## Abstract

Let *S* be a subset of a metric space *V.* A point *p* of *S* is called an *interior point* of *S* if there is a positive real number *δ* such that every point of *V* whose distance from *p* is less than *δ* belongs to *S*. The set *S* is said to be *open* in *V* if every point of *S* is an interior point of *S.* If *p* is a point in *V* then every subset *S* of *V* containing *p* as an interior point is called a *neighborhood* of *p* in *V*. A subset of *V* is said to be *closed* in *V* if its complement in *V* is open.

### Keywords

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## Copyright information

© Springer-Verlag New York Inc. 1983