# Re: What databases have taught me

From: David Cressey <dcressey_at_verizon.net>

Date: Sun, 25 Jun 2006 12:32:16 GMT

Message-ID: <knvng.2111$TC1.1128_at_trndny08>

Date: Sun, 25 Jun 2006 12:32:16 GMT

Message-ID: <knvng.2111$TC1.1128_at_trndny08>

> 2. Identify all of the regions. Regions are the empty spaces on your

*> paper, and are separated by edges from the graph. The blank space
**> outside of where you've drawn the graph DOES count as a region, so there
**> is always at least one. If the graph is a tree, for example, then there
**> is only one region, so the dual only has one vertex.
*

Doesn't region identification depend on the topology of the space? If your graph were on the surface of a torus, wouldn't you come up with possibly different regions? (viz. the seven color map theorem for the surface of a torus) Received on Sun Jun 25 2006 - 14:32:16 CEST