Nice problem
Source: Tuymaada 2016
June 28, 2017
combinatoricsgraph theory
Problem Statement
The flights map of air company presents several cities. Some cities are connected by a direct (two way) flight, the total number of flights is m. One must choose two non-intersecting groups of r cities each so that every city of the first group is connected by a flight with every city of the second group. Prove that number of possible choices does not exceed .