stuck at the part where you show that there exists only one orbit $\{\alpha \}$ of length $1$.

Let $P\leq G$ a maximal subgroup of $G$ with $|P|=p^s$., $H:=N(P)$ the normalizer of $P$. Let $\Omega:= cos(G:H)$ and $\alpha:=H$ so that $H=Stab(\alpha)$. Consider the action of the $P$ on $\Omega$. As $P\leq H$ there is one $P$-Orbit of length $1$ which is $\{\alpha\}$. Be $\{\beta\}$ any $P$- Orbit of length $1$.

Then $\beta=x^{-1}\alpha x$ for some $x\in G$.

What is the reason for this last statement?

I know that if $G$ operates on its subgroups the conjugation is a transitive operation and thus it would be clear. Cosets aren't subgroups though. So I thought it probably has to do with how conjugation is an inner automorphism, thus bijective and $|\{\beta\}|=\{\alpha\}|=1$ but I can't quite put it together.

Are orbits of the same length always conjugate to each other? Is it enough to just say that you can map one coset to another through a bijection? Or can I somehow proof that the operation of $G$ operates on $\Omega$ through conjugation is transitive? I get stuck because I end up with trying to show that there exists a $g\in G$ so that the RHS is true:

$$aH \sim bH \Leftrightarrow g(aH)g^{-1} = bH$$

(where $a,b\in G$) I don't know how I would show that both cosets are the same.

Relevant part of the proof from Groups and Geometry (Neumann, Stoy, Thompson)

• The action is not conjugation, it is right multiplication. Where you see $\alpha^x$ in NST it does not mean here $x^{-1}\alpha x$ it means $\alpha\cdot x=Hx$. It is the stabilisers which are related by conjugation. OK? – ancientmathematician Mar 12 '17 at 13:55
• Thank you! I was unaware because at one point the same notation was used for conjugation. Then right translation on the cosets ist transitive and thus the Stabilizers are conjugated. I managed to show even that, so the rest should be clear. – laura_b Mar 14 '17 at 14:15