我们称o为加性单位元素。
群：二元结合运算为闭合的数学集合，具有单位元素。集合中每一元素都有逆元素。
Mathematics a set with an associative binary operation under which the set is closed, which contains an identity element and an inverse for every element in the set.
群：二元结合运算为闭合的数学集合，具有单位元素。集合中每一元素都有逆元素。
Mathematics a set with an associative binary operation under which the set is closed, which contains an identity element and an inverse for every element in the set.
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