MathDB

The participants to an international conference are native and foreign scientist. Each native scientist sends a message to a foreign scientist and each foreign scientist sends a message to a native scientist. There are native scientists who did not receive a message. Prove that there exists a set

S

of native scientists such that the outer

S

scientists are exactly those who received messages from those foreign scientists who did not receive messages from scientists belonging to

S

. Radu Niculescu

Problem Statement