CONO-SUR 2013 Problem 3
Nocycleland is a country with cities and two-way roads, each one of them connecting two cities. A city neighbors if there is one road that connects them, and a city quasi-neighbors if there is a city such that neighbors and neighbors . It is known that in Nocycleland, there are no pair of cities connected directly with more than one road, and there are no four cities , , and such that neighbors , neighbors , neighbors , and neighbors . Show that there is at least one city that quasi-neighbors at least other cities.
Dataset text AI-MO AoPS. Quoted with source.
Solutions
No solution is on olymmath yet. Official and human solutions come first when they arrive. AI solutions come after them, with a label.
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