Definition:Internal Group Direct Product/Definition 2

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Let $\struct {H_1, \circ {\restriction_{H_1} } }, \struct {H_2, \circ {\restriction_{H_2} } }$ be subgroups of a group $\struct {G, \circ}$

where $\circ {\restriction_{H_1} }, \circ {\restriction_{H_2} }$ are the restrictions of $\circ$ to $H_1, H_2$ respectively.

The group $\struct {G, \circ}$ is the internal group direct product of $H_1$ and $H_2$ if and only if:

$(1): \quad \struct {H_1, \circ {\restriction_{H_1} } }$ and $\struct {H_2, \circ {\restriction_{H_2} } }$ are both normal subgroups of $\struct {G, \circ}$
$(2): \quad$ every element of $G$ can be expressed uniquely in the form:
$g = h_1 \circ h_2$

Also known as

Some authors refer to the internal group direct product $H_1 \times H_2$ as the normal product of $H_1$ by $H_2$.

Other sources use the term semidirect product.

Some authors call it just the group direct product, but it should not be confused with the external group direct product.

Although this is just a more specific example of the internal direct product of general algebraic structures, it is usually defined and treated separately because of its considerable conceptual importance.


$C_2 \times C_3$ is Internal Group Direct Product of $C_6$

The direct product of the cyclic groups $C_2$ and $C_3$ is isomorphic to the cyclic groups $C_6$.

Hence it is seen to be an internal group direct product.

Also see

  • Results about (internal) group direct products can be found here.