Source: 2022 ISL/C9, Proposed by Ankan Bhattacharya
July 9, 2023
combinatorics
Problem Statement
Let Z≥0 be the set of non-negative integers, and let f:Z≥0×Z≥0→Z≥0 be a bijection such that whenever f(x1,y1)>f(x2,y2), we have f(x1+1,y1)>f(x2+1,y2) and f(x1,y1+1)>f(x2,y2+1).Let N be the number of pairs of integers (x,y) with 0≤x,y<100, such that f(x,y) is odd. Find the smallest and largest possible values of N.