MathDB

Some pairs of cities in the country are connected by airlines, and some are not. But every city has an airport, from which you can get to any other city, making no more than one transfer. A tourist who wants to make a round trip through several cities of the country will have to fly around at least five cities. Prove that the same number of airlines depart from each city of the country (If there is an airline from one city to another, then there is also one from the second to the first. A circular trip is a route that passes through at least three cities, starting and ending in same city, other cities are not repeated in it)

Problem Statement