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Open set
set that does not contain any of its boundary points / From Wikipedia, the free encyclopedia
In set theory an open set is a set where all elements have the same properties. Simply put, an open set is a set that does not include its edges or endpoints. For each point in the set, you can make a bubble around that point, such that all points in the bubble are also in the set.[1]
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On the other hand, a closed set includes all its edges or endpoints. A set that includes some of its edges or endpoints is neither open nor closed.[2]
An open set is very similar to an open interval.