1977 IMO 第 3 题

(BUL 3) In a company of $n$ persons, each person has no more than $d$ acquaintances, and in that company there exists a group of $k$ persons, $k \geq d$, who are not acquainted with each other. Prove that the number of acquainted pairs is not greater than $\left[n^{2} / 4\right]$.

(BUL 3) 在一个由 $n$ 人组成的公司中，每个人的熟人不超过 $d$，并且该公司中存在一组 $k$ 人，$k \geq d$，他们彼此不认识。证明熟悉对的数量不大于$\left[n^{2} / 4\right]$。