The set of points, known as boundary points, which are members of the set closure of a given set and the set closure of its complement set. The boundary is sometimes called the frontier.
See alsoBoundary Conditions, Boundary Map, Boundary Point, Boundary Set, Natural Boundary, Surgery
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Weisstein, Eric W. "Boundary." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Boundary.html