# Thread: Sylow subgroup of group of order 56

1. ## Sylow subgroup of group of order 56

Let $G$ be a group of order 56, $T \in \text{Syl}_2(G), S \in \text{Syl}_7(G)$ be given. Then $|T| = 8, |S| = 7$. Assume $S$ is not normal, so then $T \lhd G$. Show that $T \cong C_2^3$.

Here's what I have so far. My idea is to show that each element of $T$ has order dividing 2. Let $z \in S$ be given such that $\langle z \rangle = S$. Then $G / T = \{T z^i : i=0,\dots,6\}$ and $\forall x \in T, (Tz)^x = Tz$. I've also got that $G = T \rtimes S$, although I fail to seen how any of this is helping.

2. Originally Posted by Giraffro
Let $G$ be a group of order 56, $T \in \text{Syl}_2(G), S \in \text{Syl}_7(G)$ be given. Then $|T| = 8, |S| = 7$. Assume $S$ is not normal, so then $T \lhd G$. Show that $T \cong C_2^3$.

Here's what I have so far. My idea is to show that each element of $T$ has order dividing 2. Let $z \in S$ be given such that $\langle z \rangle = S$. Then $G / T = \{T z^i : i=0,\dots,6\}$ and $\forall x \in T, (Tz)^x = Tz$. I've also got that $G = T \rtimes S$, although I fail to seen how any of this is helping.

So you have that $T\rtimes S\Longrightarrow$ there exists a non-trivial homomorphism $S\rightarrow Aut(T)$ . Well, what are the possible orders of automorphisms groups of groups of order 8?

Tonio

3. Originally Posted by tonio
So you have that $T\rtimes S\Longrightarrow$ there exists a non-trivial homomorphism $S\rightarrow Aut(T)$ . Well, what are the possible orders of automorphisms groups of groups of order 8?

Tonio
So $\exists \phi : S \to \text{Aut}(T)$ a group homomorphism such that $\forall x, y \in T, \forall i, j \in \mathbb{Z}, x z^i y z^j = x \phi(z^i)(y) z^{i+j}$. If $\phi(z) = \text{id}_T$ then $\forall x \in T, z^x = x^{-1} z x = x^{-1} \phi(z)(x) z = z$, so as $S = \langle z \rangle, S \lhd G$, which is a contradiction. Therefore $\phi(z) \ne \text{id}_T$ and so as $o(\phi(z)) \ne o(z), o(\phi(z)) = 7$. So it follows that $\exists x \in T$ such that $x,\phi(z)(x),\dots,\phi(z^6)(x)$ are distinct and have the same order. So $T$ has at least 7 elements of a specific order.

$
\begin{array}{c|ccc}
& \text{Order} \, 2 & \text{Order} \, 4 & \text{Order} \, 8 \\
\hline
C_8 & 1 & 2 & 4 \\
C_4 \times C_2 & 3 & 4 & 0 \\
C_2^3 & 7 & 0 & 0 \\
D_8 & 5 & 2 & 0 \\
Q & 1 & 6 & 0
\end{array}$

Therefore $T \cong C_2^3$.

Thanks for the help!