Definition:Ring Direct Product
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Definition
Finite Case
Let $\left({R_1, +_1, \circ_1}\right), \left({R_2, +_2, \circ_2}\right), \ldots, \left({R_n, +_n, \circ_n}\right)$ be rings.
Let:
- $\displaystyle \left({R, +, \circ}\right) = \prod_{k \mathop = 1}^n \left({R_k, +_k, \circ_k}\right)$
be the external direct product on two operations, such that:
- $(1):\quad$ The operation $+$ induced on $R$ by $+_1, \ldots, +_n$ is defined as:
- $\left({s_1, s_2, \ldots, s_n}\right) + \left({t_1, t_2, \ldots, t_n}\right) = \left({s_1 +_1 t_1, s_2 +_2 t_2, \ldots, s_n +_n t_n}\right)$
- $(2):\quad$ The operation $\circ $ induced on $R$ by $\circ_1, \ldots, \circ_n$ is defined as:
- $\left({s_1, s_2, \ldots, s_n}\right) \circ \left({t_1, t_2, \ldots, t_n}\right) = \left({s_1 \circ_1 t_1, s_2 \circ_2 t_2, \ldots, s_n \circ_n t_n}\right)$
Then $\left({R, +, \circ}\right)$ is referred to as the (external) direct product of $\left({R_1, +_1, \circ_1}\right), \left({R_2, +_2, \circ_2}\right), \ldots, \left({R_n, +_n, \circ_n}\right)$.
The external direct product of $R_1, R_2, \ldots, R_n$ is often denoted
- $R_1 \times R_2 \times \cdots \times R_n$