Maths Olympiad Prep

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Combinatorics Difficulty 5.6 AIME, harder Prove it JBMO

Problem:

In a country with nn cities, all direct airlines are two-way. There are r>2014r > 2014 routes between pairs of different cities that include no more than one intermediate stop (the direction of each route matters). Find the least possible nn and the least possible rr for that value of nn.

Solution

Solution:

Denote by X1,X2,,XnX_{1}, X_{2}, \ldots, X_{n} the cities in the country and let XiX_{i} be connected to exactly mim_{i} other cities by direct two-way airline. Then XiX_{i} is a final destination of mim_{i} direct routes and an intermediate stop of mi(mi1)m_{i}(m_{i}-1) non-direct routes. Thus r=m12++mn2r = m_{1}^{2} + \ldots + m_{n}^{2}. As each mim_{i} is at most n1n-1 and 13122<201413 \cdot 12^{2} < 2014, we deduce n14n \geq 14.

Consider n=14n = 14. As each route appears in two opposite directions, rr is even, so r2016r \geq 2016. We can achieve r=2016r = 2016 by arranging the 14 cities uniformly on a circle and connect (by direct two-way airlines) all of them, except the diametrically opposite pairs. This way, there are exactly 14122=201614 \cdot 12^{2} = 2016 routes.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.