Show that if a subset A ⊂ E n has volume, then the interior of A (cf. Prob. 15, Chap. III) has the same volume.
Prob 15.Let S be a subset of the metric space E. A point p ∈ S is called an interior point of S if there is an open ball in E of center p which is contained in S. Prove that the set of interior points of S is an open subset of E (called the interior of S) that contains all other open subsets of E that are contained in S.