Pullback of Quotient Group Isomorphism is Subgroup

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Let $\struct {G, \circ}$ be a group whose identity element is $e_G$.

Let $\struct {H, *}$ be a group whose identity element is $e_H$.

Let $N \lhd G, K \lhd H$ be normal subgroups of $G$ and $H$ respectively.


$G / N \cong H / K$


$G / N$ denotes the quotient of $G$ by $N$
$\cong$ denotes group isomorphism.

Let $\theta: G / N \to H / K$ be such a group isomorphism.

Let $G \times^\theta H$ be the pullback of $G$ and $H$ via $\theta$.

Then $G \times^\theta H$ is a subgroup of $G \times H$.


This result is proved by an application of the Two-Step Subgroup Test:

Condition $(1)$

From the definition of pullback:

$\tuple {e_G, e_H} \in G \times^\theta H$

if and only if:

$\map \theta {e_G \circ N} = e_H * K$

By Coset by Identity, $e_G \circ N, e_H * K$ are the identities of $G / N$ and $H / K$

From Group Homomorphism Preserves Identity:

$\map \theta {e_G \circ N} = e_H * K$

So $\tuple {e_G, e_H} \in G \times^\theta H$

Thus $G \times^\theta H$ is non-empty.


Condition $(2)$

Let $\tuple {g, h}$ and $\tuple {g', h'}$ be elements of $G \times^\theta H$.

It follows by definition of $\theta$ that:

$\map \theta {g \circ N} = h * K$


$\map \theta {g' \circ N} = h' * K$

By the morphism property:

$\map \theta {g \circ N} * \map \theta {g' \circ N} = \map \theta {g \circ N \circ g' \circ N} = \map \theta {\paren {g \circ g'} \circ N}$


\(\ds \paren {h * h'} * K\) \(=\) \(\ds h * K * h' * K\)
\(\ds \) \(=\) \(\ds \map \theta {g \circ N} * \map \theta {g' \circ N}\)
\(\ds \) \(=\) \(\ds \map \theta {\paren {g \circ g'} \circ N}\)


$\tuple {g \circ g', h * h'} \in G \times^\theta H$

Hence $G \times^\theta H$ is closed under the operation.


Condition $(3)$

Let $\tuple {g, h} \in G \times^\theta H$.


\(\ds \map \theta {g \circ N}\) \(=\) \(\ds h * K\)
\(\ds \leadsto \ \ \) \(\ds \map \theta {g \circ N}^{-1}\) \(=\) \(\ds h^{-1} * K\)


\(\ds \map \theta {g \circ N}^{-1}\) \(=\) \(\ds \map \theta {g^{-1} \circ N}\) Group Homomorphism Preserves Inverses
\(\ds \leadsto \ \ \) \(\ds \map \theta {g^{-1} \circ N}\) \(=\) \(\ds h^{-1} * K\)
\(\ds \leadsto \ \ \) \(\ds \tuple {g^{-1}, h^{-1} }\) \(\in\) \(\ds G \times^\theta H\)

Thus $G \times^\theta H$ is closed under inverses.


Therefore by the Two-Step Subgroup Test:

$G \times^\theta H \le G \times H$