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| | Binary operation |
 | | In the definition of a group on mathworld, http://mathworld.wolfram.com/Group.html, it is stated that the group operation is a binary operation, and it is stated that elements of a group must satisfy the four properties, including closure. |
 | | What do you mean "isn't it sufficient to talk about the property of closure in the definition of a group?" It is not necessary to talk about the property of closure in the defintion of a group. |
 | | Anyways, although it is not necessary, in theory, to talk about the property of closure, you are often just given a set S with a function * with domain SxS, and you have to verify that * is indeed an operation, that is, that closure does indeed hold. |
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