问题2：从这个定理中，你能解释我们为 什么研究“同构”吗？ Theorem 9.1 Let G-H be an isomorphism of two groups.Then the following statements are true. 1.-1:HG is an isomorphism. 2.G=H. 3.If G is abelian,then H is abelian. 4.If G is cyclic,then H is cyclic. 5.If G has a subgroup of order n,then H has a subgroup of order n.问题2：从这个定理中，你能解释我们为 什么研究“同构”吗？