Interior

Imprecisely speaking, the interior of a set contains all the points of the set except its bounds. The interior of some sets is empty. For example, a line in $$\mathbf R^2$$ has empty interior. Similarly, any point has zero interior. A set that has nonempty interior in one ambient space can have empty interior in another. For example, a 2D disk has nonempty interior in $$\mathbf R^2$$ but empty interior in $$\mathbf R^3.$$

Sometimes, we want to expand sets so that their interior is nonempty. To this end, we define the $$\textbf{relint}$$