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