Ultrametric space
In mathematics, an ultrametric space is a metric space in which the triangle inequality is strengthened to for all , , and . Sometimes the associated metric is also called a non-Archimedean metric or super-metric.
From the above definition, one can conclude several typical properties of ultrametrics. For example, for all , at least one of the three equalities or or holds. That is, every triple of points in the space forms an isosceles triangle, so the whole space is an isosceles set.
Defining the (open) ball of radius centred at as , we have the following properties:
Proving these statements is an instructive exercise.[2] All directly derive from the ultrametric triangle inequality. Note that, by the second statement, a ball may have several center points that have non-zero distance. The intuition behind such seemingly strange effects is that, due to the strong triangle inequality, distances in ultrametrics do not add up.