Direct Product of Normal Subgroups is Normal
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Theorem
Let $G$ and $G'$ be groups.
Let:
- $H \lhd G$
- $H' \lhd G'$
where $\lhd$ denotes the relation of being a normal subgroup.
Then:
- $\paren {H \times H'} \lhd \paren {G \times G'}$
where $H \times H'$ denotes the group direct product of $H$ and $H'$
Proof
Let $\tuple {x, x'} \in G \times G'$ and $\tuple {y, y'} \in H \times H'$.
Then:
\(\ds \tuple {x, x'} \tuple {y, y'} \tuple {x, x'}^{-1}\) | \(=\) | \(\ds \tuple {x, x'} \tuple {y, y'} \tuple {x^{-1}, x'^{-1} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \tuple {x y x^{-1}, x' y' x'^{-1} }\) | ||||||||||||
\(\ds \) | \(\in\) | \(\ds H \times H'\) | Definition of Normal Subgroup |
Hence:
- $\paren {H \times H'} \lhd {G \times G'}$
$\blacksquare$
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {II}$: New Structures from Old: $\S 13$: Compositions Induced on Cartesian Products and Function Spaces: Exercise $13.14 \ \text{(a)}$
- 1971: Allan Clark: Elements of Abstract Algebra ... (previous) ... (next): Chapter $2$: Conjugacy, Normal Subgroups, and Quotient Groups: $\S 47 \theta$
- 1996: John F. Humphreys: A Course in Group Theory ... (previous) ... (next): Chapter $11$: The Sylow Theorems: Exercise $2$