The interior of a set is the union of all its open subsets.

More informally, the interior of geometric structure is that portion of a region lying "inside" a specified boundary. For example, the interior of the sphere is an (open) ball and the interior of a circle is an (open) disk.

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Cite this as:

Weisstein, Eric W. "Interior." From MathWorld--A Wolfram Web Resource.

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