Distance Function
# Definition 1
Let $S$ be a Set. A Distance Function on $S$ is a Function of the form $d: S^{2} \to \mathbb{R}_{\geq 0}$ that satisfies the following properties:
- $d(x, y) = 0$ If and Only If $x = y$ $\forall x,y \in S$.
- $d(x, y) = d(y, x)$ $\forall x,y \in S$
- Triangle Inequality: $d(x, z) \leq d(x, y) + d(y, z)$ $\forall x,y,z \in S$
# Definition 2
An Extended Distance Function which is never $\infty$.