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In mathematics, one can often define a direct product of objects already known, giving a new one. This generalizes the Cartesian product of the underlying sets, together with a suitably defined structure on the product set. More abstractly, one talks about the product in category theory, which formalizes these notions. Examples are the product of sets (see Cartesian product), groups (described below), the product of rings and of other algebraic structures. The product of topological spaces is another instance. There is also the direct sum – in some areas this is used interchangeably, in others it is a different concept. == Examples == * If we think of as the set of real numbers, then the direct product is precisely just the cartesian product, . * If we think of as the group of real numbers under addition, then the direct product still consists of . The difference between this and the preceding example is that is now a group. We have to also say how to add their elements. This is done by letting . * If we think of as the ring of real numbers, then the direct product again consists of . To make this a ring, we say how their elements are added, , and how they are multiplied . * However, if we think of as the field of real numbers, then the direct product does not exist – naively defining in a similar manner to the above examples would not result in a field since the element does not have a multiplicative inverse. In a similar manner, we can talk about the product of more than two objects, e.g. . We can even talk about product of infinitely many objects, e.g. . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「direct product」の詳細全文を読む スポンサード リンク
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