A set X equipped with a metric $$d:X \times X \to {R_ + }$$ which satisfies the conditions (the axioms of the metric) $$d\left( {x,y} \right) = 0 \Leftrightarrow x = y,\forall x,y \in X$$ (1) $$d\left( {x,y} \right) = d\left( {y,x} \right)\forall x,y \in X$$ (2) $$d\left( {x,z} \right) \leqslant d\left( {x,y} \right) + d\left( {y,z} \right),\forall x,y,z, \in X\left( {thetriangleinequality} \right)$$ (3) is called the metric space.