两个群的外直积ThegroupG x H is called the erternal direct product of G and H.Noticethat thereis nothing special about the fact that we have used only two groups to build a new group.The direct productnIIG; = Gi × G2 × ... × Gni1of the groups Gi,G2,...,Gn is defined in exactly the same manner.If G =Gi = G2 == Gn, we often write Gn instead of Gi × G2 × ... × Gn
两个群的外直积
问题4：为什么下面的结论不叫“定理”？Proposition 9.7 Let G and H be groups. The set G x H is a group underthe operation (g1, hi)(g2, h2)= (g192,hih2) where g1,g2 E G and h1, h2 E H.Example 9. The group Z, considered as a set, is just the set of all binaryn-tuples.The group operation is the“exclusive or"of two binary n-tuples.For example,(01011101)+(01001011)=(0Q010110)后会有期个操作从何而来？问题5.1：这个符号是
问题4：为什么下面的结论不叫“定理”？ 问题5.1：这个符号是什么意思？后会有期问题5.2：这个操作从何而来？