# HDU 4702. Group

http://acm.hust.edu.cn/vjudge/problem/viewProblem.action?id=46279

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### Analysis:

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「A subset H of the group G is a subgroup of G if and only if it is nonempty and closed under products and inverses. (The closure conditions mean the following: whenever a and b are in H, then ab and a−1 are also in H. These two conditions can be combined into one equivalent condition: whenever a and b are in H, then ab−1 is also in H.)"


http://en.wikipedia.org/wiki/Subgroup

##### 拉格朗日定理

http://zh.wikipedia.org/wiki/%E6%8B%89%E6%A0%BC%E6%9C%97%E6%97%A5%E5%AE%9A%E7%90%86_(%E7%BE%A4%E8%AB%96)

### References:

Group 解題報告 by ftiasch