MathDB
CGMO6: Airline companies and cities

Source: 2012 China Girl's Mathematical Olympiad

August 13, 2012
inductioncombinatorics unsolvedcombinatorics

Problem Statement

There are nn cities, 22 airline companies in a country. Between any two cities, there is exactly one 22-way flight connecting them which is operated by one of the two companies. A female mathematician plans a travel route, so that it starts and ends at the same city, passes through at least two other cities, and each city in the route is visited once. She finds out that wherever she starts and whatever route she chooses, she must take flights of both companies. Find the maximum value of nn.