两个群的外直积 The group Gx H is called the external direct product of G and H.Notice that there is nothing special about the fact that we have used only two groups to build a new group. The direct product ΠG,=G1×G2×…×Gn i=1 of the groups G1,G2,...,Gn is defined in exactly the same manner.If G=G1 =G2 ·=Gn,we often write Gm instead of Gi×G2X·×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 Gx H is a group under the operation (g1,h1)(g2,h2)=(91g2,hih2)where g1,92 EG and h1,h2EH. Example 9.The group Z2,considered as a set,is just the set of all binary n-tuples.The group operation is the "exclusive or"of two binary n-tuples. For example, (01011101)+(01001011)=(00010110). 问题5.1：这个符号是 后会有期 个操作从何而来？
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？