数学概念合集（不定期更新）
Metric space 度量空间
In mathematics, a metric space is a set together with a metric on the set. The metric is a function that defines a concept of distance between any two members of the set, which are usually called points. The metric satisfies a few simple properties. Informally:
- the distance from a point to itself is zero,
- the distance between two distinct points is positive,
- the distance from A to B is the same as the distance from B to A, and the distance from A to B (directly) is less than or equal to the distance from A to B via any third point C.