Definition:Internal Group Direct Product/General Definition

Definition
Let $\left \langle {H_n} \right \rangle = \left({H_1, \circ {\restriction_{H_1}}}\right), \ldots, \left({H_n, \circ {\restriction_{H_n}}}\right)$ be a (finite) sequence of subgroups of a group $\left({G, \circ}\right)$

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

The group $\left({G, \circ}\right)$ is the internal group direct product of $\left \langle {H_n} \right \rangle$ the mapping:


 * $\displaystyle C: \prod_{k \mathop = 1}^n H_k \to G: C \left({h_1, \ldots, h_n}\right) = \prod_{k \mathop = 1}^n h_k$

is a group isomorphism from the group direct product $\left({H_1, \circ {\restriction_{H_1}}}\right) \times \cdots \times \left({H_n, \circ {\restriction_{H_n}}}\right)$ onto $\left({G, \circ}\right)$.

Also known as
Some authors call this 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.