olymmath

CONO-SUR 2013 Problem 3

    Nocycleland is a country with 500500 cities and 20132013 two-way roads, each one of them connecting two cities. A city AA neighbors BB if there is one road that connects them, and a city AA quasi-neighbors BB if there is a city CC such that AA neighbors CC and CC neighbors BB . 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 AA , BB , CC and DD such that AA neighbors BB , BB neighbors CC , CC neighbors DD , and DD neighbors AA . Show that there is at least one city that quasi-neighbors at least 5757 other cities.

    Dataset text AI-MO AoPS. Quoted with source.

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