The function d used to define a distance between two points in constructing a metric space, with the following properties:
d(x, y)=0 if and only if x=y,
d(x, y)=d(y, x),
d(x, y)+d(y, z)≥d(x, z) for any points x, y, z.
Subjects: Mathematics — Astronomy and Astrophysics.