Problem

Source: 2019 RMM Shortlist N1

Tags: number theory, Even



Let $p$ and $q$ be relatively prime positive odd integers such that $1 < p < q$. Let $A$ be a set of pairs of integers $(a, b)$, where $0 \le a \le p - 1, 0 \le b \le q - 1$, containing exactly one pair from each of the sets $$\{(a, b),(a + 1, b + 1)\}, \{(a, q - 1), (a + 1, 0)\}, \{(p - 1,b),(0, b + 1)\}$$whenever $0 \le a \le p - 2$ and $0 \le b \le q - 2$. Show that $A$ contains at least $(p - 1)(q + 1)/8$ pairs whose entries are both even. Agnijo Banerjee and Joe Benton, United Kingdom