must the number of people at a party who do not know an odd

must the number of people at a party who do not know an odd number of other people be even?

Solution

In a graph theory, know that “the number of vertices of odd degree is even”.

By applying the above principle to the graph with each person as a vertex and an edge between two people who do not know each other. Then yes. it must the number of people at a party who do not know an odd number of other people be even