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Problem of the Month – November 2019

In a country between any two cities there is a direct round flight operated by only one of
r ≥ 3 air companies. Suppose that only by using of flights of any of these r companies
it is possible to visit any city from any other city (directly or by using of several flights).
Prove that there are three cities such that direct flights between them are operated by
three different companies.


Correct Solution by,

  • Toshihiro Shimizu, Kawasaki, Japan