# Definition:Internal Direct Product

## Definition

Let $\struct {S_1, \circ {\restriction_{S_1} } }, \struct {S_2, \circ {\restriction_{S_2} } }$ be closed algebraic substructures of an algebraic structure $\struct {S, \circ}$

where $\circ {\restriction_{S_1}}, \circ {\restriction_{S_2}}$ are the operations induced by the restrictions of $\circ$ to $S_1, S_2$ respectively.

The structure $\struct {S, \circ}$ is the **internal direct product of $S_1$ and $S_2$** if and only if the mapping:

- $C: S_1 \times S_2 \to S: \map C {s_1, s_2} = s_1 \circ s_2$

is an isomorphism from the cartesian product $\struct {S_1, \circ {\restriction_{S_1} } } \times \struct {S_2, \circ {\restriction_{S_2} } }$ onto $\struct {S, \circ}$.

The operation $\circ$ on $S$ is the operation induced on $S$ by $\circ {\restriction_{S_1} }$ and $\circ {\restriction_{S_2} }$.

It can be seen that the mapping $C$ is the restriction of the mapping $\circ$ of $S \times S$ to the subset $S_1 \times S_2$.

### General Definition

Let $\struct {S_1, \circ {\restriction_{S_1} } }, \ldots, \struct {S_n, \circ {\restriction_{S_n} } }$ be closed algebraic substructures of an algebraic structure $\struct {S, \circ}$

where $\circ {\restriction_{S_1} }, \ldots, \circ {\restriction_{S_n} }$ are the operations induced by the restrictions of $\circ$ to $S_1, \ldots, S_n$ respectively.

The structure $\struct {S, \circ}$ is the **internal direct product of $\sequence {S_n}$** if the mapping:

- $\ds C: \prod_{k \mathop = 1}^n S_k \to S: \map C {s_1, \ldots, s_n} = \prod_{k \mathop = 1}^n s_k$

is an isomorphism from the cartesian product $\struct {S_1, \circ {\restriction_{S_1} } } \times \cdots \times \struct {S_n, \circ {\restriction_{S_n} } }$ onto $\struct {S, \circ}$.

The operation $\circ$ on $S$ is the operation induced on $S$ by $\circ {\restriction_{S_1} }, \circ {\restriction_{S_2} }, \ldots, \circ {\restriction_{S_n} }$.

### Decomposition

The set of algebraic substructures $\left({S_1, \circ {\restriction_{S_1}}}\right), \left({S_2, \circ {\restriction_{S_2}}}\right), \ldots, \left({S_n, \circ {\restriction_{S_n}}}\right)$ whose direct product is isomorphic with $\left({S, \circ}\right)$ is called a **decomposition** of $S$.

## Also known as

Some authors call this just the **direct product**.

Some authors call it the **direct composite**.

## Also see

- Definition:External Direct Product
- Definition:Internal Group Direct Product
- Definition:Ring Direct Sum

- Results about
**internal direct products**can be found here.

## 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