MathDB

Two countries (

A

and

B

) organize a conference, and they send an equal number of participants. Some of them have known each other from a previous conference. Prove that one can choose a nonempty subset

C

of the participants from

A

such that one of the following holds: [*]the participants from

B

each know an even number of people in

C

, [*]the participants from

B

each know an odd number of participants in

C

Problem Statement