MathLabs

第2题

In a competition, there 是 a a contestants 且 b b judges, 其中 b≥3 b\ge3 是an odd 整数. Each judge rates 每个contestant as either “pass” 或“fail”. 设 k k 是a 数 使得, 对于any two judges, their ratings coincide 对于在most k k contestants. 证明 ka≥b−12b\frac{k}{a}\ge\frac{b-1}{2b}.
第 1/4 步:计数 agreements 由pairs 的judges
通俗地说

计数 same objects 从judges’ side.

N≤k(b2)N\le k\binom b2
详细分析

设 N N be 数 的triples consisting 的a contestant 且an unordered pair 的distinct judges who give same rating. Every pair 的judges agrees 上在most k k 译文: contestants, so N≤k(b2)=kb(b−1)2 N\le k\binom b2=\frac{kb(b-1)}2.