Three cottage problem
From Wikipedia, the free encyclopedia.

The three cottage problem is a problem in mathematical graph theory:

Suppose there are three cottages that each need to be connected to the gas, water, and electric companies. Is there a way to do so without any of the lines crossing each other?

In more formal terms, this asks whether the bipartite graph K_3,3 is planar. Kazimierz Kuratowski proved in 1930 that K_3,3 is nonplanar, and thus that the three cottage problem has no solution.

The original article can be found at: http://en.wikipedia.org/wiki/Three_cottage_problem.
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