Re: Canonical DB

From: Dmitry A. Kazakov <>
Date: Sat, 24 Jun 2006 10:58:36 +0200
Message-ID: <4lsoa08lt2qu$.1mefkf7jftxz4$>

On Fri, 23 Jun 2006 15:51:07 -0700, Gene Wirchenko wrote:

> On Fri, 23 Jun 2006 21:59:52 +0200, "Dmitry A. Kazakov"
> <> wrote:

>>On Fri, 23 Jun 2006 15:37:54 +0200, mAsterdam wrote:

>>> These requirements establish K as a clean point type.
>>> Aside: With the last condition deleted one can make a
>>> circular K, having distance(k1, k2) <> distance(k2, k1).
>>That won't be formally a distance, which is required to be symmetric.

> Why must it be symmetric? A to B and B to A may be different
> distances if there are one-way routes involved.

Rather when the space is anisotropic. If you just have many routes, you still could get a metric distance by choosing the shortest path.

I found in Wikipedia that d(x,y) /= d(y,x) is called quasimetric space (I
never met such thing anywhere else). Wikipedia also states that any quasimetric space can be made metric, by defining

   d*(x,y) = (d(x,y) + d(y,x)) / 2

So it can be made metric.

Dmitry A. Kazakov
Received on Sat Jun 24 2006 - 10:58:36 CEST

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