Date: Jul 1, 2011 4:59 PM Author: Gang Subject: A Graph Theorem Question (4-coloring related) Hi,

Could Anyone here help me recall the name of a graph theorem? Basically it states like following:

In a 2D planar map, at most 4 regions/countries can be bordered with each other. Namely, you cannot find such 5 regions/countries, among which any one is bordered with all other 4 regions/countries.

Thanks many tons in advance!

-Gang