Direct Product of Sylow p-Subgroups is Sylow p-Subgroup
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Theorem
Let $G_1$ and $G_2$ be groups.
Let $H_1$ and $H_2$ be subgroups of $G_1$ and $G_2$ respectively.
Let $H_1$ be a Sylow $p$-subgroup of $G_1$.
Let $H_2$ be a Sylow $p$-subgroup of $G_2$.
Then $H_1 \times H_2$ is a Sylow $p$-subgroup of $G_1 \times G_2$.
Proof
By definition of Sylow $p$-subgroup:
- $\order {H_1} = p^r$, where $p^r$ is the highest power of $p$ which is a divisor of $\order {G_1}$.
- $\order {H_2} = p^s$, where $p^s$ is the highest power of $p$ which is a divisor of $\order {G_2}$.
We have that:
- $\order {H_1 \times H_2} = p^{r + s}$
We also have that $p^{r + s}$ is the highest power of $p$ which is a divisor of $\order {G_1 \times G_2}$.
Hence it follows by definition that $H_1 \times H_2$ is a Sylow $p$-subgroup of $G_1 \times G_2$.
$\blacksquare$
Sources
- 1996: John F. Humphreys: A Course in Group Theory ... (previous) ... (next): Chapter $11$: The Sylow Theorems: Exercise $2$