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A quasimetric on a set $X$ is a real-valued function $d: A \times A \to \R$ which satisfies the following conditions:

\((\text M 1)\)   $:$     \(\ds \forall x \in A:\) \(\ds \map d {x, x} = 0 \)             
\((\text M 2)\)   $:$     \(\ds \forall x, y, z \in A:\) \(\ds \map d {x, y} + \map d {y, z} \ge \map d {x, z} \)             
\((\text M 4)\)   $:$     \(\ds \forall x, y \in A:\) \(\ds x \ne y \implies \map d {x, y} > 0 \)             

Note the numbering system of these conditions. They are numbered this way so as to retain consistency with the metric space axioms, of which these are a subset.

The difference between a quasimetric and a metric is that a quasimetric does not insist that the distance function between distinct elements is commutative, that is, that $\map d {x, y} = \map d {y, x}$.

Quasimetric Space

A quasimetric space $M = \struct {A, d}$ is an ordered pair consisting of a set $A \ne \O$ followed by a quasimetric $d: A \times A \to \R$ which acts on that set.

Also known as

A quasimetric on a quasimetric space can be referred to as a distance function in the same way as a metric on a metric space.

Also see

  • Results about quasimetric spaces can be found here.