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Distance is a numerical description of how far apart things lie. In Physics or everyday discussion, distance may refer to a physical length, a period of time, or an estimation based on other criteria (e.g. “two counties over”). In Mathematics , distance must meet more rigorous criteria.


FORMAL TREATMENT


Mathematical analysis


General case

In M which meets the following criteria:
  • One can find the distance between any two points.

  • That distance is a distinct Real Number .

  • It is Positive Definite . d(''x'',''y'') ≥ 0, and d(''x'',''y'') = 0 If And Only If ''x'' = ''y''. (Distance is always positive, and it is zero precisely when measuring the distance from a point to itself).

  • It is Symmetric . d(''x'',''y'') = d(''y'',''x''). (The distance from ''x'' to ''y'' is the same as that from ''y'' to ''x'').

  • It satisfies the Triangle Inequality , d(''x'',''z'') ≤ d(''x'',''y'') + d(''y'',''z''). (The shortest distance between two points is on a straight line).

  • Such a function is known as a Metric . Together with the set, it makes up a Metric Space .

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