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 | | The boundary points in it, which are empty, are in the empty set, and the set is therefore closed, while the interior points in it, which are empty again, are the subset of the empty set, and the set is therefore open. |
 | | It may stem, in part, from the gap between intuitive structures that are generally modelled by sets, such as piles of objects, and the formal definition of a set. |
 | | Zermelo-Fraenkel set theory, the existence of the empty set is assured by the axiom of empty set. |
| en-cyclopedia.com /wiki/Empty_set (1098 words) |
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