MathDB
why does C shortlist have 9 problems

Source: 2022 ISL/C9, Proposed by Ankan Bhattacharya

July 9, 2023
combinatorics

Problem Statement

Let Z0\mathbb Z_{\ge 0} be the set of non-negative integers, and let f:Z0×Z0Z0f:\mathbb Z_{\ge 0}\times \mathbb Z_{\ge 0} \to \mathbb Z_{\ge 0} be a bijection such that whenever f(x1,y1)>f(x2,y2)f(x_1,y_1) > f(x_2, y_2), we have f(x1+1,y1)>f(x2+1,y2)f(x_1+1, y_1) > f(x_2 + 1, y_2) and f(x1,y1+1)>f(x2,y2+1)f(x_1, y_1+1) > f(x_2, y_2+1).
Let NN be the number of pairs of integers (x,y)(x,y) with 0x,y<1000\le x,y<100, such that f(x,y)f(x,y) is odd. Find the smallest and largest possible values of NN.